This work reveals the significant effects of cobalt(Co)on the microstructure and impact toughness of as-quenched highstrength steels by experimental characterizations and thermo-kinetic analyses.The results show that ...This work reveals the significant effects of cobalt(Co)on the microstructure and impact toughness of as-quenched highstrength steels by experimental characterizations and thermo-kinetic analyses.The results show that the Co-bearing steel exhibits finer blocks and a lower ductile-brittle transition temperature than the steel without Co.Moreover,the Co-bearing steel reveals higher transformation rates at the intermediate stage with bainite volume fraction ranging from around 0.1 to 0.6.The improved impact toughness of the Co-bearing steel results from the higher dense block boundaries dominated by the V1/V2 variant pair.Furthermore,the addition of Co induces a larger transformation driving force and a lower bainite start temperature(BS),thereby contributing to the refinement of blocks and the increase of the V1/V2 variant pair.These findings would be instructive for the composition,microstructure design,and property optimization of high-strength steels.展开更多
The effect ofωiso andαprecipitation on microstructure,microhardness,tensile properties and impact toughness of Ti-25Nb-10Ta-1Zr-0.2Fe(TNTZF)alloy was investigated.The results showed that the solution treated TNTZF a...The effect ofωiso andαprecipitation on microstructure,microhardness,tensile properties and impact toughness of Ti-25Nb-10Ta-1Zr-0.2Fe(TNTZF)alloy was investigated.The results showed that the solution treated TNTZF alloy with a small amount of nano-sizedωath particles inβmatrix possesses tensile strength of 697 MPa,elongation of~34%,Young’s modulus(YM)of 75 GPa,and impact toughness of 58.7 J/cm^(2).After aging at relatively lower temperatures of 400℃,the hardness and modulus of the alloy increased significantly,while the plasticity and toughness dropped sharply due to the precipitation ofωiso phase.ωiso phase displayed an ellipsoidal morphology with high volume fraction and a size of about 50 nm after aging at 400℃,leading to the highest hardness of 364 HV and YM of 108 GPa,along with completely embrittlement since elongation and toughness were almost zero.A brittle impact fracture morphology was observed in the alloy,which is dominated by intergranular fracture,with a mixed fracture characteristics of cleavage surfaces,terraces and tiny dimples.When aged at 550℃,plate-likeαdistributed inβmatrix uniformly and inβgrain boundaries in parallel,resulting in the high strength of 804 MPa,as well as lowest YM of 72 GPa,elongation of 9%and toughness of 35.8 J/cm^(2).The fracture morphology of the alloy aged at 550℃showed a ductile fracture mechanism with a large number of dimples.展开更多
For P91 steel weldment,performing post-weld heat treatment immediately after welding can enhance welding efficiency and reduce the risk of hydrogen-induced cracking.However,determining the post-weld cooling temperatur...For P91 steel weldment,performing post-weld heat treatment immediately after welding can enhance welding efficiency and reduce the risk of hydrogen-induced cracking.However,determining the post-weld cooling temperature(PWCT)is challenging due to potential impact on the mechanical properties of weldment after subsequent heat treatment.A systematic investigation is conducted to explore the effect of PWCT on the impact toughness of P91 steel welded joints.It has been demonstrated that the impact energy of the weld metal gradually increases from 40.2 to 49.5 J as the PWCT decreases from 300 to 100℃.Microstructural analysis reveals that PWCTs above 100℃ led to an increased dislocation density in the weld metal.Furthermore,in situ observations using a high-temperature confocal laser scanning microscope confirm that excessively high PWCTs result in the formation of untempered martensite after post-weld heat treatment.Such untempered martensites are identified as the primary cause of the reduced impact toughness in the weld metal.These findings underscore the importance of carefully controlling PWCT in welding procedures for P91 steel and similar grades.展开更多
This work used 5CrMnMo steel and titanium carbide(TiC)powders to fabricate particulate metal matrix composites(PMMCS).The composites’microstructure,hardness,and impact toughness were compared with four different tita...This work used 5CrMnMo steel and titanium carbide(TiC)powders to fabricate particulate metal matrix composites(PMMCS).The composites’microstructure,hardness,and impact toughness were compared with four different titanium carbide ceramic particle sizes.The phase composition and microstructure of composites were studied.Vickers hardness and Charpy impact tests were employed to analyze composites’hardness and impact ductility,respectively.The results showed that the four groups of composites are mainly composed of martensite,trace residual austenite,and titanium carbide(undissolved TiC and primary TiC particles).With the growth of the ceramic particle dimension in the composite layer,the number of primary titanium carbide ceramics gradually decreased.When the initial ceramic particle size was small,it tended to generate dendritic primary TiC,and when the particle size was large,it tended to generate polygons and ellipsoids.Furthermore,with the growth of titanium carbide ceramic particle dimension in the composites,the hardness of the composites decreased but the impact toughness of the composites rose first and then descended.When the ceramic particle size was 50-75μm,the composite had the highest hardness,and the impact energy of the composites was the highest,which is 8 J.This was because there were more undissolved titanium carbide ceramics in the composite,and there was a thicker matrix metal between the ceramic particles.展开更多
Microstructures of as-cast and extruded ZK60-xRE （RE=Dy, Ho and Gd, x=0-5, mass fraction） alloys were investigated. Meanwhile, the impact toughness was tested and then the relationship was discussed. The results sho...Microstructures of as-cast and extruded ZK60-xRE （RE=Dy, Ho and Gd, x=0-5, mass fraction） alloys were investigated. Meanwhile, the impact toughness was tested and then the relationship was discussed. The results show that as-cast microstructure is refined gradually with increasing the RE content. Mg-Zn-RE new phase increases gradually, while MgZn2 phase decreases gradually to disappear. Second phase tends to distribute along grain boundary in continuous network. Extruded microstructure is refined obviously to reach the micron level. Broken second phase tends to distribute along the extrusion direction in zonal shape. Impact toughness value -nK increases from 9-17 J/cm2 for as-cast state to 26-54 J/cm2 for extruded state. With increasing the value of -nK, fracture macro-morphology changes from a rough plane via multi-plane with step to V-type plane; and from single radiation zone to two zones of fiber and shear lip, respectively. Fracture micro-morphology changes from the brittle fracture to the ductile fracture. Fine grain and few fine dispersed second phase can enhance the impact toughness of magnesium alloys effectively.展开更多
The hierarchical martensitic features in ultra-high strength stainless steel(UHSSS),including the prior austenite grains,martensite packets,blocks and laths with the descending size,were refined to various extents by ...The hierarchical martensitic features in ultra-high strength stainless steel(UHSSS),including the prior austenite grains,martensite packets,blocks and laths with the descending size,were refined to various extents by employing different thermomechanical processes and then carefully characterized.Their relation to yield strength and impact toughness was analyzed.We conclude that the refinement of martensitic structures could lead to the significant increase of yield strength,which follows the Hall-Petch relation with the effect grain size defined by high angle boundaries(HABs).Impact toughness of UHSSS depends on the frequency and capability for retained austenite(RA)grains at both HABs and martensite lath boundaries to trap the propagating cracks via strain-induced transformation,in which the film-like RA grains at lath boundaries appear to make the greater contribution.展开更多
Clean high carbon heavy rail steel was prepared by the process of vacuum induction furnace smelting,forging and rolling.Mechanisms of RE on the impact toughness and fracture toughness for clean high carbon steel were ...Clean high carbon heavy rail steel was prepared by the process of vacuum induction furnace smelting,forging and rolling.Mechanisms of RE on the impact toughness and fracture toughness for clean high carbon steel were investigated.In addition,the appropriate range of RE content for clean high carbon steel was determined.Both the austenite grain size and pearlite lamellar spacing decreased due to small amount of RE,consequently the impact toughness and fracture toughness were improved evidently.When the RE content exceeded a critical value,the pearlite lamellar spacing was increased,because RE was segregated on the austenite grain boundaries,damaged the orientation relationship of pearlite transformation,caused the disorder growth and morphology degenerating of pearlite.With the increasing of RE content,both the impact toughness and fracture toughness of clean high carbon steel were gradually increased at first and then decreased.It was found that when the RE content was between 0.0081% and 0.0088%,both the impact toughness and fracture toughness of clean high carbon heavy rail steel were the best.The maximum ballistic work was 21.2 J（20 ℃） and 12.2 J（-20 ℃）,respectively.The maximum plane-strain fracture toughness was 45.67 MPa·m1/2（20 ℃） and 37.04 MPa·m1/2（-20 ℃）,respectively.展开更多
The influence of rolling at liquid nitrogen temperature and annealing on the microstructure and mechanical properties of Al 5083 alloy was studied in this paper. Cryorolled samples of Al 5083 show significant improvem...The influence of rolling at liquid nitrogen temperature and annealing on the microstructure and mechanical properties of Al 5083 alloy was studied in this paper. Cryorolled samples of Al 5083 show significant improvements in strength and hardness. The ultimate tensile strength increases up to 340 MPa and 390 MPa for the 30% and 50% cryorolled samples, respectively. The cryorolled samples, with 30% and 50% reduction, were subjected to Charpy impact testing at various temperatures from 190℃ to 100℃. It is observed that increasing the percentage of reduction of samples during cryorolling has significant effect on decreasing impact toughness at all temperatures by increasing yield strength and decreasing ductility. Annealing of samples after cryorolling shows remarkable increment in impact toughness through recovery and recrystallization. The average grain size of the 50% cryorolled sample （14 μm） after annealing at 350℃ for 1 h is found to be finer than that of the 30% cryorolled sample （25 μm）. The scanning electron microscopy （SEM） analysis of fractured surfaces shows a large-size dimpled morphology, resembling the ductile fracture mechanism in the starting material and fibrous structure with very fine dimples in cryorolled samples corresponding to the brittle fracture mechanism.展开更多
The effects of rare earth(RE) on the microstructure and impact toughness of low alloy Cr-Mo-V bainitic steels have been investigated where the steels have RE content of 0 to 0.048 wt.%. The results indicate that the n...The effects of rare earth(RE) on the microstructure and impact toughness of low alloy Cr-Mo-V bainitic steels have been investigated where the steels have RE content of 0 to 0.048 wt.%. The results indicate that the normalized microstructures of the steels are typical granular bainite(GB) composed primarily of bainitic ferrite and martensite and/or austenite(M-A) constituents. The M-A constituents are transformed into ferrite and carbides and/or agglomerated carbides after tempering at 700℃ for 4 h. The addition of RE decreases the onset temperature of bainitic transformation and results in the formation of finer bainitic ferrite, and reduces the amount of carbon-rich M-A constituents. For the normalized and tempered samples, the ductile-to-brittle transition temperature(DBTT) decreases with increasing RE content to a critical value of 0.012 wt.%. Lower DBTT and higher upper shelf energy are attributed to the decreased effective grain size and lower amount of coarse agglomerated carbides from the decomposition of massive M-A constituents. However, the addition of RE in excess of 0.012 wt.% leads to a substantial increase in the volume fraction of large-sized inclusions, which are extremely detrimental to the impact toughness.展开更多
The as-cast microstructure of H13 ESR ingot and the influence of high temperature diffusion treatment on the structure and impact toughness have been investigated. The results show that the dendrite arm spacing gradua...The as-cast microstructure of H13 ESR ingot and the influence of high temperature diffusion treatment on the structure and impact toughness have been investigated. The results show that the dendrite arm spacing gradually becomes wide from the surface to the center of ingot, and the large primary carbide particles always exist in interdendritic segregation areas; by means of high temperature diffusion treatment of ingot prior to hot forging, the banded segregation is nearly eliminated, the annealed structure is more uniform and the isotropic properties have been improved remarkably.展开更多
Here we present a novel approach of intercritical heat treatment for microstructure tailoring,in which intercritical annealing is introduced between conventional quenching and tempering.This induced a heterogeneous mi...Here we present a novel approach of intercritical heat treatment for microstructure tailoring,in which intercritical annealing is introduced between conventional quenching and tempering.This induced a heterogeneous microstructure consisting of soft intercritical ferrite and hard tempered martensite,resulting in a low yield ratio(YR)and high impact toughness in a high-strength low-alloy steel.The initial yielding and subsequent work hardening behavior of the steel during tensile deformation were modified by the presence of soft intercritical ferrite after intercritical annealing,in comparison to the steel with full martensitic microstructure.The increase in YR was related to the reduction in hardness difference between the soft and hard phases due to the precipitation of nano-carbides and the recovery of dislocations during tempering.The excellent low-temperature toughness was ascribed not only to the decrease in probability of microcrack initiation for the reduction of hardness difference between two phases,but also to the increase in resistance of microcrack propagation caused by the high density of high angle grain boundaries.展开更多
The microstructure and impact behavior of A356 aluminum alloy were studied after melt treatment processes of grain refinement and modification under both non-heat treated and T6 heat treated conditions. The modificati...The microstructure and impact behavior of A356 aluminum alloy were studied after melt treatment processes of grain refinement and modification under both non-heat treated and T6 heat treated conditions. The modification and grain refinement were done with the addition of Al-10%Sr and Al-5Ti-1B master alloys, respectively. All casting parameters were kept constant in order to focus on the influence of mentioned treatments. The results indicate that the eutectic silicon morphology is the main parameter to control the impact behavior of alloy. Consequently, the individual grain refinement of as-cast alloy does not improve the impact toughness as the modification does. While, simultaneous grain refinement and modification provide higher impact toughness in comparison with individual treatments. T6 heat treatment of the alloy improves the impact toughness under all melt-treated conditions. This is related to the further modification of eutectic silicon particles. To verify the results and clarify the mechanisms, three-point bending test and fractography were used to interpret the improvement of impact toughness of the alloy.展开更多
Microstructure, tensile and impact toughness properties and fracture location of friction stir welded AISI 1018 mild steel were revealed. The AISI 1018 mild steel plates with thickness of 5 mm were friction stir welde...Microstructure, tensile and impact toughness properties and fracture location of friction stir welded AISI 1018 mild steel were revealed. The AISI 1018 mild steel plates with thickness of 5 mm were friction stir welded by tungsten based alloy tool with tool rotational speed of 1 000 r/min and welding speed of 50 ram/rain. Tensile strength of stir zone is higher （8%） compared to that of the base metal. This may be due to the formation of finer grains in the weld nugget region under the stirring action of the rotating tool. The ductility and impact toughness of the joints are decreased compared to those of the base metal owing to the inclusion of tungsten particles in the weld region.展开更多
Ti-6Al,Ti-6Al-2Mo and Ti-6Al-3Nb alloys were prepared to investigate the toughening effects ofβstabilizers Mo and Nb on impact toughness and crack resistance of titanium alloys.Instrumented Charpy impact tests showed...Ti-6Al,Ti-6Al-2Mo and Ti-6Al-3Nb alloys were prepared to investigate the toughening effects ofβstabilizers Mo and Nb on impact toughness and crack resistance of titanium alloys.Instrumented Charpy impact tests showed that the total impact absorbed energy of Ti-6Al-2Mo and Ti-6Al-3Nb(∼64 J)were two times higher than that of Ti-6Al(∼30 J),indicating the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys.Analysis of load-displacement curves revealed the similar crack initiation energy of Ti-6Al,Ti-6Al-2Mo and Ti-6Al-3Nb(15.4 J,16.1 J and 15.0 J,respectively).However,the higher crack propagation energy of Ti-6Al-2Mo and Ti-6Al-3Nb(46.7 J and 48.3 J,respectively)were about three times higher than that of Ti-6Al(14.4 J),indicating the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb.Post-mortem analysis of impact samples demonstrated that the increased dislocation density and deformation twinning were mainly responsible for the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb.Due to the invisibility of dislocation activation and deformation twinning during the Charpy impact process,a mathematical model has been proposed to evaluate the effects of Al,Mo and Nb elements on dislocation mobility based on the Yu Rui-huang electron theory.Addition of Mo and Nb elements significantly improved the dislocation mobility in Ti-6Al-2Mo and Ti-6Al-3Nb compared to that in Ti-6Al alloy.Therefore,more dislocations were activated in Ti-6Al-2Mo and Ti-6Al-3Nb which supplied the larger plastic deformation under impact loading.A dislocation-based model also has been proposed to interpret the nucleation and propagation of deformation twinning under the impact loading.Dislocation pileup atα/βinterfaces provided potential sites for nucleation of deformation twinning in Ti-6Al-2Mo and Ti-6Al-3Nb.Furthermore,deformation twinning facilitated the dislocation motion inαgrains with hard orientations.The increased dislocation mobility and deformation twinning were responsible for the stronger crack resistance as well as the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys.展开更多
Toughness is an important property for steels used in engineering applications. However, recent toughness testing has shown the existence of a significant fluctuation in toughness in a single rolled plate of titanium ...Toughness is an important property for steels used in engineering applications. However, recent toughness testing has shown the existence of a significant fluctuation in toughness in a single rolled plate of titanium micro-alloyed steel. The underlying causes of this fluctuation were investigated by fractography, analysis of microstructure and measurement of inclusions. Coarse and distributed TiN inclusions were responsible for the toughness variation, as they tended to act as the potential cleavage initiators to form micro-cracks. From a calculation of the local fracture stress, the critical size of coarse TiN inclusions for dominating micro-crack propagation was 4.93 μm, and similarly that of ferrite grains was 36.6μm. Under current casting and thermo-mechanically controlled processing schedules, the toughness fluctuation of rolled steel plates can be primarily attributed to the fraction of coarse TiN inclusions larger than 5μm. A corresponding relationship between impact energy and the proportion of coarse TiN inclusions was established. Finally, a normalizing treatment was applied to refine the ferrite grains of rolled steel plates. Despite the presence of coarse TiN inclusions, this refinement in ferrite grains minimized the toughness fluctuation and improved the uniformity of the impact properties of the steel plates.展开更多
The effects of Nb on the thermal stability and impact toughness of ultra-high intrinsic coercivity of Nd-Fe-B magnets were investigated.The results showed that the addition of Nb could improve the thermal stability,an...The effects of Nb on the thermal stability and impact toughness of ultra-high intrinsic coercivity of Nd-Fe-B magnets were investigated.The results showed that the addition of Nb could improve the thermal stability,and obviously increased the impact toughness of sintered Nd-Fe-B magnets.The optimum thermal stability of sintered Nd-Fe-B magnets was obtained when the content of Nb was 1.0 at.%.The maximum impact toughness of sintered Nd-Fe-B magnets was obtained when the content of Nb was 1.5 at.%,but the magnetic properties of sintered Nd-Fe-B magnets drastically deteriorated when the content of Nb increased from 1.0 at.% to 1.5 at.%.The microstructure showed that overfull Nb addition made many ultra-fine grains get together,which led to the density of sintered Nd-Fe-B magnets decline and drastically deteriorated the magnetic properties of sintered Nd-Fe-B magnets.展开更多
CrMnFeCoNi high-entropy alloys(HEAs)exhibit an excellent combination of tensile strength and ductility at cryogenic temperatures.This study led to the introduction of a new method for the development of high-performan...CrMnFeCoNi high-entropy alloys(HEAs)exhibit an excellent combination of tensile strength and ductility at cryogenic temperatures.This study led to the introduction of a new method for the development of high-performance CrMnFeCoNi HEAs at cryogenic temperatures by jointly utilizing additive manufacturing(AM)and the addition of interstitial atoms.The interstitial oxygen present in the powder feedstock was transformed into beneficial nano-sized oxides during AM processing.The HEA nanocomposite fabricated using laser powder bed fusion(L-PBF)not only contains heterogeneous grains and substructures but also a high number density of nano-sized oxides.The tensile results revealed that the L-PBF HEA nanocomposite has superior yield strengths of 0.77 GPa and 1.15 GPa,and tensile strengths of 0.92 GPa and 1.45 GPa at 298 K and 77 K,respectively.In addition,the Charpy impact energies of the samples tested at 298 K and 77 K were measured as 176.2 J and 103.7 J,respectively.These results indicate that the L-PBF HEA nanocomposite successfully overcomes the well-known strength-toughness trade-off.The tensile deformation microstructure contained a relatively large number of deformation twins(DTs)at cryogenic temperature,a possible consequence of the decrease in the stacking fault energy with decreasing temperature.On the other hand,cracks were found to propagate along the grain boundaries at room temperature,whereas a transgranular crack was observed at cryogenic temperature in the specimens fractured as a result of the Charpy impact.展开更多
An NM400 wear-resistant steel was hot rolled and then the plates were heat-treated by direct quenching and tempering （DQT） and reheat quenching and tempering （RQT） techniques, respectively. The Charpy impact test ...An NM400 wear-resistant steel was hot rolled and then the plates were heat-treated by direct quenching and tempering （DQT） and reheat quenching and tempering （RQT） techniques, respectively. The Charpy impact test was carried out with an instrumented Charpy impact tester. The microstructure and fracture surface were investiga-ted by a combination of optical microscopy, transmission electron microscopy and scanning electron microscopy methods. It was found that the impact toughness of DQT specimen was much higher than that of RQT specimen. The microstructure of both DQT and RQT specimens was characterized by a mixture of tempered lath martensite and lower hainite. The lower bainite in DQT specimen extended into prior austenite grains and the content was higher than that in RQT specimen. The lower bainite in DOT specimen improved the impact toughness by increasing the proportion of large-angle boundaries and relieving the stress concentration at the crack tip. A number of fine and dis-persed carbides in DQT specimen also contributed to the improvement of the impact toughness.展开更多
Alloying can make conventional metals reach ultra-high strength,but this usually comes at dramatic loss of toughness.In this work,a desirable strength–toughness combination in high-strength low-alloy steel achieved v...Alloying can make conventional metals reach ultra-high strength,but this usually comes at dramatic loss of toughness.In this work,a desirable strength–toughness combination in high-strength low-alloy steel achieved via trace carbon addition.The significance of carbon in tailoring variant pairing and tuning impact toughness was elucidated from the perspective of crystallography and thermodynamics.As the carbon content increases,the packets and blocks are refined,and the-40 impact toughness improves.The enhancement of impact toughness results from the higher density of block boundaries,and the fracture mode shifts from brittle fracture to ductile–brittle combined fractures,then to ductile fracture due to the increased carbon.Increasing the carbon content would lower the martensite start temperature(M_S)temperature and driving force for martensitic transformation,and increase the strength of austenite matrix,which in turn contributes to producing more V1/V2 variant pairs to accommodate the transformation strain.展开更多
The correlation between the impact toughness and microstructural characteristics of a large bainitic steel bloom has been investigated. The study focuses on microcrack nucleation and propagation in the basic cleavage ...The correlation between the impact toughness and microstructural characteristics of a large bainitic steel bloom has been investigated. The study focuses on microcrack nucleation and propagation in the basic cleavage plane. To analyze the phase transformation during the wind-cooling process, the temperature field of the bloom was acquired by computer simulation, and a continuous cooling transformation experiment was conducted. The results show that compared with the surface of the bloom, the toughness of the bloom’s core is decreased by the increase in proeutectoid ferrite and the coarsening of tempered martensite–austenite constituents. The proeutectoid ferrite decreases the toughness via its effects on carbide precipitation, the formation of martensite–austenite constituents, and the bainite transformation. The relatively large tempered martensite–austenite constituents are conducive to microcrack nucleation and propagation.展开更多
基金supported by the National Natural Science Foundation of China(No.52271089)the financial support from the C hina Postdoctoral Science Foundation(No.2023M732192)。
文摘This work reveals the significant effects of cobalt(Co)on the microstructure and impact toughness of as-quenched highstrength steels by experimental characterizations and thermo-kinetic analyses.The results show that the Co-bearing steel exhibits finer blocks and a lower ductile-brittle transition temperature than the steel without Co.Moreover,the Co-bearing steel reveals higher transformation rates at the intermediate stage with bainite volume fraction ranging from around 0.1 to 0.6.The improved impact toughness of the Co-bearing steel results from the higher dense block boundaries dominated by the V1/V2 variant pair.Furthermore,the addition of Co induces a larger transformation driving force and a lower bainite start temperature(BS),thereby contributing to the refinement of blocks and the increase of the V1/V2 variant pair.These findings would be instructive for the composition,microstructure design,and property optimization of high-strength steels.
基金supported by the Natural Science Foundation of Hunan Province(2023JJ50055,2023JJ30081)the Science Research Foundation of Hunan Provincial Education Department(21A0546)+1 种基金the Youth Project of the National Natural Science Foundation of China(62003056)the Open Fund of Hunan Engineering Research Center of Research and Development of Degradable Materials and Molding Technology(2023KFKT05).
文摘The effect ofωiso andαprecipitation on microstructure,microhardness,tensile properties and impact toughness of Ti-25Nb-10Ta-1Zr-0.2Fe(TNTZF)alloy was investigated.The results showed that the solution treated TNTZF alloy with a small amount of nano-sizedωath particles inβmatrix possesses tensile strength of 697 MPa,elongation of~34%,Young’s modulus(YM)of 75 GPa,and impact toughness of 58.7 J/cm^(2).After aging at relatively lower temperatures of 400℃,the hardness and modulus of the alloy increased significantly,while the plasticity and toughness dropped sharply due to the precipitation ofωiso phase.ωiso phase displayed an ellipsoidal morphology with high volume fraction and a size of about 50 nm after aging at 400℃,leading to the highest hardness of 364 HV and YM of 108 GPa,along with completely embrittlement since elongation and toughness were almost zero.A brittle impact fracture morphology was observed in the alloy,which is dominated by intergranular fracture,with a mixed fracture characteristics of cleavage surfaces,terraces and tiny dimples.When aged at 550℃,plate-likeαdistributed inβmatrix uniformly and inβgrain boundaries in parallel,resulting in the high strength of 804 MPa,as well as lowest YM of 72 GPa,elongation of 9%and toughness of 35.8 J/cm^(2).The fracture morphology of the alloy aged at 550℃showed a ductile fracture mechanism with a large number of dimples.
基金supported by the National Key R&D Program of China(No.2022YFE0123300)the National Natural Science Foundation of China(Nos.U20A20277 and 52474351)Major Project of Liaoning Province Innovation Consortium(No.2023JH1/11200012).
文摘For P91 steel weldment,performing post-weld heat treatment immediately after welding can enhance welding efficiency and reduce the risk of hydrogen-induced cracking.However,determining the post-weld cooling temperature(PWCT)is challenging due to potential impact on the mechanical properties of weldment after subsequent heat treatment.A systematic investigation is conducted to explore the effect of PWCT on the impact toughness of P91 steel welded joints.It has been demonstrated that the impact energy of the weld metal gradually increases from 40.2 to 49.5 J as the PWCT decreases from 300 to 100℃.Microstructural analysis reveals that PWCTs above 100℃ led to an increased dislocation density in the weld metal.Furthermore,in situ observations using a high-temperature confocal laser scanning microscope confirm that excessively high PWCTs result in the formation of untempered martensite after post-weld heat treatment.Such untempered martensites are identified as the primary cause of the reduced impact toughness in the weld metal.These findings underscore the importance of carefully controlling PWCT in welding procedures for P91 steel and similar grades.
基金supported by the Equipment Pre-research and Sharing Technology(41423030503)provided funding for this workThe Equipment Pre-research and Sharing Technology(41423030503)funded this work.
文摘This work used 5CrMnMo steel and titanium carbide(TiC)powders to fabricate particulate metal matrix composites(PMMCS).The composites’microstructure,hardness,and impact toughness were compared with four different titanium carbide ceramic particle sizes.The phase composition and microstructure of composites were studied.Vickers hardness and Charpy impact tests were employed to analyze composites’hardness and impact ductility,respectively.The results showed that the four groups of composites are mainly composed of martensite,trace residual austenite,and titanium carbide(undissolved TiC and primary TiC particles).With the growth of the ceramic particle dimension in the composite layer,the number of primary titanium carbide ceramics gradually decreased.When the initial ceramic particle size was small,it tended to generate dendritic primary TiC,and when the particle size was large,it tended to generate polygons and ellipsoids.Furthermore,with the growth of titanium carbide ceramic particle dimension in the composites,the hardness of the composites decreased but the impact toughness of the composites rose first and then descended.When the ceramic particle size was 50-75μm,the composite had the highest hardness,and the impact energy of the composites was the highest,which is 8 J.This was because there were more undissolved titanium carbide ceramics in the composite,and there was a thicker matrix metal between the ceramic particles.
基金Projects(2010A090200078,2011A080403008)supported by the Major Science and TechnologyProject of Guangdong Province,China
文摘Microstructures of as-cast and extruded ZK60-xRE （RE=Dy, Ho and Gd, x=0-5, mass fraction） alloys were investigated. Meanwhile, the impact toughness was tested and then the relationship was discussed. The results show that as-cast microstructure is refined gradually with increasing the RE content. Mg-Zn-RE new phase increases gradually, while MgZn2 phase decreases gradually to disappear. Second phase tends to distribute along grain boundary in continuous network. Extruded microstructure is refined obviously to reach the micron level. Broken second phase tends to distribute along the extrusion direction in zonal shape. Impact toughness value -nK increases from 9-17 J/cm2 for as-cast state to 26-54 J/cm2 for extruded state. With increasing the value of -nK, fracture macro-morphology changes from a rough plane via multi-plane with step to V-type plane; and from single radiation zone to two zones of fiber and shear lip, respectively. Fracture micro-morphology changes from the brittle fracture to the ductile fracture. Fine grain and few fine dispersed second phase can enhance the impact toughness of magnesium alloys effectively.
基金the support from the National Key Research and Development Program of China(2016YFB0300202 and 2016YFB0300102)the Fundamental Research Funds for the Central Universities(No.FRF-TP-18-002C2)。
文摘The hierarchical martensitic features in ultra-high strength stainless steel(UHSSS),including the prior austenite grains,martensite packets,blocks and laths with the descending size,were refined to various extents by employing different thermomechanical processes and then carefully characterized.Their relation to yield strength and impact toughness was analyzed.We conclude that the refinement of martensitic structures could lead to the significant increase of yield strength,which follows the Hall-Petch relation with the effect grain size defined by high angle boundaries(HABs).Impact toughness of UHSSS depends on the frequency and capability for retained austenite(RA)grains at both HABs and martensite lath boundaries to trap the propagating cracks via strain-induced transformation,in which the film-like RA grains at lath boundaries appear to make the greater contribution.
文摘Clean high carbon heavy rail steel was prepared by the process of vacuum induction furnace smelting,forging and rolling.Mechanisms of RE on the impact toughness and fracture toughness for clean high carbon steel were investigated.In addition,the appropriate range of RE content for clean high carbon steel was determined.Both the austenite grain size and pearlite lamellar spacing decreased due to small amount of RE,consequently the impact toughness and fracture toughness were improved evidently.When the RE content exceeded a critical value,the pearlite lamellar spacing was increased,because RE was segregated on the austenite grain boundaries,damaged the orientation relationship of pearlite transformation,caused the disorder growth and morphology degenerating of pearlite.With the increasing of RE content,both the impact toughness and fracture toughness of clean high carbon steel were gradually increased at first and then decreased.It was found that when the RE content was between 0.0081% and 0.0088%,both the impact toughness and fracture toughness of clean high carbon heavy rail steel were the best.The maximum ballistic work was 21.2 J（20 ℃） and 12.2 J（-20 ℃）,respectively.The maximum plane-strain fracture toughness was 45.67 MPa·m1/2（20 ℃） and 37.04 MPa·m1/2（-20 ℃）,respectively.
基金One of the authors (R. Jayaganthan) thanks DST,New Delhi for the financial support of this work throughgrant No. DST-462-MMD
文摘The influence of rolling at liquid nitrogen temperature and annealing on the microstructure and mechanical properties of Al 5083 alloy was studied in this paper. Cryorolled samples of Al 5083 show significant improvements in strength and hardness. The ultimate tensile strength increases up to 340 MPa and 390 MPa for the 30% and 50% cryorolled samples, respectively. The cryorolled samples, with 30% and 50% reduction, were subjected to Charpy impact testing at various temperatures from 190℃ to 100℃. It is observed that increasing the percentage of reduction of samples during cryorolling has significant effect on decreasing impact toughness at all temperatures by increasing yield strength and decreasing ductility. Annealing of samples after cryorolling shows remarkable increment in impact toughness through recovery and recrystallization. The average grain size of the 50% cryorolled sample （14 μm） after annealing at 350℃ for 1 h is found to be finer than that of the 30% cryorolled sample （25 μm）. The scanning electron microscopy （SEM） analysis of fractured surfaces shows a large-size dimpled morphology, resembling the ductile fracture mechanism in the starting material and fibrous structure with very fine dimples in cryorolled samples corresponding to the brittle fracture mechanism.
基金the National Natural Science Foundation of China[Grant No.U1708252]the Youth Innovation Promotion Association,Chinese Academy of Sciences[Grant No.2013126]+1 种基金Innovation Foundation of Graduate School of Institute of Metals Research,Chinese Academy of Sciences,ChinaLiaoNing Revitalization Talents Program[Grant No.XLYC 1807022]。
文摘The effects of rare earth(RE) on the microstructure and impact toughness of low alloy Cr-Mo-V bainitic steels have been investigated where the steels have RE content of 0 to 0.048 wt.%. The results indicate that the normalized microstructures of the steels are typical granular bainite(GB) composed primarily of bainitic ferrite and martensite and/or austenite(M-A) constituents. The M-A constituents are transformed into ferrite and carbides and/or agglomerated carbides after tempering at 700℃ for 4 h. The addition of RE decreases the onset temperature of bainitic transformation and results in the formation of finer bainitic ferrite, and reduces the amount of carbon-rich M-A constituents. For the normalized and tempered samples, the ductile-to-brittle transition temperature(DBTT) decreases with increasing RE content to a critical value of 0.012 wt.%. Lower DBTT and higher upper shelf energy are attributed to the decreased effective grain size and lower amount of coarse agglomerated carbides from the decomposition of massive M-A constituents. However, the addition of RE in excess of 0.012 wt.% leads to a substantial increase in the volume fraction of large-sized inclusions, which are extremely detrimental to the impact toughness.
基金Sponsored by National Key Technology Research and Development Program of China(2007BAE51B04)
文摘The as-cast microstructure of H13 ESR ingot and the influence of high temperature diffusion treatment on the structure and impact toughness have been investigated. The results show that the dendrite arm spacing gradually becomes wide from the surface to the center of ingot, and the large primary carbide particles always exist in interdendritic segregation areas; by means of high temperature diffusion treatment of ingot prior to hot forging, the banded segregation is nearly eliminated, the annealed structure is more uniform and the isotropic properties have been improved remarkably.
基金This work was financially supported by the National Key Research and Development Program of China(No.2017YFB 0304800)One of the authors,H.Guo,would like to express her gratitude for the financial support of China Scholarship Council(award for one year visiting at Northwestern University in the USA,No.201706465056).
文摘Here we present a novel approach of intercritical heat treatment for microstructure tailoring,in which intercritical annealing is introduced between conventional quenching and tempering.This induced a heterogeneous microstructure consisting of soft intercritical ferrite and hard tempered martensite,resulting in a low yield ratio(YR)and high impact toughness in a high-strength low-alloy steel.The initial yielding and subsequent work hardening behavior of the steel during tensile deformation were modified by the presence of soft intercritical ferrite after intercritical annealing,in comparison to the steel with full martensitic microstructure.The increase in YR was related to the reduction in hardness difference between the soft and hard phases due to the precipitation of nano-carbides and the recovery of dislocations during tempering.The excellent low-temperature toughness was ascribed not only to the decrease in probability of microcrack initiation for the reduction of hardness difference between two phases,but also to the increase in resistance of microcrack propagation caused by the high density of high angle grain boundaries.
文摘The microstructure and impact behavior of A356 aluminum alloy were studied after melt treatment processes of grain refinement and modification under both non-heat treated and T6 heat treated conditions. The modification and grain refinement were done with the addition of Al-10%Sr and Al-5Ti-1B master alloys, respectively. All casting parameters were kept constant in order to focus on the influence of mentioned treatments. The results indicate that the eutectic silicon morphology is the main parameter to control the impact behavior of alloy. Consequently, the individual grain refinement of as-cast alloy does not improve the impact toughness as the modification does. While, simultaneous grain refinement and modification provide higher impact toughness in comparison with individual treatments. T6 heat treatment of the alloy improves the impact toughness under all melt-treated conditions. This is related to the further modification of eutectic silicon particles. To verify the results and clarify the mechanisms, three-point bending test and fractography were used to interpret the improvement of impact toughness of the alloy.
文摘Microstructure, tensile and impact toughness properties and fracture location of friction stir welded AISI 1018 mild steel were revealed. The AISI 1018 mild steel plates with thickness of 5 mm were friction stir welded by tungsten based alloy tool with tool rotational speed of 1 000 r/min and welding speed of 50 ram/rain. Tensile strength of stir zone is higher （8%） compared to that of the base metal. This may be due to the formation of finer grains in the weld nugget region under the stirring action of the rotating tool. The ductility and impact toughness of the joints are decreased compared to those of the base metal owing to the inclusion of tungsten particles in the weld region.
基金The authors acknowledge the financial supports of National Key Research and Development Program of China(2016YFB0301201)。
文摘Ti-6Al,Ti-6Al-2Mo and Ti-6Al-3Nb alloys were prepared to investigate the toughening effects ofβstabilizers Mo and Nb on impact toughness and crack resistance of titanium alloys.Instrumented Charpy impact tests showed that the total impact absorbed energy of Ti-6Al-2Mo and Ti-6Al-3Nb(∼64 J)were two times higher than that of Ti-6Al(∼30 J),indicating the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys.Analysis of load-displacement curves revealed the similar crack initiation energy of Ti-6Al,Ti-6Al-2Mo and Ti-6Al-3Nb(15.4 J,16.1 J and 15.0 J,respectively).However,the higher crack propagation energy of Ti-6Al-2Mo and Ti-6Al-3Nb(46.7 J and 48.3 J,respectively)were about three times higher than that of Ti-6Al(14.4 J),indicating the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb.Post-mortem analysis of impact samples demonstrated that the increased dislocation density and deformation twinning were mainly responsible for the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb.Due to the invisibility of dislocation activation and deformation twinning during the Charpy impact process,a mathematical model has been proposed to evaluate the effects of Al,Mo and Nb elements on dislocation mobility based on the Yu Rui-huang electron theory.Addition of Mo and Nb elements significantly improved the dislocation mobility in Ti-6Al-2Mo and Ti-6Al-3Nb compared to that in Ti-6Al alloy.Therefore,more dislocations were activated in Ti-6Al-2Mo and Ti-6Al-3Nb which supplied the larger plastic deformation under impact loading.A dislocation-based model also has been proposed to interpret the nucleation and propagation of deformation twinning under the impact loading.Dislocation pileup atα/βinterfaces provided potential sites for nucleation of deformation twinning in Ti-6Al-2Mo and Ti-6Al-3Nb.Furthermore,deformation twinning facilitated the dislocation motion inαgrains with hard orientations.The increased dislocation mobility and deformation twinning were responsible for the stronger crack resistance as well as the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys.
基金This work was financially supported by the National Natural Science Foundation of China （NSFC, Grant Nos. 51374260, 51504048 and 51611130062）. The authors thank the members of Laboratory of Metallurgy and Materials, Chongqing University, for the support of this work.
文摘Toughness is an important property for steels used in engineering applications. However, recent toughness testing has shown the existence of a significant fluctuation in toughness in a single rolled plate of titanium micro-alloyed steel. The underlying causes of this fluctuation were investigated by fractography, analysis of microstructure and measurement of inclusions. Coarse and distributed TiN inclusions were responsible for the toughness variation, as they tended to act as the potential cleavage initiators to form micro-cracks. From a calculation of the local fracture stress, the critical size of coarse TiN inclusions for dominating micro-crack propagation was 4.93 μm, and similarly that of ferrite grains was 36.6μm. Under current casting and thermo-mechanically controlled processing schedules, the toughness fluctuation of rolled steel plates can be primarily attributed to the fraction of coarse TiN inclusions larger than 5μm. A corresponding relationship between impact energy and the proportion of coarse TiN inclusions was established. Finally, a normalizing treatment was applied to refine the ferrite grains of rolled steel plates. Despite the presence of coarse TiN inclusions, this refinement in ferrite grains minimized the toughness fluctuation and improved the uniformity of the impact properties of the steel plates.
基金supported by the National High Technology Research and Development Program of China(2007AA03Z438)the National Natural Science Foundations of China(50871032)
文摘The effects of Nb on the thermal stability and impact toughness of ultra-high intrinsic coercivity of Nd-Fe-B magnets were investigated.The results showed that the addition of Nb could improve the thermal stability,and obviously increased the impact toughness of sintered Nd-Fe-B magnets.The optimum thermal stability of sintered Nd-Fe-B magnets was obtained when the content of Nb was 1.0 at.%.The maximum impact toughness of sintered Nd-Fe-B magnets was obtained when the content of Nb was 1.5 at.%,but the magnetic properties of sintered Nd-Fe-B magnets drastically deteriorated when the content of Nb increased from 1.0 at.% to 1.5 at.%.The microstructure showed that overfull Nb addition made many ultra-fine grains get together,which led to the density of sintered Nd-Fe-B magnets decline and drastically deteriorated the magnetic properties of sintered Nd-Fe-B magnets.
基金supported by a National Research Foundation of Korea(NRF)grant funded by the Korean government(MEST)(No.2019R1A2C1008904)。
文摘CrMnFeCoNi high-entropy alloys(HEAs)exhibit an excellent combination of tensile strength and ductility at cryogenic temperatures.This study led to the introduction of a new method for the development of high-performance CrMnFeCoNi HEAs at cryogenic temperatures by jointly utilizing additive manufacturing(AM)and the addition of interstitial atoms.The interstitial oxygen present in the powder feedstock was transformed into beneficial nano-sized oxides during AM processing.The HEA nanocomposite fabricated using laser powder bed fusion(L-PBF)not only contains heterogeneous grains and substructures but also a high number density of nano-sized oxides.The tensile results revealed that the L-PBF HEA nanocomposite has superior yield strengths of 0.77 GPa and 1.15 GPa,and tensile strengths of 0.92 GPa and 1.45 GPa at 298 K and 77 K,respectively.In addition,the Charpy impact energies of the samples tested at 298 K and 77 K were measured as 176.2 J and 103.7 J,respectively.These results indicate that the L-PBF HEA nanocomposite successfully overcomes the well-known strength-toughness trade-off.The tensile deformation microstructure contained a relatively large number of deformation twins(DTs)at cryogenic temperature,a possible consequence of the decrease in the stacking fault energy with decreasing temperature.On the other hand,cracks were found to propagate along the grain boundaries at room temperature,whereas a transgranular crack was observed at cryogenic temperature in the specimens fractured as a result of the Charpy impact.
基金Item Sponsored by National Natural Science Foundation of China(51074052)
文摘An NM400 wear-resistant steel was hot rolled and then the plates were heat-treated by direct quenching and tempering （DQT） and reheat quenching and tempering （RQT） techniques, respectively. The Charpy impact test was carried out with an instrumented Charpy impact tester. The microstructure and fracture surface were investiga-ted by a combination of optical microscopy, transmission electron microscopy and scanning electron microscopy methods. It was found that the impact toughness of DQT specimen was much higher than that of RQT specimen. The microstructure of both DQT and RQT specimens was characterized by a mixture of tempered lath martensite and lower hainite. The lower bainite in DQT specimen extended into prior austenite grains and the content was higher than that in RQT specimen. The lower bainite in DOT specimen improved the impact toughness by increasing the proportion of large-angle boundaries and relieving the stress concentration at the crack tip. A number of fine and dis-persed carbides in DQT specimen also contributed to the improvement of the impact toughness.
基金financially supported by the National Key Research and Development Program of China(No.2017YFB0304800)financial support of China Scholarship Council(award for one year visiting in Northwestern University in USA,No.201706465056)。
文摘Alloying can make conventional metals reach ultra-high strength,but this usually comes at dramatic loss of toughness.In this work,a desirable strength–toughness combination in high-strength low-alloy steel achieved via trace carbon addition.The significance of carbon in tailoring variant pairing and tuning impact toughness was elucidated from the perspective of crystallography and thermodynamics.As the carbon content increases,the packets and blocks are refined,and the-40 impact toughness improves.The enhancement of impact toughness results from the higher density of block boundaries,and the fracture mode shifts from brittle fracture to ductile–brittle combined fractures,then to ductile fracture due to the increased carbon.Increasing the carbon content would lower the martensite start temperature(M_S)temperature and driving force for martensitic transformation,and increase the strength of austenite matrix,which in turn contributes to producing more V1/V2 variant pairs to accommodate the transformation strain.
文摘The correlation between the impact toughness and microstructural characteristics of a large bainitic steel bloom has been investigated. The study focuses on microcrack nucleation and propagation in the basic cleavage plane. To analyze the phase transformation during the wind-cooling process, the temperature field of the bloom was acquired by computer simulation, and a continuous cooling transformation experiment was conducted. The results show that compared with the surface of the bloom, the toughness of the bloom’s core is decreased by the increase in proeutectoid ferrite and the coarsening of tempered martensite–austenite constituents. The proeutectoid ferrite decreases the toughness via its effects on carbide precipitation, the formation of martensite–austenite constituents, and the bainite transformation. The relatively large tempered martensite–austenite constituents are conducive to microcrack nucleation and propagation.
