The impact of heavy reduction on dendritic morphology was explored by combining experimental research and numerical simulation in metallurgy,including a detailed three-dimensional(3D)analysis and reconstruction of den...The impact of heavy reduction on dendritic morphology was explored by combining experimental research and numerical simulation in metallurgy,including a detailed three-dimensional(3D)analysis and reconstruction of dendritic solidification structures.Combining scanning electron microscopy and energy-dispersive scanning analysis and ANSYS simulation,the high-precision image processing software Mimics Research was utilized to conduct the extraction of dendritic morphologies.Reverse engineering software NX Imageware was employed for the 3D reconstruction of two-dimensional dendritic morphologies,restoring the dendritic characteristics in three-dimensional space.The results demonstrate that in a two-dimensional plane,dendrites connect with each other to form irregularly shaped“ring-like”structures.These dendrites have a thickness greater than 0.1 mm along the Z-axis direction,leading to the envelopment of molten steel by dendrites in a 3D space of at least 0.1 mm.This results in obstructed flow,confirming the“bridging”of dendrites in three-dimensional space,resulting in a tendency for central segregation.Dense and dispersed tiny dendrites,under the influence of heat flow direction,interconnect and continuously grow,gradually forming primary and secondary dendrites in three-dimensional space.After the completion of dendritic solidification and growth,these microdendrites appear dense and dispersed on the two-dimensional plane,providing the nuclei for the formation of new dendrites.When reduction occurs at a solid fraction of 0.46,there is a noticeable decrease in dendritic spacing,resulting in improved central segregation.展开更多
Tungsten heavy alloys(WHAs)prepared using laser additive manufacturing(AM)exhibit intricate ge-ometries,albeit with limited mechanical properties.Here we designed a high-strength WHA featuring a FeCrCoNi high entropy ...Tungsten heavy alloys(WHAs)prepared using laser additive manufacturing(AM)exhibit intricate ge-ometries,albeit with limited mechanical properties.Here we designed a high-strength WHA featuring a FeCrCoNi high entropy alloy(HEA)binder via the laser metal deposition(LMD)technique.Due to the distinctive thermal cycle and rapid cooling rate,the as-deposited alloys exhibit microstructures with hy-poeutectic,eutectic-like,and spot-like characteristics.To elucidate this phenomenon,the solidification paths were delineated and analyzed by combining microstructural characterization and phase equilib-rium simulation.Theμphase precipitated out from the supersaturated solid solution,thereby nucleating massive dislocations on the FeCrCoNi matrix to increase the work hardening rate.Furthermore,theμphase formed an ultrafine intermetallic compound(IMC)layer around the W grain,reducing the hole or crack between the W grain and FeCrCoNi matrix.Attributed to the precipitation strengthening,the solid solution of the FeCrCoNi binder,along with the load-bearing strength of W,the developed alloy achieved ultrahigh compressive stress and strain of 2047 MPa and 32%respectively at room temperature.These findings contribute valuable insights to the advancement of additive manufacturing for tungsten alloys,leveraging their excellent properties.展开更多
In 316L austenitic stainless steel,the presence of ferrite phase severely affects the non-magnetic properties.316L austenitic stainless steel with low-alloy type(L-316L)and high-alloy type(H-316L)has been studied.The ...In 316L austenitic stainless steel,the presence of ferrite phase severely affects the non-magnetic properties.316L austenitic stainless steel with low-alloy type(L-316L)and high-alloy type(H-316L)has been studied.The microstructure and solidification kinetics of the two as-cast grades were in situ observed by high temperature confocal laser scanning microscopy(HT-CLSM).There are significant differences in the as-cast microstructures of the two 316L stainless steel compositions.In L-316L steel,ferrite morphology appears as the short rods with a ferrite content of 6.98%,forming a dual-phase microstructure consisting of austenite and ferrite.Conversely,in H-316L steel,the ferrite appears as discontinuous network structures with a content of 4.41%,forming a microstructure composed of austenite and sigma(σ)phase.The alloying elements in H-316L steel exhibit a complex distribution,with Ni and Mo enriching at the austenite grain boundaries.HT-CLSM experiments provide the real-time observation of the solidification processes of both 316L specimens and reveal distinct solidification modes:L-316L steel solidifies in an FA mode,whereas H-316L steel solidifies in an AF mode.These differences result in ferrite and austenite predominantly serving as the nucleation and growth phases,respectively.The solidification mode observed by experiments is similar to the thermodynamic calculation results.The L-316L steel solidified in the FA mode and showed minimal element segregation,which lead to a direct transformation of ferrite to austenite phase(δ→γ)during phase transformation after solidification.Besides,the H-316L steel solidified in the AF mode and showed severe element segregation,which lead to Mo enrichment at grain boundaries and transformation of ferrite into sigma and austenite phases through the eutectoid reaction(δ→σ+γ).展开更多
Carbon fibres have been produced from hydroxypropyl-modified lignin(TcC)/bio-based polyamide 1010(PA1010)blended filaments.Two grades of PA1010,with different molecular weights and rheological properties,were used for...Carbon fibres have been produced from hydroxypropyl-modified lignin(TcC)/bio-based polyamide 1010(PA1010)blended filaments.Two grades of PA1010,with different molecular weights and rheological properties,were used for blending with TcC.An oxidative thermal stabilisation step was used prior to carbonisation in an inert atmosphere to prevent the fusion of the filaments during the latter step.Thermal stabilisation was not possible using a one-step stabilisation process reported in the literature for lignin and other lignin/synthetic polymer blends.As a consequence,a cyclic process involving an additional isothermal phase at a lower temperature than the precursor filaments’melting point,was introduced to increase the cross-linking reactions between the lignin and polyamide.Thermally stabilised filaments were characterised by DSC,TGA,TGA-FTIR,ATR,and SEM techniques.Polymer rheology and heating rate used during thermal stabilisation influenced the thermal stabilisation process and mechanical properties of the derived filaments.Thermally stabilised filaments using optimised conditions(heating in the air atmosphere at 0.25℃/min to 180℃;isothermal for 1 h,cooling back down to ambient at 5℃/min;heating to 250℃ at 0.25℃/min,isothermal for 2 h)could be successfully carbonised.Carbon fibres pro-duced had void-free morphologies and mechanical properties comparable to similarly thermally stabilised and carbonised polyacrylonitrile(PAN)filaments.展开更多
This study used steel slag,fly ash,and metakaolin as raw materials(SFM materials)to create silica-alumina-based geopolymers that can solidify Hg^(2+)when activated with sodiumbased water glass.The experiments began wi...This study used steel slag,fly ash,and metakaolin as raw materials(SFM materials)to create silica-alumina-based geopolymers that can solidify Hg^(2+)when activated with sodiumbased water glass.The experiments began with a triangular lattice point mixing design experiment,and the results were fitted,analyzed,and predicted.The optimum SFM material mass ratio was found to be 70%steel slag,25%fly ash,and 5%metakaolin.The optimum modulus of the activator was identified by comparing the unconfined compressive strength and solidifying impact on Hg^(2+)of geosynthetics with different modulus.The SFM geopolymer was then applied in the form of potting to cure the granulated mercury tailings.The inclusion of 50%SFM material generated a geosynthetic that reduced mercury transport to the surface soil by roughly 90%.The mercury concentration of herbaceous plant samples was also reduced by 78%.It indicates that the SFM material can effectively attenuate the migration transformation of mercury.Finally,characterization methods such as XPS and FTIR were used to investigate the mechanism of Hg^(2+)solidification by geopolymers generated by SFM materials.The possible solidification mechanisms were proposed as alkaline environment-induced mercury precipitation,chemical bonding s,surface adsorption of Hg^(2+)and its precipitates by the geopolymer,and physical encapsulation.展开更多
By applying the rapid solidification technique of deep undercooling,Cu65Ni35 and Cu60Ni40 alloys achieved maximum undercoolings of 284 and 222 K,respectively.Microstructural images captured reveal grain refinement in ...By applying the rapid solidification technique of deep undercooling,Cu65Ni35 and Cu60Ni40 alloys achieved maximum undercoolings of 284 and 222 K,respectively.Microstructural images captured reveal grain refinement in both alloys across both large and small undercooling ranges.High-speed photography was used to analyze the relationship between solidification front morphology and undercooling,showing that dendrite remelting and fragmentation caused grain refinement under small undercooling,while stress-induced recrystallization is responsible under large undercooling.Microhardness testing further demonstrates a sudden drop in microhardness near the critical undercooling point,providing evidence for grain refinement due to recrystallization in large undercooling tissues.展开更多
The evolution of the microstructure and morphology of Cu55Ni45 and Cu60Ni40 alloys under varying degrees of undercooling was investigated through molten glass purification and cyclic superheating technology.By increas...The evolution of the microstructure and morphology of Cu55Ni45 and Cu60Ni40 alloys under varying degrees of undercooling was investigated through molten glass purification and cyclic superheating technology.By increasing the Cu content,the effect of Cu on the evolution of the microstructure and morphology of the Cu-Ni alloy during undercooling was studied.The mechanism of grain refinement at different degrees of undercooling and the effect of Cu content on its solidification behaviour were investigated.The solidification behaviour of Cu55Ni45 and Cu60Ni40 alloys was investigated using infrared thermometry and high-speed photography.The results indicate that both Cu55Ni45 and Cu60Ni40 alloy melts undergo only one recalescence during rapid solidification.The degree of recalescence increases approximately linearly with increasing undercooling.The solidification front of the alloy melts undergoes a transition process from a small-angle plane to a sharp front and then to a smooth arc.However,the growth of the subcooled melt is constrained to a narrow range,facilitating the formation of a coarse dendritic crystal morphology in the Cu-Ni alloy.At large undercooling,the stress breakdown of the directionally growing dendrites is primarily caused by thermal diffusion.The strain remaining in the dendritic fragments provides the driving force for recrystallisation of the tissue to occur,which in turn refines the tissue.展开更多
The Mg-Y-Zn magnesium alloy system is known for the presence of Long-Period Stacking Ordered(LPSO)phases that improves strength and ductility with minimal amounts of alloying elements.Even better improvements are asso...The Mg-Y-Zn magnesium alloy system is known for the presence of Long-Period Stacking Ordered(LPSO)phases that improves strength and ductility with minimal amounts of alloying elements.Even better improvements are associated with the specific microstructure known as the Mille-Feuille(MF)structure that can occur in this alloy as well after proper heat treatment.This study systematically compares the traditional ingot metallurgy method with the Bridgman method(slow cooling),coupled with diverse heat treatments and extrusion process.Microscopic analyses reveal variations in the presence of LPSO phases,MF structure,and especially grain size,leading to divergent mechanical and corrosion properties.The Bridgman approach surprisingly stands out,ensuring superior mechanical properties due to kink and texture strengthening.展开更多
The microstructure formed during solidification has a significant impact on the mechanical properties of materials.In this study,a two-dimensional(2D)cellular automaton(CA)-finite difference(FD)-CALPHAD model was deve...The microstructure formed during solidification has a significant impact on the mechanical properties of materials.In this study,a two-dimensional(2D)cellular automaton(CA)-finite difference(FD)-CALPHAD model was developed to simulate the formation of microstructure and solute segregation in the solidification processes of ternary alloys.In the model,dendritic growth is simulated using the CA technique,while solute diffusion is solved by the FD method,and the CALPHAD method is employed to calculate thermodynamic phase equilibrium during solidification.The CA-FD-CALPHAD coupled model is capable of reproducing the evolution of continuous nucleation and growth of grains as well as the evolution of the microstructure and solute distribution during solidification of ternary alloys.In this study,Al–Zn–Mg ternary alloy is taken as an example to simulate the growth of equiaxed and columnar grains and the columnar-to-equiaxed transition(CET)under different solidification conditions.The simulation results are compared with experimental data from the literature,showing a good agreement.Besides,the study also investigates the evolution of temperature and multicomponent solute fields during solidification and the effects of alloy composition and cooling rate on the microstructure morphology.The results reveal that the initial alloy composition and cooling rate significantly affect dendritic morphology and solute segregation.Higher initial alloy concentrations promote the growth of side branches in equiaxed grains,leading to more pronounced solute segregation between dendrites.As the cooling rate increases,the average grain size of the equiaxed grains decreases accordingly.Additionally,a higher cooling rate accelerates the columnar-to-equiaxed transition,leading to a finer grain structure.展开更多
Microporosity formed in the solidification process of Al alloys is detrimental to the alloy properties.A two-dimensional cellular automaton(CA)model was developed to simulate the microstructure and microporosity forma...Microporosity formed in the solidification process of Al alloys is detrimental to the alloy properties.A two-dimensional cellular automaton(CA)model was developed to simulate the microstructure and microporosity formation in Al-Cu alloys,considering variations in Cu content and solidification rate.The results indicate that the Cu content primarily influences the growth of microporosity.To validate the model,directional solidification experiments were conducted on Al-Cu alloys with varing Cu contents and withdrawal rates.The experimental results of dendrites and microporosity characteristics agree well with the predictions from the developed model,thus confirming the validity of the model.The alloy’s liquidus temperature,dendrite morphology,and hydrogen saturation solubility arising from different Cu contents have significant effects on microporosity morphology.The withdrawal rate primarily affects the nucleation of hydrogen microporosity by altering cooling rates and dendritic growth rates,resulting in different microporosity characteristics.展开更多
One of the major challenges in the application of microbially induced carbonate precipitation(MICP)is achieving"bacteria freedom",as it necessitates a substantial volume of bacterial solutions.Nevertheless,b...One of the major challenges in the application of microbially induced carbonate precipitation(MICP)is achieving"bacteria freedom",as it necessitates a substantial volume of bacterial solutions.Nevertheless,both insitu bacterial cultivation and transportation of bacterial solutions have proven to be inefficient.In this study,we suggested the utilization of bacteria in the form of dry powder,enabling easy on-site activation and achieving a relatively high urease activity.We conducted MICP curing experiments on gold mine tailings(GMT)using steel slag(SS)as an additive.The results showed that the average unconfined compressive strength(UCS)values of the tailings treated with MICP and MICP+SS reached 0.51 and 0.71 MPa,respectively.In addition,the average leaching reduction rates of Cu,Pb,Cr,Zn,and T-CN in GMT after MICP treatment reached 98.54%,100%,70.94%,59.25%,and 98.02%,respectively,and the average reduction rates after MICP+SS treatment reached 98.77%,100%,88.03%,72.59%,and 98.63%,respectively.SEM,XRD,FT-IR analyses,and ultra-deep field microscopy results confirmed that the MICP treatment produced calcite-based calcium carbonate that filled the inter-tailing pores and cemented them together,and the hydration mechanism was the main reason for the increased curing efficiency of SS.Our research findings demonstrate that bacterial powder can efficiently achieve the objectives of heavy metal removal and tailing solidification.This approach can substantially de-crease the expenses associated with bacterial cultivation and solution transportation,thereby playing a crucial role in advancing the practical implementation of MICP.展开更多
Nickel-based single-crystal(SX)superalloys are the key metallic materials of aeroengines.However,thermomechanical deformation always occurs during the directional solidification of SX superalloys,negatively influencin...Nickel-based single-crystal(SX)superalloys are the key metallic materials of aeroengines.However,thermomechanical deformation always occurs during the directional solidification of SX superalloys,negatively influencing the SX structure.Casting deformation is simulated in most of the previous studies,whereas the direct simulation of dendritic thermomechanical deformation has been largely ignored,resulting in a lack of comprehensive understanding of this process.In this study,we systematically investigate dendritic thermomechanical deformation with a model coupled with dendrite growth,fluid flow,and thermomechanical deformation behavior.Results reveal that the dendritic thermomechanical deformation-induced dendrite bending is not randomly distributed but is mainly concentrated on the casting surface.The dendritic thermal stress increases as dendrite grows and accumulates after dendrite bridging.Transverse thermal contraction mainly occurs at the edge of casting in the corner,and axial thermal contraction is larger than transverse contraction.The high-stress region of the primary dendrite trunk is mainly distributed below the dendrite bridging near the solidified part,and the stress along the transverse direction reaches its maximum value on the casting surface.Stress concentrated on the casting surface is mainly attributed to variations in transverse temperature gradients caused by heat dissipation on the lateral mold wall,and inconsistent constraints in the lateral mold walls.展开更多
This work studies the impact of the carbon diffusion on the growth kinetics of austenite and the solute segregation,by utilizing the phase-field(PF)method to simulate the solidification of a Fe-C binary alloy.It is re...This work studies the impact of the carbon diffusion on the growth kinetics of austenite and the solute segregation,by utilizing the phase-field(PF)method to simulate the solidification of a Fe-C binary alloy.It is revealed that increasing the ratio of the carbon diffusion coefficient in solid to that in liquid is advantageous in reducing the solute segregation,and a novel microsegregation model is developed based on the quantitative analysis of the results from PF simulations.The simplified one-dimensional diffusion simulation is employed to analyse the quantitative relationship between the parameters of the proposed microsegregation model and the properties of materials.The universality and reliability of the new microsegregation model are then validated by comparing with the experimental data of various alloy systems.These findings contribute to our comprehension of the fundamental theory of solidification and also provide a potential and promising approach to controlling the solidification microstructure.展开更多
This paper investigated the use of magnesium phosphate cement (MPC) for solidifying sludge with different humic acid (HA) content (ranging from 0 to 4.5%) and explored the solidification mechanism. Fluidity, setting t...This paper investigated the use of magnesium phosphate cement (MPC) for solidifying sludge with different humic acid (HA) content (ranging from 0 to 4.5%) and explored the solidification mechanism. Fluidity, setting time, unconfined compressive strength (UCS), the strength formation mechanism, and the spontaneous imbibition process of solidified sludge (SS) were studied. The results indicate that MPC can be used as a low-alkalinity curing agent. As the HA content increases, fluidity and setting time also increase, while hydration temperature and strength decrease. Additionally, the failure mode of SS transitions from brittleness to ductility. The strength of SS is composed of the cementation strength provided by MPC hydration products, matric suction, osmotic suction, and the structural strength of the sludge. MPC reduces the structural strength caused by the shrinkage of pure sludge under the action of matric suction, but the incorporation of MPC significantly improved the strength when the sludge is eroded by water. X-ray diffraction (XRD) and scanning electron microscopy (SEM) show that the sludge and MPC can form a dense solid body, forming various hydration products, and synergistically improve the mechanical properties of the sludge.展开更多
A novel droplet solidification technique was developed to emulate sub-rapid solidification and facilitate the formation of deposited films during the strip casting of silicon steels(w(Si):2.5 and 3.5 wt.%).With the in...A novel droplet solidification technique was developed to emulate sub-rapid solidification and facilitate the formation of deposited films during the strip casting of silicon steels(w(Si):2.5 and 3.5 wt.%).With the increasing number of droplet ejection experiments,the peak heat fluxes between droplet and substrate decreased firstly(1rd–5th ejection),then increased(5th–7th ejection),and finally decreased again(>7th ejection).In the first five experiments,the interfacial thermal resistance between the droplet and the substrate improved with increasing film thickness.However,at the onset of the 6th droplet ejection experiment,the deposited film initiated its melting process due to the accumulated thermal resistance,which has the potential to eradicate the cavity or air space existing between the droplet and the substrate.Consequently,the interfacial contact condition was improved gradually with the increasing melting area from 5th to 7th droplet ejection experiments,leading to an increase in heat fluxes.Increased SiO_(2) content in deposited films for 3.5 wt.%Si steel led to lower peak heat fluxes than for 2.5 wt.%Si steel.The solidification structure of the 2.5 wt.%Si steel droplet sample comprised a fine grain zone at the base,a columnar grain zone in the center,and an equiaxed grain zone at the top.However,the solidification structure of the 3.5 wt.%Si steel droplet only contained columnar grains and equiaxed grains,with a larger average grain size due to the lower interfacial heat flux.展开更多
ThMn_(12)-type iron-rich rare-earth permanent magnetic materials have garnered significant attention due to their exceptional intrinsic magnetic properties.However,challenges such as the metastable nature of the ThMn1...ThMn_(12)-type iron-rich rare-earth permanent magnetic materials have garnered significant attention due to their exceptional intrinsic magnetic properties.However,challenges such as the metastable nature of the ThMn12-type phase,excessively small single-domain grain size,and complex fabrication processes have hindered the achievement of high phase purity,uniform microstructure,and desirable extrinsic performance.In this study,we directly synthesized ThMn_(12)-type Sm_(0.8)Zr_(0.2)Fe_(11)SiB_(x)(x=0-0.2)ribbon magnets via boron doping combined with a one-step rapid solidification method.This approach not only simplifies the fabrication process but also enhances phase stability and achieves a uniform microstructure with high ThMn12-type phase purity.By optimizing the boron content and cooling rate,the resulting magnets exhibit a coercivity(H_(c))of 6222 Oe,a remanence(M_(r))of 80 emu/g,and a remanence ratio(M_(r)/M_(s))of 0.71.This work demonstrates a streamlined approach to producing high-performance ThMn12-type magnets and provides insights into their practical application potential.展开更多
Solidification structure of casting strands significantly impacts the subsequent processing and service properties of the steel products,which correlates closely with the melt flow during the solidification process.Se...Solidification structure of casting strands significantly impacts the subsequent processing and service properties of the steel products,which correlates closely with the melt flow during the solidification process.Several abnormal solidification phenomena and segregation characteristics observed in slab casting are elucidated by referencing to their related flow patterns of molten steel calculated by a multi-field coupling model for actual casting conditions.Eventually,the effect of forced convection on the solidification structure was discussed.The results show that the forced convection generated by electromagnetic stirring and/or nozzle jet will remove the solute-enriched molten steel between the dendrite in front of the solidifying shell,and change solute distribution at the interface of dendrite tips,leading to the white bands and dendrite deflection.In the white band region,a dense dendrite structure without dendrite segregation appears.Moreover,forced convection results in a higher growth rate on the upstream side than the backflow side of the dendrite tip,promoting the columnar crystal deflection.In addition,dendrite fragmentation upon the forced convection during solidification will increase the equiaxed crystal ratio of the as-cast slab and the number of the spot-like semi-macrosegregation.The carbon extreme range decreased with the change in electromagnetic stirring process,indicating a significant improvement in the composition uniformity of the slab casting.It is suggested that the final quality of rolled products could be improved from the very beginning of casting and solidification through regulating the as-cast solidification structure.展开更多
The low frequency electromagnetic casting(LFEC)was used to prevent hot cracking during the solidification process of GH4742 superalloy ingot.The effects of LFEC on the solidification macrostructure of the ingot were i...The low frequency electromagnetic casting(LFEC)was used to prevent hot cracking during the solidification process of GH4742 superalloy ingot.The effects of LFEC on the solidification macrostructure of the ingot were investigated through experiments and simulation.The results show that the average grain size decreases after application of LFEC.At the same time,the fraction of equiaxed grains increases compared with the ingots that without LFEC.In addition,the average grain size decreases and the fraction of equiaxed grains increases with increasing the current frequency.When the current frequency increases from 5 Hz to 20 Hz,the average grain size decreases from 5.39 mm to 4.74 mm,and the fraction of equiaxed grains increases from 41.21%to 55.24%.The distribution of Lorentz force,melt flow field and temperature field in the melt was simulated using COMSOL Multiphysics software.It is found that the Lorentz force increases and the forced convection is enhanced with increasing the current frequency,thus the melt flow velocity and heat transfer in the melt are promoted.It can facilitate the heterogenous nucleation in the melt,resulting in grain refinement,and further preventing hot cracking of large size ingots.展开更多
The cooling rate of the center and edge of vacuum induction melting(VIM)or vacuum arc remelting(VAR)ingots exhibit substantial difference,leading to markedly distinct dendritic structures and precipitates.The current ...The cooling rate of the center and edge of vacuum induction melting(VIM)or vacuum arc remelting(VAR)ingots exhibit substantial difference,leading to markedly distinct dendritic structures and precipitates.The current lack of precise predictions for dendritic segregation and the distribution of precipitates in ingot makes it difficult to determine the annealing and homogenization heat treatment process.Thus,clarifying the impact of cooling rate on the solidification behavior of alloy is significantly important.The dendritic structure and precipitation characteristics of as-cast C-HRA-3 Ni–Cr–Co–Mo-based heat-resistant alloy were investigated using Thermo-Calc thermodynamic calculations,scanning electron microscopy observations,and electron probe microanalyzer.Based on high temperature observation system,the effects of cooling rate on the dendritic structure,dendritic segregation,and precipitation in this alloy were explored.The results showed that the precipitates in the as-cast C-HRA-3 alloy primarily consist of blocky Ti(C,N)phases,large-sized Ti(C,N)–M_(6)C–M_(23)C_(6) symbiotic phases and M_(6)C–M_(23)C_(6) carbides,and small-sized dispersed M_(6)C and M_(23)C_(6) carbides surronding these symbiotic phases.The primary constituent elements of these precipitates are Mo,Cr,C,and Ti,which predominantly concentrate in the interdendritic regions of the as-cast alloy.There is a clear power-law relationship between the secondary dendrite arm spacing and the cooling rate.The dendritic segregation ratio of Mo,Cr,and Ti exhibits a piecewise functional relationship with the cooling rate,under equiaxed dendritic solidification condition.These predictive models and theoretical analyses were validated using numerical simulations and experimental results from the 200 kg grade VIM electrode.展开更多
The effect of melt superheating treatment on the solidification microstructure and mechanical properties of theγ'phase precipitation-strengthened K424 superalloy was investigated.Differential scanning calorimetry...The effect of melt superheating treatment on the solidification microstructure and mechanical properties of theγ'phase precipitation-strengthened K424 superalloy was investigated.Differential scanning calorimetry(DSC)experiments were conducted to explore the influence of melt treatment temperature on the undercooling of the superalloy.Additionally,pouring experiments were carried out to assess how alterations in both the temperature and duration of melt treatment impacted the grain size,secondary dendrite arm spacing(SDAS),elemental segregation,and mechanical properties of the alloy.Metallographic analysis,scanning electron microscopy,energy dispersive spectroscopy(EDS)and Thermo-Calc software were employed for microstructure characterization.The test specimens were subjected to tensile testing at room temperature and stress rupture testing at 975℃ under 196 MPa.The findings reveal that appropriate melt treatment conditions result in decreased grain size,refined SDAS,minimized elemental segregation,and significant improvements in mechanical properties.Specifically,the study demonstrates that a melt treatment at 1,650℃ for 5 min results in the smallest average grain size of 949μm and the smallest SDAS of 25.38μm.Furthermore,the room temperature tensile properties and creep resistance are notably affected by the melt treatment parameters.It is shown that specific melt treatment conditions,such as holding at 1,650℃ for 5 min,result in superior room temperature strength and extended stress rupture life of the K424 superalloy,while a balance between strength and stability is achieved at 1,600℃ with a holding time of 10 min.These findings offer guidance for optimizing the melt treatment parameters for the K424 superalloy,laying a foundation for further investigations.展开更多
基金supported by Open Foundation of the State Key Laboratory of Refractories and Metallurgy(No.G201711)the National Natural Science Foundation of China(Nos.52104317 and 51874001).
文摘The impact of heavy reduction on dendritic morphology was explored by combining experimental research and numerical simulation in metallurgy,including a detailed three-dimensional(3D)analysis and reconstruction of dendritic solidification structures.Combining scanning electron microscopy and energy-dispersive scanning analysis and ANSYS simulation,the high-precision image processing software Mimics Research was utilized to conduct the extraction of dendritic morphologies.Reverse engineering software NX Imageware was employed for the 3D reconstruction of two-dimensional dendritic morphologies,restoring the dendritic characteristics in three-dimensional space.The results demonstrate that in a two-dimensional plane,dendrites connect with each other to form irregularly shaped“ring-like”structures.These dendrites have a thickness greater than 0.1 mm along the Z-axis direction,leading to the envelopment of molten steel by dendrites in a 3D space of at least 0.1 mm.This results in obstructed flow,confirming the“bridging”of dendrites in three-dimensional space,resulting in a tendency for central segregation.Dense and dispersed tiny dendrites,under the influence of heat flow direction,interconnect and continuously grow,gradually forming primary and secondary dendrites in three-dimensional space.After the completion of dendritic solidification and growth,these microdendrites appear dense and dispersed on the two-dimensional plane,providing the nuclei for the formation of new dendrites.When reduction occurs at a solid fraction of 0.46,there is a noticeable decrease in dendritic spacing,resulting in improved central segregation.
基金financially suppoted by the National Natural Sci-ence Foundation of China(No.52371041).
文摘Tungsten heavy alloys(WHAs)prepared using laser additive manufacturing(AM)exhibit intricate ge-ometries,albeit with limited mechanical properties.Here we designed a high-strength WHA featuring a FeCrCoNi high entropy alloy(HEA)binder via the laser metal deposition(LMD)technique.Due to the distinctive thermal cycle and rapid cooling rate,the as-deposited alloys exhibit microstructures with hy-poeutectic,eutectic-like,and spot-like characteristics.To elucidate this phenomenon,the solidification paths were delineated and analyzed by combining microstructural characterization and phase equilib-rium simulation.Theμphase precipitated out from the supersaturated solid solution,thereby nucleating massive dislocations on the FeCrCoNi matrix to increase the work hardening rate.Furthermore,theμphase formed an ultrafine intermetallic compound(IMC)layer around the W grain,reducing the hole or crack between the W grain and FeCrCoNi matrix.Attributed to the precipitation strengthening,the solid solution of the FeCrCoNi binder,along with the load-bearing strength of W,the developed alloy achieved ultrahigh compressive stress and strain of 2047 MPa and 32%respectively at room temperature.These findings contribute valuable insights to the advancement of additive manufacturing for tungsten alloys,leveraging their excellent properties.
基金support of the Research Project Supported by Shanxi Scholarship Council of China(2022-040)"Chunhui Plan"Collaborative Research Project by the Ministry of Education of China(HZKY20220507)+2 种基金National Natural Science Foundation of China(52104338)Applied Fundamental Research Programs of Shanxi Province(202303021221036)Shandong Postdoctoral Science Foundation(SDCX-ZG-202303027,SDBX2023054).
文摘In 316L austenitic stainless steel,the presence of ferrite phase severely affects the non-magnetic properties.316L austenitic stainless steel with low-alloy type(L-316L)and high-alloy type(H-316L)has been studied.The microstructure and solidification kinetics of the two as-cast grades were in situ observed by high temperature confocal laser scanning microscopy(HT-CLSM).There are significant differences in the as-cast microstructures of the two 316L stainless steel compositions.In L-316L steel,ferrite morphology appears as the short rods with a ferrite content of 6.98%,forming a dual-phase microstructure consisting of austenite and ferrite.Conversely,in H-316L steel,the ferrite appears as discontinuous network structures with a content of 4.41%,forming a microstructure composed of austenite and sigma(σ)phase.The alloying elements in H-316L steel exhibit a complex distribution,with Ni and Mo enriching at the austenite grain boundaries.HT-CLSM experiments provide the real-time observation of the solidification processes of both 316L specimens and reveal distinct solidification modes:L-316L steel solidifies in an FA mode,whereas H-316L steel solidifies in an AF mode.These differences result in ferrite and austenite predominantly serving as the nucleation and growth phases,respectively.The solidification mode observed by experiments is similar to the thermodynamic calculation results.The L-316L steel solidified in the FA mode and showed minimal element segregation,which lead to a direct transformation of ferrite to austenite phase(δ→γ)during phase transformation after solidification.Besides,the H-316L steel solidified in the AF mode and showed severe element segregation,which lead to Mo enrichment at grain boundaries and transformation of ferrite into sigma and austenite phases through the eutectoid reaction(δ→σ+γ).
文摘Carbon fibres have been produced from hydroxypropyl-modified lignin(TcC)/bio-based polyamide 1010(PA1010)blended filaments.Two grades of PA1010,with different molecular weights and rheological properties,were used for blending with TcC.An oxidative thermal stabilisation step was used prior to carbonisation in an inert atmosphere to prevent the fusion of the filaments during the latter step.Thermal stabilisation was not possible using a one-step stabilisation process reported in the literature for lignin and other lignin/synthetic polymer blends.As a consequence,a cyclic process involving an additional isothermal phase at a lower temperature than the precursor filaments’melting point,was introduced to increase the cross-linking reactions between the lignin and polyamide.Thermally stabilised filaments were characterised by DSC,TGA,TGA-FTIR,ATR,and SEM techniques.Polymer rheology and heating rate used during thermal stabilisation influenced the thermal stabilisation process and mechanical properties of the derived filaments.Thermally stabilised filaments using optimised conditions(heating in the air atmosphere at 0.25℃/min to 180℃;isothermal for 1 h,cooling back down to ambient at 5℃/min;heating to 250℃ at 0.25℃/min,isothermal for 2 h)could be successfully carbonised.Carbon fibres pro-duced had void-free morphologies and mechanical properties comparable to similarly thermally stabilised and carbonised polyacrylonitrile(PAN)filaments.
基金supported by the National Key R&D Projects of China(No.2018YFC1801706-01)the National Natural Science Foundation of China(No.22162007)+2 种基金the Science and Technology Supporting Project of Guizhou Province(No.[2021]480,[2023]379)the Wengfu(Group)Co.,Ltd.Technology Development Project(No.WH-220787(YF))the Project from Guizhou Institute of Innovation and Development of Dual-carbon and New Energy Technologies(No.DCRE-2023-05)。
文摘This study used steel slag,fly ash,and metakaolin as raw materials(SFM materials)to create silica-alumina-based geopolymers that can solidify Hg^(2+)when activated with sodiumbased water glass.The experiments began with a triangular lattice point mixing design experiment,and the results were fitted,analyzed,and predicted.The optimum SFM material mass ratio was found to be 70%steel slag,25%fly ash,and 5%metakaolin.The optimum modulus of the activator was identified by comparing the unconfined compressive strength and solidifying impact on Hg^(2+)of geosynthetics with different modulus.The SFM geopolymer was then applied in the form of potting to cure the granulated mercury tailings.The inclusion of 50%SFM material generated a geosynthetic that reduced mercury transport to the surface soil by roughly 90%.The mercury concentration of herbaceous plant samples was also reduced by 78%.It indicates that the SFM material can effectively attenuate the migration transformation of mercury.Finally,characterization methods such as XPS and FTIR were used to investigate the mechanism of Hg^(2+)solidification by geopolymers generated by SFM materials.The possible solidification mechanisms were proposed as alkaline environment-induced mercury precipitation,chemical bonding s,surface adsorption of Hg^(2+)and its precipitates by the geopolymer,and physical encapsulation.
基金Funded by the Basic Research Project in Shanxi Province(No.202103021224183)the 2024 Science and Technology Plan Project of Jiaozuo City,Henan Province(No.2024410001)。
文摘By applying the rapid solidification technique of deep undercooling,Cu65Ni35 and Cu60Ni40 alloys achieved maximum undercoolings of 284 and 222 K,respectively.Microstructural images captured reveal grain refinement in both alloys across both large and small undercooling ranges.High-speed photography was used to analyze the relationship between solidification front morphology and undercooling,showing that dendrite remelting and fragmentation caused grain refinement under small undercooling,while stress-induced recrystallization is responsible under large undercooling.Microhardness testing further demonstrates a sudden drop in microhardness near the critical undercooling point,providing evidence for grain refinement due to recrystallization in large undercooling tissues.
基金Funded by the Basic Research Project in Shanxi Province(No.202103021224183)the 2024 Science and Technology PlanProject of Jiaozuo City,Henan Province(No.2024410001)。
文摘The evolution of the microstructure and morphology of Cu55Ni45 and Cu60Ni40 alloys under varying degrees of undercooling was investigated through molten glass purification and cyclic superheating technology.By increasing the Cu content,the effect of Cu on the evolution of the microstructure and morphology of the Cu-Ni alloy during undercooling was studied.The mechanism of grain refinement at different degrees of undercooling and the effect of Cu content on its solidification behaviour were investigated.The solidification behaviour of Cu55Ni45 and Cu60Ni40 alloys was investigated using infrared thermometry and high-speed photography.The results indicate that both Cu55Ni45 and Cu60Ni40 alloy melts undergo only one recalescence during rapid solidification.The degree of recalescence increases approximately linearly with increasing undercooling.The solidification front of the alloy melts undergoes a transition process from a small-angle plane to a sharp front and then to a smooth arc.However,the growth of the subcooled melt is constrained to a narrow range,facilitating the formation of a coarse dendritic crystal morphology in the Cu-Ni alloy.At large undercooling,the stress breakdown of the directionally growing dendrites is primarily caused by thermal diffusion.The strain remaining in the dendritic fragments provides the driving force for recrystallisation of the tissue to occur,which in turn refines the tissue.
基金supported by Japan Society for the Promotion of Science(KAKENHI Grant-in-Aid for Scientific Research,18H05475,18H05476 and JP20H00312)MRC International Collaborative Research Grant+4 种基金The authors would like to thank the Czech Science Foundation(Project No.22-22248S)specific university research(A1_FCHT_2024_007)for financial supportsupported by the Ministry of Education,Youth,and Sports of the Czech Republic.Project No.CZ.02.01.01/00/22_008/0004591co-funded by the European UnionCzechNanoLab project LM2023051 funded by MEYS CR is gratefully acknowledged for the financial support of the measurements/sample fabrication at LNSM Research Infrastructure。
文摘The Mg-Y-Zn magnesium alloy system is known for the presence of Long-Period Stacking Ordered(LPSO)phases that improves strength and ductility with minimal amounts of alloying elements.Even better improvements are associated with the specific microstructure known as the Mille-Feuille(MF)structure that can occur in this alloy as well after proper heat treatment.This study systematically compares the traditional ingot metallurgy method with the Bridgman method(slow cooling),coupled with diverse heat treatments and extrusion process.Microscopic analyses reveal variations in the presence of LPSO phases,MF structure,and especially grain size,leading to divergent mechanical and corrosion properties.The Bridgman approach surprisingly stands out,ensuring superior mechanical properties due to kink and texture strengthening.
基金supported by the National Natural Science Foundation of China(Grant No.52301035)the Natural Science Foundation of Jiangsu Province,China(Grant No.BK20230844)the National Key Research and Development Program of China(Grant No.2023YFB3710202).
文摘The microstructure formed during solidification has a significant impact on the mechanical properties of materials.In this study,a two-dimensional(2D)cellular automaton(CA)-finite difference(FD)-CALPHAD model was developed to simulate the formation of microstructure and solute segregation in the solidification processes of ternary alloys.In the model,dendritic growth is simulated using the CA technique,while solute diffusion is solved by the FD method,and the CALPHAD method is employed to calculate thermodynamic phase equilibrium during solidification.The CA-FD-CALPHAD coupled model is capable of reproducing the evolution of continuous nucleation and growth of grains as well as the evolution of the microstructure and solute distribution during solidification of ternary alloys.In this study,Al–Zn–Mg ternary alloy is taken as an example to simulate the growth of equiaxed and columnar grains and the columnar-to-equiaxed transition(CET)under different solidification conditions.The simulation results are compared with experimental data from the literature,showing a good agreement.Besides,the study also investigates the evolution of temperature and multicomponent solute fields during solidification and the effects of alloy composition and cooling rate on the microstructure morphology.The results reveal that the initial alloy composition and cooling rate significantly affect dendritic morphology and solute segregation.Higher initial alloy concentrations promote the growth of side branches in equiaxed grains,leading to more pronounced solute segregation between dendrites.As the cooling rate increases,the average grain size of the equiaxed grains decreases accordingly.Additionally,a higher cooling rate accelerates the columnar-to-equiaxed transition,leading to a finer grain structure.
基金financially supported by the National Natural Science Foundation of China(Grant No.51875211)the Beijing Natural Science Foundation(Grant No.L223001)。
文摘Microporosity formed in the solidification process of Al alloys is detrimental to the alloy properties.A two-dimensional cellular automaton(CA)model was developed to simulate the microstructure and microporosity formation in Al-Cu alloys,considering variations in Cu content and solidification rate.The results indicate that the Cu content primarily influences the growth of microporosity.To validate the model,directional solidification experiments were conducted on Al-Cu alloys with varing Cu contents and withdrawal rates.The experimental results of dendrites and microporosity characteristics agree well with the predictions from the developed model,thus confirming the validity of the model.The alloy’s liquidus temperature,dendrite morphology,and hydrogen saturation solubility arising from different Cu contents have significant effects on microporosity morphology.The withdrawal rate primarily affects the nucleation of hydrogen microporosity by altering cooling rates and dendritic growth rates,resulting in different microporosity characteristics.
基金supported by the Ordos City Science and Technology Major Project(2021ZD14-16)the National Key Research and Development Program(2018YFC1802904)the Discipline Signature Achievements of the Shanghai Polytechnic University(A10GY23G004-14).
文摘One of the major challenges in the application of microbially induced carbonate precipitation(MICP)is achieving"bacteria freedom",as it necessitates a substantial volume of bacterial solutions.Nevertheless,both insitu bacterial cultivation and transportation of bacterial solutions have proven to be inefficient.In this study,we suggested the utilization of bacteria in the form of dry powder,enabling easy on-site activation and achieving a relatively high urease activity.We conducted MICP curing experiments on gold mine tailings(GMT)using steel slag(SS)as an additive.The results showed that the average unconfined compressive strength(UCS)values of the tailings treated with MICP and MICP+SS reached 0.51 and 0.71 MPa,respectively.In addition,the average leaching reduction rates of Cu,Pb,Cr,Zn,and T-CN in GMT after MICP treatment reached 98.54%,100%,70.94%,59.25%,and 98.02%,respectively,and the average reduction rates after MICP+SS treatment reached 98.77%,100%,88.03%,72.59%,and 98.63%,respectively.SEM,XRD,FT-IR analyses,and ultra-deep field microscopy results confirmed that the MICP treatment produced calcite-based calcium carbonate that filled the inter-tailing pores and cemented them together,and the hydration mechanism was the main reason for the increased curing efficiency of SS.Our research findings demonstrate that bacterial powder can efficiently achieve the objectives of heavy metal removal and tailing solidification.This approach can substantially de-crease the expenses associated with bacterial cultivation and solution transportation,thereby playing a crucial role in advancing the practical implementation of MICP.
基金financially sponsored by the National Natural Science Foundation of China(Nos.U2441268 and 52304406)the Natural Science Foundation of Shanghai,China(No.23TS1401900)+2 种基金the Science Foundation of Aeronautics(PSSFA),China(No.2024Z053057002)the Science and Technology Cooperation Program of Shanghai Jiao Tong University in Inner Mongolia Autonomous Region-Action Plan of Shanghai Jiao Tong University for“Revitalizing Inner Mongolia through Science and Technology”,ChinaLuwei Yang would like to thank the financial support from the Chinese Scholarship Council(No.202306230337).
文摘Nickel-based single-crystal(SX)superalloys are the key metallic materials of aeroengines.However,thermomechanical deformation always occurs during the directional solidification of SX superalloys,negatively influencing the SX structure.Casting deformation is simulated in most of the previous studies,whereas the direct simulation of dendritic thermomechanical deformation has been largely ignored,resulting in a lack of comprehensive understanding of this process.In this study,we systematically investigate dendritic thermomechanical deformation with a model coupled with dendrite growth,fluid flow,and thermomechanical deformation behavior.Results reveal that the dendritic thermomechanical deformation-induced dendrite bending is not randomly distributed but is mainly concentrated on the casting surface.The dendritic thermal stress increases as dendrite grows and accumulates after dendrite bridging.Transverse thermal contraction mainly occurs at the edge of casting in the corner,and axial thermal contraction is larger than transverse contraction.The high-stress region of the primary dendrite trunk is mainly distributed below the dendrite bridging near the solidified part,and the stress along the transverse direction reaches its maximum value on the casting surface.Stress concentrated on the casting surface is mainly attributed to variations in transverse temperature gradients caused by heat dissipation on the lateral mold wall,and inconsistent constraints in the lateral mold walls.
基金supported by the National Science and Technology Major Project(Grant No.J2019-VI-0019-0134)the National Natural Science Foundation of China(Grant No.52203301)+1 种基金the China Postdoctoral Science Foundation(Grant No.2021TQ0335)the Liaoning Province Science and Technology Plan Joint Fund(Doctoral Research Initiation Project)(Grant No.2024-BSLH-195).
文摘This work studies the impact of the carbon diffusion on the growth kinetics of austenite and the solute segregation,by utilizing the phase-field(PF)method to simulate the solidification of a Fe-C binary alloy.It is revealed that increasing the ratio of the carbon diffusion coefficient in solid to that in liquid is advantageous in reducing the solute segregation,and a novel microsegregation model is developed based on the quantitative analysis of the results from PF simulations.The simplified one-dimensional diffusion simulation is employed to analyse the quantitative relationship between the parameters of the proposed microsegregation model and the properties of materials.The universality and reliability of the new microsegregation model are then validated by comparing with the experimental data of various alloy systems.These findings contribute to our comprehension of the fundamental theory of solidification and also provide a potential and promising approach to controlling the solidification microstructure.
基金This research work was financially supported by the National Natural Science Foundation of China(Grant No.51972209).
文摘This paper investigated the use of magnesium phosphate cement (MPC) for solidifying sludge with different humic acid (HA) content (ranging from 0 to 4.5%) and explored the solidification mechanism. Fluidity, setting time, unconfined compressive strength (UCS), the strength formation mechanism, and the spontaneous imbibition process of solidified sludge (SS) were studied. The results indicate that MPC can be used as a low-alkalinity curing agent. As the HA content increases, fluidity and setting time also increase, while hydration temperature and strength decrease. Additionally, the failure mode of SS transitions from brittleness to ductility. The strength of SS is composed of the cementation strength provided by MPC hydration products, matric suction, osmotic suction, and the structural strength of the sludge. MPC reduces the structural strength caused by the shrinkage of pure sludge under the action of matric suction, but the incorporation of MPC significantly improved the strength when the sludge is eroded by water. X-ray diffraction (XRD) and scanning electron microscopy (SEM) show that the sludge and MPC can form a dense solid body, forming various hydration products, and synergistically improve the mechanical properties of the sludge.
基金supports from National Natural Science Foundation of China(Nos.52304361 and 52130408)Natural Science Foundation of Hunan Province(2023JJ40737)are great acknowledged.
文摘A novel droplet solidification technique was developed to emulate sub-rapid solidification and facilitate the formation of deposited films during the strip casting of silicon steels(w(Si):2.5 and 3.5 wt.%).With the increasing number of droplet ejection experiments,the peak heat fluxes between droplet and substrate decreased firstly(1rd–5th ejection),then increased(5th–7th ejection),and finally decreased again(>7th ejection).In the first five experiments,the interfacial thermal resistance between the droplet and the substrate improved with increasing film thickness.However,at the onset of the 6th droplet ejection experiment,the deposited film initiated its melting process due to the accumulated thermal resistance,which has the potential to eradicate the cavity or air space existing between the droplet and the substrate.Consequently,the interfacial contact condition was improved gradually with the increasing melting area from 5th to 7th droplet ejection experiments,leading to an increase in heat fluxes.Increased SiO_(2) content in deposited films for 3.5 wt.%Si steel led to lower peak heat fluxes than for 2.5 wt.%Si steel.The solidification structure of the 2.5 wt.%Si steel droplet sample comprised a fine grain zone at the base,a columnar grain zone in the center,and an equiaxed grain zone at the top.However,the solidification structure of the 3.5 wt.%Si steel droplet only contained columnar grains and equiaxed grains,with a larger average grain size due to the lower interfacial heat flux.
基金Project supported by the National Key R&D Program of China(Grant Nos.2021YFB3500300 and 2023YFB3507000)the Scientific Research Foundation of the High Education Institutions for Distinguished Young Scholars in Anhui Province(Grant No.2022AH020012)+1 种基金partially supported by the Innovation Project for Overseas Researcher in Anhui Province(Grant No.2022LCX004)the facilities at the Center of Free Electron Laser&High Magnetic Field(FEL&HMF)in Anhui University。
文摘ThMn_(12)-type iron-rich rare-earth permanent magnetic materials have garnered significant attention due to their exceptional intrinsic magnetic properties.However,challenges such as the metastable nature of the ThMn12-type phase,excessively small single-domain grain size,and complex fabrication processes have hindered the achievement of high phase purity,uniform microstructure,and desirable extrinsic performance.In this study,we directly synthesized ThMn_(12)-type Sm_(0.8)Zr_(0.2)Fe_(11)SiB_(x)(x=0-0.2)ribbon magnets via boron doping combined with a one-step rapid solidification method.This approach not only simplifies the fabrication process but also enhances phase stability and achieves a uniform microstructure with high ThMn12-type phase purity.By optimizing the boron content and cooling rate,the resulting magnets exhibit a coercivity(H_(c))of 6222 Oe,a remanence(M_(r))of 80 emu/g,and a remanence ratio(M_(r)/M_(s))of 0.71.This work demonstrates a streamlined approach to producing high-performance ThMn12-type magnets and provides insights into their practical application potential.
基金The authors are grateful to Weifang Science and Technology Development Plan Project(2023ZJ1166)for supporting this work.
文摘Solidification structure of casting strands significantly impacts the subsequent processing and service properties of the steel products,which correlates closely with the melt flow during the solidification process.Several abnormal solidification phenomena and segregation characteristics observed in slab casting are elucidated by referencing to their related flow patterns of molten steel calculated by a multi-field coupling model for actual casting conditions.Eventually,the effect of forced convection on the solidification structure was discussed.The results show that the forced convection generated by electromagnetic stirring and/or nozzle jet will remove the solute-enriched molten steel between the dendrite in front of the solidifying shell,and change solute distribution at the interface of dendrite tips,leading to the white bands and dendrite deflection.In the white band region,a dense dendrite structure without dendrite segregation appears.Moreover,forced convection results in a higher growth rate on the upstream side than the backflow side of the dendrite tip,promoting the columnar crystal deflection.In addition,dendrite fragmentation upon the forced convection during solidification will increase the equiaxed crystal ratio of the as-cast slab and the number of the spot-like semi-macrosegregation.The carbon extreme range decreased with the change in electromagnetic stirring process,indicating a significant improvement in the composition uniformity of the slab casting.It is suggested that the final quality of rolled products could be improved from the very beginning of casting and solidification through regulating the as-cast solidification structure.
基金financially supported by the Major Projects in Aviation Engines and Gas Turbines (Grant No.2019-VI-0020-0136)the National Key Research and Development Program of China (Grant Nos.2022YFB3705101&2022YFB3705102)+1 种基金the National Natural Science Foundation of China (Grant No.U1708253)the Fundamental Research Funds for the Central Universities,China (Grant No.N2302005)。
文摘The low frequency electromagnetic casting(LFEC)was used to prevent hot cracking during the solidification process of GH4742 superalloy ingot.The effects of LFEC on the solidification macrostructure of the ingot were investigated through experiments and simulation.The results show that the average grain size decreases after application of LFEC.At the same time,the fraction of equiaxed grains increases compared with the ingots that without LFEC.In addition,the average grain size decreases and the fraction of equiaxed grains increases with increasing the current frequency.When the current frequency increases from 5 Hz to 20 Hz,the average grain size decreases from 5.39 mm to 4.74 mm,and the fraction of equiaxed grains increases from 41.21%to 55.24%.The distribution of Lorentz force,melt flow field and temperature field in the melt was simulated using COMSOL Multiphysics software.It is found that the Lorentz force increases and the forced convection is enhanced with increasing the current frequency,thus the melt flow velocity and heat transfer in the melt are promoted.It can facilitate the heterogenous nucleation in the melt,resulting in grain refinement,and further preventing hot cracking of large size ingots.
基金funded by the National Key R&D Program Funded Projects(No.2021YFB3704102).
文摘The cooling rate of the center and edge of vacuum induction melting(VIM)or vacuum arc remelting(VAR)ingots exhibit substantial difference,leading to markedly distinct dendritic structures and precipitates.The current lack of precise predictions for dendritic segregation and the distribution of precipitates in ingot makes it difficult to determine the annealing and homogenization heat treatment process.Thus,clarifying the impact of cooling rate on the solidification behavior of alloy is significantly important.The dendritic structure and precipitation characteristics of as-cast C-HRA-3 Ni–Cr–Co–Mo-based heat-resistant alloy were investigated using Thermo-Calc thermodynamic calculations,scanning electron microscopy observations,and electron probe microanalyzer.Based on high temperature observation system,the effects of cooling rate on the dendritic structure,dendritic segregation,and precipitation in this alloy were explored.The results showed that the precipitates in the as-cast C-HRA-3 alloy primarily consist of blocky Ti(C,N)phases,large-sized Ti(C,N)–M_(6)C–M_(23)C_(6) symbiotic phases and M_(6)C–M_(23)C_(6) carbides,and small-sized dispersed M_(6)C and M_(23)C_(6) carbides surronding these symbiotic phases.The primary constituent elements of these precipitates are Mo,Cr,C,and Ti,which predominantly concentrate in the interdendritic regions of the as-cast alloy.There is a clear power-law relationship between the secondary dendrite arm spacing and the cooling rate.The dendritic segregation ratio of Mo,Cr,and Ti exhibits a piecewise functional relationship with the cooling rate,under equiaxed dendritic solidification condition.These predictive models and theoretical analyses were validated using numerical simulations and experimental results from the 200 kg grade VIM electrode.
基金financially supported by the Natural Science Foundation Joint Fund of Liaoning Province,China(No.2023-MSLH-342).
文摘The effect of melt superheating treatment on the solidification microstructure and mechanical properties of theγ'phase precipitation-strengthened K424 superalloy was investigated.Differential scanning calorimetry(DSC)experiments were conducted to explore the influence of melt treatment temperature on the undercooling of the superalloy.Additionally,pouring experiments were carried out to assess how alterations in both the temperature and duration of melt treatment impacted the grain size,secondary dendrite arm spacing(SDAS),elemental segregation,and mechanical properties of the alloy.Metallographic analysis,scanning electron microscopy,energy dispersive spectroscopy(EDS)and Thermo-Calc software were employed for microstructure characterization.The test specimens were subjected to tensile testing at room temperature and stress rupture testing at 975℃ under 196 MPa.The findings reveal that appropriate melt treatment conditions result in decreased grain size,refined SDAS,minimized elemental segregation,and significant improvements in mechanical properties.Specifically,the study demonstrates that a melt treatment at 1,650℃ for 5 min results in the smallest average grain size of 949μm and the smallest SDAS of 25.38μm.Furthermore,the room temperature tensile properties and creep resistance are notably affected by the melt treatment parameters.It is shown that specific melt treatment conditions,such as holding at 1,650℃ for 5 min,result in superior room temperature strength and extended stress rupture life of the K424 superalloy,while a balance between strength and stability is achieved at 1,600℃ with a holding time of 10 min.These findings offer guidance for optimizing the melt treatment parameters for the K424 superalloy,laying a foundation for further investigations.
