Due to the low content of alloying elements and the lack of effective nucleation sites,the fusion zone(FZ)of tungsten inert gas(TIG)welded AZ31 alloy typically exhibits undesirable coarse columnar grains,which can res...Due to the low content of alloying elements and the lack of effective nucleation sites,the fusion zone(FZ)of tungsten inert gas(TIG)welded AZ31 alloy typically exhibits undesirable coarse columnar grains,which can result in solidification defects and reduced mechanical properties.In this work,a novel welding wire containing MgO particles has been developed to promote columnar-to-equiaxed transition(CET)in the FZ of TIG-welded AZ31 alloy.The results show the achievement of a fully equiaxed grain structure in the FZ,with a significant 71.9%reduction in grain size to 41 μm from the original coarse columnar dendrites.Furthermore,the combination of using MgO-containing welding wire and pulse current can further refine the grain size to 25.6 μm.Microstructural analyses reveal the homogeneous distribution of MgO particles in the FZ.The application of pulse current results in an increase in the number density of MgO(1-2 μm)from 5.16 × 10^(4) m^(-3) to 6.18 × 10^(4) m^(-3).The good crystallographic matching relationship between MgO and α-Mg matrix,characterized by the orientation relationship of[11(2)0]α-Mg//[0(1)1]MgO and(0002)_(α-Mg)//(111)_(MgO),indicates that the MgO particles can act as effective nucleation sites for α-Mg to reduce nucleation undercooling.According to the Hunt criteria,the critical temperature gradient for CET is greatly enhanced due to the significantly increased number density of MgO nucleation sites.In addition,the correlation with the thermal simulation results reveals a transition in the solidification conditions within the welding pool from the columnar grain zone to the equiaxed grain zone in the CET map,leading to the realization of CET.The exceptional grain refinement has contributed to a simultaneous improvement in the strength and plasticity of welded joints.This study presents a novel strategy for controlling equiaxed microstructure and optimizing mechanical properties in fusion welding or wire and arc additive manufacturing of Mg alloy components.展开更多
Large solidification ranges and coarse columnar grains in the additively manufacturing of Al-Mg-Si alloys are normally involved in hot cracks during solidification.In this work,we develop novel crack-free Al-Mg_(2) Si...Large solidification ranges and coarse columnar grains in the additively manufacturing of Al-Mg-Si alloys are normally involved in hot cracks during solidification.In this work,we develop novel crack-free Al-Mg_(2) Si alloys fabricated by laser powder-bed fusion(L-PBF).The results indicate that the eutectic Mg_(2) Si phase possesses a strong ability to reduce crack susceptibility.It can enhance the grain growth restriction factor in the initial stage of solidification and promote eutectic filling in the terminal stage of solidifica-tion.The crack-free L-PBFed Al-x Mg_(2) Si alloys(x=6 wt.%,9 wt.%,and 12 wt.%)exhibit the combination of low crack susceptibility index(CSI),superior ability for liquid filling,and grain refinement.Particularly,the L-PBFed Al-9Mg_(2) Si alloy shows improved mechanical properties(e.g.yield strength of 397 MPa and elongation of 7.3%).However,the cracks are more likely to occur in the region near the columnar grain boundaries of the L-PBFed Al-3Mg_(2) Si alloy with a large solidification range and low eutectic content for liquid filling.Correspondingly,the L-PBFed Al-3Mg_(2) Si alloy shows poor bearing capacity of mechanical properties.The precise tuning of Mg_(2) Si eutectic content can offer an innovative strategy for eliminating cracks in additively manufactured Al-Mg-Si alloy.展开更多
In this study,the effect of inclination angles relative to the building direction in the additively manufactured eutectic Al-5Mg-2Si alloy was investigated through the laser powder bed fusion(LPBF).The microstructures...In this study,the effect of inclination angles relative to the building direction in the additively manufactured eutectic Al-5Mg-2Si alloy was investigated through the laser powder bed fusion(LPBF).The microstructures and mechanical properties of the Al-5Mg-2Si alloy manufactured with different inclination angles(0°,30°,45°,60°and 90°)were reported and discussed.It is found that the“semicircular”melt pool(MP)in the load bearing face of 0°sample was eventually transformed into“stripe-like”MP in the 90°sample,accompanied by an increased fraction of melt pool boundaries(MPBs).Moreover,the microstructural analysis revealed that the columnar-to-equiaxed transition(CET)of theα-Al grains and eutectic Mg2Si was completed in the 90°sample,which were significantly refined with the average size of 10.6μm and 0.44μm,respectively.It is also found that the 90°sample exhibited good combination of strength and elongation(i.e.yield strength of 393 MPa,ultimate tensile strength of 483 MPa and elongation of 8.1%).The anisotropic mechanical properties were highly associated with the refined microstructures,thermal stress,and density of MPBs.Additionally,the CET driven by inclination angles was attributed to the variation of thermal conditions inside the local MPs.展开更多
The effect of addition temperature of MgO particles(MgOp)on their dispersion behavior and the efficiency of grain refinement in AZ31 Mg alloy was investigated.In addition,the grain refinement mechanism was systematica...The effect of addition temperature of MgO particles(MgOp)on their dispersion behavior and the efficiency of grain refinement in AZ31 Mg alloy was investigated.In addition,the grain refinement mechanism was systematically studied by microstructure characterization,thermodynamic calculation,and analysis of solidification curves.The results show that the grain size of AZ31 Mg alloy initially decreases and then increases as the MgOp addition temperature is increased from 720 to 810℃,exhibiting a minimum value of 136μm at 780℃.The improved grain refinement efficiency with increasing MgOp addition temperature can be attributed to the reduced Mg melt viscosity and enhanced wettability between MgOp and Mg melt.Furthermore,a corresponding physical model describing the solidification behavior and grain refinement mechanism was proposed.展开更多
In Mg-Ca alloys the grain refining mechanism,in particular regarding the role of nucleant substrates,remains the object of debates.Although native MgO is being recognised as a nucleating substrate accounting for grain...In Mg-Ca alloys the grain refining mechanism,in particular regarding the role of nucleant substrates,remains the object of debates.Although native MgO is being recognised as a nucleating substrate accounting for grain refinement of Mg alloys,the possible interactions of MgO with alloying elements that may alter the nucleation potency have not been elucidated yet.Herein,we design casting experiments of Mg-xCa alloys varied qualitatively in number density of native MgO,which are then comprehensively studied by advanced electron microscopy.The results show that grain refinement is enhanced as the particle number density of MgO increases.The native MgO particles are modified by interfacial layers due to the co-segregation of Ca and N solute atoms at the MgO/Mg interface.Using aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy,we reveal the nature of these Ca/N interfacial layers at the atomic scale.Irrespective of the crystallographic termination of MgO,Ca and N co-segregate at the MgO/Mg interface and occupy Mg and O sites,respectively,forming an interfacial structure of a few atomic layers.The interfacial structure is slightly expanded,less ordered and defective compared to the MgO matrix due to compositional deviations,whereby the MgO substrate is altered as a poorer template to nucleate Mg solid.Upon solidification in a TP-1 mould,the impotent MgO particles account for the grain refining mechanism,where they are suggested to participate into nucleation and grain initiation processes in an explosive manner.This work not only reveals the atomic engineering of a substrate through interfacial segregation but also demonstrates the effectiveness of a strategy whereby native MgO particles can be harnessed for grain refinement in Mg-Ca alloys.展开更多
Magnesium is the lightest constructional metal,which makes it an important material for different applications like automotive,transportation,aviation and aerospace.There are several studies about developing propertie...Magnesium is the lightest constructional metal,which makes it an important material for different applications like automotive,transportation,aviation and aerospace.There are several studies about developing properties of existing Mg alloys and introducing new alloy systems to industrial producers.An important way to improve properties of metallic materials is to decrease grain size that results almost in increasing all kind of properties of the material.This review paper aims to summarize the literature about grain refining of magnesium alloys.The text is consisting of three sections,which focused on the(1)grain refining methods used in the past,which are not used today,(2)grain refining methods currently being used in the industry and(3)novel and newly developed methods that may find usage in the industry in future.Before explaining grain refining methods of magnesium alloys a general summary about grain refinement of metals is also provided.展开更多
A fine and equiaxed solidification process delivers multidimensional benefits to Mg-alloys, such as improved castability, reduced casting defects, enhanced mechanical properties, increased corrosion resistance and pot...A fine and equiaxed solidification process delivers multidimensional benefits to Mg-alloys, such as improved castability, reduced casting defects, enhanced mechanical properties, increased corrosion resistance and potential for increased recycled contents. Despite extensive research on grain refinement of Mg-alloys in the last few decades, currently, there is no effective grain refiner available for refining Mg-Al alloys, and our current understanding of grain refining mechanisms is not adequate to facilitate the development of effective grain refiners.Under the EPSRC(UK) Li ME Hub’s research program, substantial advances have been made in understanding the early stages of solidification covering prenucleation, heterogeneous nucleation, grain initiation and grain refinement. In this paper, we provide a comprehensive overview of grain refinement of Mg-alloys by native MgO particles. We show that native MgO particles can be made available for effective grain refinement of Mg-alloys by intensive melt shearing regardless of the alloy compositions. More importantly, we demonstrate that(1) the addition of more potent exogenous particles will not be more effective than native MgO;and(2) MgO particles are difficult to be made more impotent for grain refinement through promoting explosive grain initiation. We suggest that the most effective approach to grain refinement of Mg-alloys is to make more native MgO particles available for grain refinement through dispersion, such as by intensive melt shearing.展开更多
Recently, Sn-Zn-Bi alloys have been reported to be the sheath material for miniature detonating cords,due to appropriate mechanical properties, ease of manufacturing, and low cost. Bi addition was found beneficial to ...Recently, Sn-Zn-Bi alloys have been reported to be the sheath material for miniature detonating cords,due to appropriate mechanical properties, ease of manufacturing, and low cost. Bi addition was found beneficial to the mechanical performance of Sn-Zn. However, limited information about the influence of Bi on the corrosion properties of Sn-Zn alloys has been provided. In this work, electrochemical corrosion behaviours of Sn-3Zn-xBi(x=0, 1, 3, 5, 7 wt%) alloys were investigated using potentiodynamic polarization and electrochemical impedance spectroscopy(EIS) techniques, to explore the effects of Bi on the corrosion performance of Sn-Zn alloys. The corrosion mechanism of Sn-Zn-Bi alloys was analysed through microstructure examination on the surface of alloys after corrosion measurements. Results indicated that the addition of 1 wt% Bi increased the corrosion susceptibility of the Sn-3Zn alloy, mainly attributed to the coarsened and more uniformly distributed corrosion-vulnerable Zn-rich precipitates, while further increasing the Bi contents decreased the corrosion susceptibility of Sn-3Zn-xBi alloys due to the higher fraction of nobler Bi particles serving as anodic barriers. The Sn-3Zn-7Bi possessed the best corrosion resistance among all Sn-Zn-Bi alloys investigated. The role of Bi on corrosion was considerably discussed.展开更多
The development of lightweight magnesium(Mg)alloys capable of operating at elevated temperatures of 200-300℃and the ability of using high pressure die casting for high-volume manufacturing are the most advanced devel...The development of lightweight magnesium(Mg)alloys capable of operating at elevated temperatures of 200-300℃and the ability of using high pressure die casting for high-volume manufacturing are the most advanced developments in manufacturing critical parts for internal combustion engines used in power tools.Here we report the microstructure and mechanical properties of a newly developed die-cast Mg-RE(La,Ce,Nd,Gd)-Al alloy capable of working at higher elevated temperatures of 200-300℃.The new alloy delivers the yield strength of 94 MPa at 300℃,which demonstrates a 42%increase over the benchmark AE44 high temperature die-cast Mg alloy.The new alloy also has good stiffness at elevated temperatures with its modulus only decreasing linearly by 13%from room temperature up to 300℃.Thermal analysis shows a minor peak at 364.7℃in the specific heat curve of the new alloy,indicating a good phase stability of the alloy up to 300℃.Nd and Gd have more affinity to Al for the formation of the minority of divorced Al-RE(Nd,Gd)based compounds,and the stable Al-poor Mg_(12)RE(La_(0.22)Ce_(0.13)Nd_(0.31)Gd_(0.31))Zn_(0.39)Al_(0.13)compound acts as the continuous inter-dendritic network,which contribute to the high mechanical performance and stability of the new die-cast Mg alloy at 200-300℃.展开更多
The influence of annealing cycles up to 650 °C on the specific conductivity and hardness（HV） of hot-rolled sheets of Al alloys containing up to 0.5% Zr（mass fraction） was studied.Using analytical calculations...The influence of annealing cycles up to 650 °C on the specific conductivity and hardness（HV） of hot-rolled sheets of Al alloys containing up to 0.5% Zr（mass fraction） was studied.Using analytical calculations of phase composition and experimental methods（scanning electron microscopy,transmission electron microscopy,electron microprobe analysis,etc）,it is demonstrated that the conductivity depends on the content of Zr in the Al solid solution which is the minimum after holding at 450 °C for 3 h.On the other hand,the hardness of the alloy is mainly caused by the amount of nanoparticles of the L12（Al3Zr） phase that defines the retention of strain hardening.It is shown that the best combination of electrical conductivity and hardness values can be reached within an acceptable holding time at the temperature about 450 °C.展开更多
Intensive melt shearing has a significant grain refining effect on some light alloys.However,the persistence of the grain refining effect during isothermal holding and remelting is still unclear,although it is very im...Intensive melt shearing has a significant grain refining effect on some light alloys.However,the persistence of the grain refining effect during isothermal holding and remelting is still unclear,although it is very important for the practical application.In this study,intensive melt shearing was achieved in a twin-screw mechanism to investigate its grain refining effect on AZ91D magnesium alloy.The refinement mechanism was discussed and the persistence of grain refinement after remelting and isothermal holding was also studied.A Zeiss imaging system with polarized light was used for quantitative measurement of grain size.The results show that the intensive melt shearing has a significant grain refining effect on AZ91D magnesium alloy.With the application of intensive melt shearing,the grain size of AZ91D magnesium alloy can be reduced from 530 μm(for a typical as-cast microstructure) to 170 μm,which is about 70% size reduction.The grain refinement achieved by the intensive melt shearing can be partially kept after isothermal holding and remelting.It is believed that the refinement effect was mainly due to the finer and well dispersed oxide particles formed by high intensive shearing.The smaller size of oxide particles and their slow motion velocity in the sheared melt could make important contributions to the remained grain refinement.展开更多
AlP has been widely used as an effective heterogenous nucleus for primary Si phase in hypereutectic AlSi alloys,but the morphological correlation between AlP and primary Si is still confusing.In the present work,the m...AlP has been widely used as an effective heterogenous nucleus for primary Si phase in hypereutectic AlSi alloys,but the morphological correlation between AlP and primary Si is still confusing.In the present work,the morphologies of AlP crystals were studied comprehensively by experimental observation and theorical prediction.It is found that AlP collected from an Al-0.03 P melt could be divided into two categories:spinel twin crystals and non-twin crystals.During the nucleation process,these two kinds of AlP crystals triggered morphologically templated nucleation of primary Si phase,resulting in the formation of hexagonal primary Si twin and octahedral non-twin crystals,respectively.As such,the percentage of primary Si twin crystals in the experimental Al-18 Si alloy was also increased obviously after the morphologically templated nucleation via Al P.The morphologically templated nucleation also eliminated the dendritic growth of primary Si phase and the formation of hopper structures inside primary Si,forcing primary Si to maintain to be faceted solid crystals through layer-by-layer growing mechanism.The insight into morphologically templated nucleation offers a new view in understanding the mechanism of heterogeneous nucleation of primary Si phase on AlP nuclei.展开更多
A novel Al-5 Mg2 Si-2 Mg alloy was processed by selective laser melting(SLM) to understand its representative features of microstructural evolution and mechanical properties during additive manufacturing(AM). The as-S...A novel Al-5 Mg2 Si-2 Mg alloy was processed by selective laser melting(SLM) to understand its representative features of microstructural evolution and mechanical properties during additive manufacturing(AM). The as-SLM fabricated Al-5 Mg_(2) Si-2 Mg alloy is found to deliver much less hot cracks and other defects. Meanwhile, excellent mechanical property is also achieved, i.e. 452 ± 11 MPa for ultimate tensile strength, 295 ± 14 MPa for yield strength, and 9.3 ± 2.5% for elongation. Clearly, these mechanical properties are better than that obtained by high pressure die casting(HPDC), and better than some other alloys obtained by SLM. The as-SLM fabricated Al-5 Mg_(2) Si-2 Mg alloy is featured by the significantly refined microstructures in the compact primary α-Al, the divorced Mg_(2) Si eutectic networks distributing atα-Al grain boundaries, and some α-Al Fe Mn Si phase in association with eutectic Mg_(2) Si phase. The high strength and ductility of the alloy are attributed to its unique features including(a) the reduced solidification range,(b) the possible increase of the eutectic level in the microstructure, and(c) the shift of eutectic point and the maximum solubility point of Mg2 Si in Al matrix.展开更多
The selective laser melting(SLM)with subsequent cold rolling and annealing is used to produce high-density lattice defects and grain refinement in the CoCrNi medium-entropy alloys(MEAs).The superior comprehensive mech...The selective laser melting(SLM)with subsequent cold rolling and annealing is used to produce high-density lattice defects and grain refinement in the CoCrNi medium-entropy alloys(MEAs).The superior comprehensive mechanical properties have been achieved in the as-SLMed CoCrNi alloy after rolling and annealing.The as-SLMed alloys delivered the yield strength of 693.4 MPa,the ultimate tensile strength of 912.7 MPa and the fracture strain of 54.4%.After rolling with 70%reduction in thickness and annealing at 700℃for 2 h.the yield strength,ultimate tensile strength and fracture strain reached 1161.6 MPa,1390.8 MPa and 31.5%,respectively.The exceptional strength-ductility synergy is mainly attributed to the refined hierarchical microstructures with coarsening grains at a level of 30μm and ultrafine grains at a level of 1μm,and the heritage of dislocation-formed sub-grains and other lattice defects.This investigation demonstrates that the SLM with subsequent rolling and annealing is beneficial to fabricate high strength and ductile MEAs with single face-centered cubic(fcc)structure.展开更多
Effect of Si addition on microstructure,hardness and wear resistance of as cast Mg-5Sn and Mg-10Sn alloys were investigated.For this purpose alloys with wt.-%0,0.5,1,1.5 Si were cast using an induction furnace.Additio...Effect of Si addition on microstructure,hardness and wear resistance of as cast Mg-5Sn and Mg-10Sn alloys were investigated.For this purpose alloys with wt.-%0,0.5,1,1.5 Si were cast using an induction furnace.Addition of Si to both alloys caused an increasement in the amount of intermetallic phases at the grain boundaries.Also refined grain structure was reported with increasing Si content in the cast alloys.Both increased amount of intermetallics and decreased grain size caused an increase in the hardness of the alloys.Wear resistance of the binary alloys first increased with Si addition and then decreased.Increased amounts of Si in the alloys caused crack formation during wear tests and decreased wear resistance.Sliding speed also has a dramatical effect on the wear resistance of the alloys.Increased sliding speed resulted with higher wear resistance.Worn surfaces of the alloys also investigated under scanning electron microscope to understand the wear mechanisms operated during wear tests.展开更多
The coarsening-grained single-phase face-centered cubic(fcc)medium-entropy alloys(MEAs)normally exhibit insufficient strength for some engineering applications.Here,superior mechanical properties with ultimate tensile...The coarsening-grained single-phase face-centered cubic(fcc)medium-entropy alloys(MEAs)normally exhibit insufficient strength for some engineering applications.Here,superior mechanical properties with ultimate tensile strength of 1.6 GPa and fracture strain of 13.1%at ambient temperature have been achieved in a(CoCrNi)_(94)Ti_(3)Al_(3)MEA by carefully architecting the multi-scale heterogeneous structures.Electron microscopy characterization indicates that the superior mechanical properties mainly originated from the favorable heterogeneous fcc matrix(1-40μm)and the coherent sphericalγ’precipitates(10-100 nm),together with a high number density of crystalline defects(2-10 nm),including dislocations,small stacking faults,Lomer-Cottrell locks,and ultrafine deformation twins.展开更多
The retrogression and re-aging(RRA)processes,aimed mainly at tailoring intergranular precipitates,could significantly improve the corrosion resistance(i.e.,stress corrosion cracking resistance)without considerably dec...The retrogression and re-aging(RRA)processes,aimed mainly at tailoring intergranular precipitates,could significantly improve the corrosion resistance(i.e.,stress corrosion cracking resistance)without considerably decreasing the strength,which signifies that an efficient control of the size,distribution and evolution of intergranular and intragranular precipitates becomes critical for the integrated properties of the(mid-)thick high-strength Al alloy plates.Compared to RRA process with retrogression at200℃ (T77),this study investigated the impact of a modified RRA process(MT77)with lower retrogression temperatures(155-175℃ )and first-stage under-aging on the properties of a high-strength AA7050 Al alloy,in combination with detailed precipitate characterization.The study showed that the strength/microhardness of the RRA-treated alloys decreased with raising retrogression temperature and/or prolonging retrogression time,along with the increased electrical conductivity.The rapid responsiveness of microstructure/property typical of retrogression at 200℃ was obviously postponed or decreased by using MT77 process with longer retrogression time that was more suitable for treating the(mid-)thick plates.On the other hand,higher retrogression temperature facilitated more intragranularηprecipitates,coarse intergranular precipitates and wide precipitate free zones,which prominently increased the electrical conductivity alongside a considerable strength loss as compared to the MT77-treated alloys.With the preferred MT77 process,the high strength approaching T6 level as well as good corrosion resistance was achieved.However,though a relatively homogeneous through-thickness strength was obtained,some small discrepancies of properties between the central and surface areas of an 86-mm thick 7050 Al alloy plate were observed,possibly related to the quenching sensitivity.The precipitate evolution and mechanistic connection to the properties were discussed and reviewed for high-strength Al alloys along with suggestions for further RRA optimization.展开更多
In this work,a process of cold rolling with 70%thickness reduction and different annealing temperatures was selected to regulate the microstructure of Ti-3wt%Cu alloy.Microstructural evolution,mechanical properties an...In this work,a process of cold rolling with 70%thickness reduction and different annealing temperatures was selected to regulate the microstructure of Ti-3wt%Cu alloy.Microstructural evolution,mechanical properties and antibacterial properties of the Ti-3wt%Cu alloy under different conditions were systematically investigated in terms of X-ray diffraction(XRD),scanning electron microscope(SEM),transmission electron microscope(TEM),tensile and antibacterial test.The results indicated that cold rolling could dramatically increase the ultimate tensile stress(UTS)from 520 to 928 MPa,but reduce the fracture strain from 15.3%to 3.8%.With the annealing temperature increasing from 400 to 800C for 1 h,the UTS decreased from 744 to 506 MPa and the fracture strain increased from12.7%to 24.4%.Moreover,the antibacterial properties of the Ti-3wt%Cu alloy under different conditions showed excellent antibacterial rate(>96.69%).The results also indicated that the excellent combination of strength and ductility of the Ti-3wt%Cu alloy with cold rolling and following annealing could be achieved in a trade-off by tuning the size and distribution of Ti2Cu phase,which could increase the applicability of the alloy in clinical practice.More importantly,the antibacterial properties maintained a good stability for the Ti-3wt%Cu alloy under different conditions.The excellent combination of mechanical properties and antibacterial properties could make the Ti-3wt%Cu alloy a good candidate for long-term orthopaedic implant application.展开更多
基金supported by the National Natural Science Foundation of China(No.51871155).
文摘Due to the low content of alloying elements and the lack of effective nucleation sites,the fusion zone(FZ)of tungsten inert gas(TIG)welded AZ31 alloy typically exhibits undesirable coarse columnar grains,which can result in solidification defects and reduced mechanical properties.In this work,a novel welding wire containing MgO particles has been developed to promote columnar-to-equiaxed transition(CET)in the FZ of TIG-welded AZ31 alloy.The results show the achievement of a fully equiaxed grain structure in the FZ,with a significant 71.9%reduction in grain size to 41 μm from the original coarse columnar dendrites.Furthermore,the combination of using MgO-containing welding wire and pulse current can further refine the grain size to 25.6 μm.Microstructural analyses reveal the homogeneous distribution of MgO particles in the FZ.The application of pulse current results in an increase in the number density of MgO(1-2 μm)from 5.16 × 10^(4) m^(-3) to 6.18 × 10^(4) m^(-3).The good crystallographic matching relationship between MgO and α-Mg matrix,characterized by the orientation relationship of[11(2)0]α-Mg//[0(1)1]MgO and(0002)_(α-Mg)//(111)_(MgO),indicates that the MgO particles can act as effective nucleation sites for α-Mg to reduce nucleation undercooling.According to the Hunt criteria,the critical temperature gradient for CET is greatly enhanced due to the significantly increased number density of MgO nucleation sites.In addition,the correlation with the thermal simulation results reveals a transition in the solidification conditions within the welding pool from the columnar grain zone to the equiaxed grain zone in the CET map,leading to the realization of CET.The exceptional grain refinement has contributed to a simultaneous improvement in the strength and plasticity of welded joints.This study presents a novel strategy for controlling equiaxed microstructure and optimizing mechanical properties in fusion welding or wire and arc additive manufacturing of Mg alloy components.
基金financially supported by the National Natural Science Foundation of China(Grant No.52071343)the Leading Innovation and Entrepreneurship Team of Zhejiang Province-Automotive Light Alloy Innovation Team(No.2022R01018).
文摘Large solidification ranges and coarse columnar grains in the additively manufacturing of Al-Mg-Si alloys are normally involved in hot cracks during solidification.In this work,we develop novel crack-free Al-Mg_(2) Si alloys fabricated by laser powder-bed fusion(L-PBF).The results indicate that the eutectic Mg_(2) Si phase possesses a strong ability to reduce crack susceptibility.It can enhance the grain growth restriction factor in the initial stage of solidification and promote eutectic filling in the terminal stage of solidifica-tion.The crack-free L-PBFed Al-x Mg_(2) Si alloys(x=6 wt.%,9 wt.%,and 12 wt.%)exhibit the combination of low crack susceptibility index(CSI),superior ability for liquid filling,and grain refinement.Particularly,the L-PBFed Al-9Mg_(2) Si alloy shows improved mechanical properties(e.g.yield strength of 397 MPa and elongation of 7.3%).However,the cracks are more likely to occur in the region near the columnar grain boundaries of the L-PBFed Al-3Mg_(2) Si alloy with a large solidification range and low eutectic content for liquid filling.Correspondingly,the L-PBFed Al-3Mg_(2) Si alloy shows poor bearing capacity of mechanical properties.The precise tuning of Mg_(2) Si eutectic content can offer an innovative strategy for eliminating cracks in additively manufactured Al-Mg-Si alloy.
基金Project(52071343)supported by the National Natural Science Foundation of China。
文摘In this study,the effect of inclination angles relative to the building direction in the additively manufactured eutectic Al-5Mg-2Si alloy was investigated through the laser powder bed fusion(LPBF).The microstructures and mechanical properties of the Al-5Mg-2Si alloy manufactured with different inclination angles(0°,30°,45°,60°and 90°)were reported and discussed.It is found that the“semicircular”melt pool(MP)in the load bearing face of 0°sample was eventually transformed into“stripe-like”MP in the 90°sample,accompanied by an increased fraction of melt pool boundaries(MPBs).Moreover,the microstructural analysis revealed that the columnar-to-equiaxed transition(CET)of theα-Al grains and eutectic Mg2Si was completed in the 90°sample,which were significantly refined with the average size of 10.6μm and 0.44μm,respectively.It is also found that the 90°sample exhibited good combination of strength and elongation(i.e.yield strength of 393 MPa,ultimate tensile strength of 483 MPa and elongation of 8.1%).The anisotropic mechanical properties were highly associated with the refined microstructures,thermal stress,and density of MPBs.Additionally,the CET driven by inclination angles was attributed to the variation of thermal conditions inside the local MPs.
基金the National Natural Science Foundation of China(No.51871155).
文摘The effect of addition temperature of MgO particles(MgOp)on their dispersion behavior and the efficiency of grain refinement in AZ31 Mg alloy was investigated.In addition,the grain refinement mechanism was systematically studied by microstructure characterization,thermodynamic calculation,and analysis of solidification curves.The results show that the grain size of AZ31 Mg alloy initially decreases and then increases as the MgOp addition temperature is increased from 720 to 810℃,exhibiting a minimum value of 136μm at 780℃.The improved grain refinement efficiency with increasing MgOp addition temperature can be attributed to the reduced Mg melt viscosity and enhanced wettability between MgOp and Mg melt.Furthermore,a corresponding physical model describing the solidification behavior and grain refinement mechanism was proposed.
基金financial support under grant number EP/N007638/1supported by EPSRC under grant number EP/W021080/1
文摘In Mg-Ca alloys the grain refining mechanism,in particular regarding the role of nucleant substrates,remains the object of debates.Although native MgO is being recognised as a nucleating substrate accounting for grain refinement of Mg alloys,the possible interactions of MgO with alloying elements that may alter the nucleation potency have not been elucidated yet.Herein,we design casting experiments of Mg-xCa alloys varied qualitatively in number density of native MgO,which are then comprehensively studied by advanced electron microscopy.The results show that grain refinement is enhanced as the particle number density of MgO increases.The native MgO particles are modified by interfacial layers due to the co-segregation of Ca and N solute atoms at the MgO/Mg interface.Using aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy,we reveal the nature of these Ca/N interfacial layers at the atomic scale.Irrespective of the crystallographic termination of MgO,Ca and N co-segregate at the MgO/Mg interface and occupy Mg and O sites,respectively,forming an interfacial structure of a few atomic layers.The interfacial structure is slightly expanded,less ordered and defective compared to the MgO matrix due to compositional deviations,whereby the MgO substrate is altered as a poorer template to nucleate Mg solid.Upon solidification in a TP-1 mould,the impotent MgO particles account for the grain refining mechanism,where they are suggested to participate into nucleation and grain initiation processes in an explosive manner.This work not only reveals the atomic engineering of a substrate through interfacial segregation but also demonstrates the effectiveness of a strategy whereby native MgO particles can be harnessed for grain refinement in Mg-Ca alloys.
基金National Natural Science Foundation of China(51371126)Science and Technology Supporting Program in Tianjin(14ZCZDGX00007)+1 种基金International Exchange-NSFC-RS(5131113013)Self-Create Program of Science and Technology Plan in Tianjin Binhai New Area(2012-BK120024)
文摘Magnesium is the lightest constructional metal,which makes it an important material for different applications like automotive,transportation,aviation and aerospace.There are several studies about developing properties of existing Mg alloys and introducing new alloy systems to industrial producers.An important way to improve properties of metallic materials is to decrease grain size that results almost in increasing all kind of properties of the material.This review paper aims to summarize the literature about grain refining of magnesium alloys.The text is consisting of three sections,which focused on the(1)grain refining methods used in the past,which are not used today,(2)grain refining methods currently being used in the industry and(3)novel and newly developed methods that may find usage in the industry in future.Before explaining grain refining methods of magnesium alloys a general summary about grain refinement of metals is also provided.
文摘A fine and equiaxed solidification process delivers multidimensional benefits to Mg-alloys, such as improved castability, reduced casting defects, enhanced mechanical properties, increased corrosion resistance and potential for increased recycled contents. Despite extensive research on grain refinement of Mg-alloys in the last few decades, currently, there is no effective grain refiner available for refining Mg-Al alloys, and our current understanding of grain refining mechanisms is not adequate to facilitate the development of effective grain refiners.Under the EPSRC(UK) Li ME Hub’s research program, substantial advances have been made in understanding the early stages of solidification covering prenucleation, heterogeneous nucleation, grain initiation and grain refinement. In this paper, we provide a comprehensive overview of grain refinement of Mg-alloys by native MgO particles. We show that native MgO particles can be made available for effective grain refinement of Mg-alloys by intensive melt shearing regardless of the alloy compositions. More importantly, we demonstrate that(1) the addition of more potent exogenous particles will not be more effective than native MgO;and(2) MgO particles are difficult to be made more impotent for grain refinement through promoting explosive grain initiation. We suggest that the most effective approach to grain refinement of Mg-alloys is to make more native MgO particles available for grain refinement through dispersion, such as by intensive melt shearing.
基金Financial support from the National Aerospace Technology Exploitation Programme (NATEP)Chemring Energetics UK [grant number WEAF058]
文摘Recently, Sn-Zn-Bi alloys have been reported to be the sheath material for miniature detonating cords,due to appropriate mechanical properties, ease of manufacturing, and low cost. Bi addition was found beneficial to the mechanical performance of Sn-Zn. However, limited information about the influence of Bi on the corrosion properties of Sn-Zn alloys has been provided. In this work, electrochemical corrosion behaviours of Sn-3Zn-xBi(x=0, 1, 3, 5, 7 wt%) alloys were investigated using potentiodynamic polarization and electrochemical impedance spectroscopy(EIS) techniques, to explore the effects of Bi on the corrosion performance of Sn-Zn alloys. The corrosion mechanism of Sn-Zn-Bi alloys was analysed through microstructure examination on the surface of alloys after corrosion measurements. Results indicated that the addition of 1 wt% Bi increased the corrosion susceptibility of the Sn-3Zn alloy, mainly attributed to the coarsened and more uniformly distributed corrosion-vulnerable Zn-rich precipitates, while further increasing the Bi contents decreased the corrosion susceptibility of Sn-3Zn-xBi alloys due to the higher fraction of nobler Bi particles serving as anodic barriers. The Sn-3Zn-7Bi possessed the best corrosion resistance among all Sn-Zn-Bi alloys investigated. The role of Bi on corrosion was considerably discussed.
文摘The development of lightweight magnesium(Mg)alloys capable of operating at elevated temperatures of 200-300℃and the ability of using high pressure die casting for high-volume manufacturing are the most advanced developments in manufacturing critical parts for internal combustion engines used in power tools.Here we report the microstructure and mechanical properties of a newly developed die-cast Mg-RE(La,Ce,Nd,Gd)-Al alloy capable of working at higher elevated temperatures of 200-300℃.The new alloy delivers the yield strength of 94 MPa at 300℃,which demonstrates a 42%increase over the benchmark AE44 high temperature die-cast Mg alloy.The new alloy also has good stiffness at elevated temperatures with its modulus only decreasing linearly by 13%from room temperature up to 300℃.Thermal analysis shows a minor peak at 364.7℃in the specific heat curve of the new alloy,indicating a good phase stability of the alloy up to 300℃.Nd and Gd have more affinity to Al for the formation of the minority of divorced Al-RE(Nd,Gd)based compounds,and the stable Al-poor Mg_(12)RE(La_(0.22)Ce_(0.13)Nd_(0.31)Gd_(0.31))Zn_(0.39)Al_(0.13)compound acts as the continuous inter-dendritic network,which contribute to the high mechanical performance and stability of the new die-cast Mg alloy at 200-300℃.
基金Project(RMEF157814X0004)supported by the Ministry of Education and Science of the Russian Federation
文摘The influence of annealing cycles up to 650 °C on the specific conductivity and hardness（HV） of hot-rolled sheets of Al alloys containing up to 0.5% Zr（mass fraction） was studied.Using analytical calculations of phase composition and experimental methods（scanning electron microscopy,transmission electron microscopy,electron microprobe analysis,etc）,it is demonstrated that the conductivity depends on the content of Zr in the Al solid solution which is the minimum after holding at 450 °C for 3 h.On the other hand,the hardness of the alloy is mainly caused by the amount of nanoparticles of the L12（Al3Zr） phase that defines the retention of strain hardening.It is shown that the best combination of electrical conductivity and hardness values can be reached within an acceptable holding time at the temperature about 450 °C.
基金supported by the Engineering and Physical Sciences Research Council(EPSRC) of the UK and the National Natural Science Foundation of China(Grant No.51104043)
文摘Intensive melt shearing has a significant grain refining effect on some light alloys.However,the persistence of the grain refining effect during isothermal holding and remelting is still unclear,although it is very important for the practical application.In this study,intensive melt shearing was achieved in a twin-screw mechanism to investigate its grain refining effect on AZ91D magnesium alloy.The refinement mechanism was discussed and the persistence of grain refinement after remelting and isothermal holding was also studied.A Zeiss imaging system with polarized light was used for quantitative measurement of grain size.The results show that the intensive melt shearing has a significant grain refining effect on AZ91D magnesium alloy.With the application of intensive melt shearing,the grain size of AZ91D magnesium alloy can be reduced from 530 μm(for a typical as-cast microstructure) to 170 μm,which is about 70% size reduction.The grain refinement achieved by the intensive melt shearing can be partially kept after isothermal holding and remelting.It is believed that the refinement effect was mainly due to the finer and well dispersed oxide particles formed by high intensive shearing.The smaller size of oxide particles and their slow motion velocity in the sheared melt could make important contributions to the remained grain refinement.
基金Financial supports from Innovate UK(grant number 11019)National Natural Science Foundation of China(grant number 51571133,51731007 and 52071189)。
文摘AlP has been widely used as an effective heterogenous nucleus for primary Si phase in hypereutectic AlSi alloys,but the morphological correlation between AlP and primary Si is still confusing.In the present work,the morphologies of AlP crystals were studied comprehensively by experimental observation and theorical prediction.It is found that AlP collected from an Al-0.03 P melt could be divided into two categories:spinel twin crystals and non-twin crystals.During the nucleation process,these two kinds of AlP crystals triggered morphologically templated nucleation of primary Si phase,resulting in the formation of hexagonal primary Si twin and octahedral non-twin crystals,respectively.As such,the percentage of primary Si twin crystals in the experimental Al-18 Si alloy was also increased obviously after the morphologically templated nucleation via Al P.The morphologically templated nucleation also eliminated the dendritic growth of primary Si phase and the formation of hopper structures inside primary Si,forcing primary Si to maintain to be faceted solid crystals through layer-by-layer growing mechanism.The insight into morphologically templated nucleation offers a new view in understanding the mechanism of heterogeneous nucleation of primary Si phase on AlP nuclei.
基金financially supported by the National Key Research and Development Program of China (No.2020YFB0311300ZL)the National Natural Science Foundation of China (No. 52071343)。
文摘A novel Al-5 Mg2 Si-2 Mg alloy was processed by selective laser melting(SLM) to understand its representative features of microstructural evolution and mechanical properties during additive manufacturing(AM). The as-SLM fabricated Al-5 Mg_(2) Si-2 Mg alloy is found to deliver much less hot cracks and other defects. Meanwhile, excellent mechanical property is also achieved, i.e. 452 ± 11 MPa for ultimate tensile strength, 295 ± 14 MPa for yield strength, and 9.3 ± 2.5% for elongation. Clearly, these mechanical properties are better than that obtained by high pressure die casting(HPDC), and better than some other alloys obtained by SLM. The as-SLM fabricated Al-5 Mg_(2) Si-2 Mg alloy is featured by the significantly refined microstructures in the compact primary α-Al, the divorced Mg_(2) Si eutectic networks distributing atα-Al grain boundaries, and some α-Al Fe Mn Si phase in association with eutectic Mg_(2) Si phase. The high strength and ductility of the alloy are attributed to its unique features including(a) the reduced solidification range,(b) the possible increase of the eutectic level in the microstructure, and(c) the shift of eutectic point and the maximum solubility point of Mg2 Si in Al matrix.
基金the National Key Research and Development Program of China(No.2020YFB0311300ZL)the National Natural Science Foundation of China(No.52071343)。
文摘The selective laser melting(SLM)with subsequent cold rolling and annealing is used to produce high-density lattice defects and grain refinement in the CoCrNi medium-entropy alloys(MEAs).The superior comprehensive mechanical properties have been achieved in the as-SLMed CoCrNi alloy after rolling and annealing.The as-SLMed alloys delivered the yield strength of 693.4 MPa,the ultimate tensile strength of 912.7 MPa and the fracture strain of 54.4%.After rolling with 70%reduction in thickness and annealing at 700℃for 2 h.the yield strength,ultimate tensile strength and fracture strain reached 1161.6 MPa,1390.8 MPa and 31.5%,respectively.The exceptional strength-ductility synergy is mainly attributed to the refined hierarchical microstructures with coarsening grains at a level of 30μm and ultrafine grains at a level of 1μm,and the heritage of dislocation-formed sub-grains and other lattice defects.This investigation demonstrates that the SLM with subsequent rolling and annealing is beneficial to fabricate high strength and ductile MEAs with single face-centered cubic(fcc)structure.
文摘Effect of Si addition on microstructure,hardness and wear resistance of as cast Mg-5Sn and Mg-10Sn alloys were investigated.For this purpose alloys with wt.-%0,0.5,1,1.5 Si were cast using an induction furnace.Addition of Si to both alloys caused an increasement in the amount of intermetallic phases at the grain boundaries.Also refined grain structure was reported with increasing Si content in the cast alloys.Both increased amount of intermetallics and decreased grain size caused an increase in the hardness of the alloys.Wear resistance of the binary alloys first increased with Si addition and then decreased.Increased amounts of Si in the alloys caused crack formation during wear tests and decreased wear resistance.Sliding speed also has a dramatical effect on the wear resistance of the alloys.Increased sliding speed resulted with higher wear resistance.Worn surfaces of the alloys also investigated under scanning electron microscope to understand the wear mechanisms operated during wear tests.
基金This work was financially supported by the National Key Research and Development Program of China(No.2020YFB0311300ZL)the National Natural Science Foundation of China(No.52071343).
文摘The coarsening-grained single-phase face-centered cubic(fcc)medium-entropy alloys(MEAs)normally exhibit insufficient strength for some engineering applications.Here,superior mechanical properties with ultimate tensile strength of 1.6 GPa and fracture strain of 13.1%at ambient temperature have been achieved in a(CoCrNi)_(94)Ti_(3)Al_(3)MEA by carefully architecting the multi-scale heterogeneous structures.Electron microscopy characterization indicates that the superior mechanical properties mainly originated from the favorable heterogeneous fcc matrix(1-40μm)and the coherent sphericalγ’precipitates(10-100 nm),together with a high number density of crystalline defects(2-10 nm),including dislocations,small stacking faults,Lomer-Cottrell locks,and ultrafine deformation twins.
基金financial support from the Constructed Project for Key Laboratory of Beijing,China(No.BJSJ2019004)the State Key Laboratory for Advanced Metals and Materials of China(No.2018Z-23)+2 种基金the Major State Research and Development Program of China(No.2016YFB0300801)the National Natural Science Foundation of China(No.51401016)the supports from International S&T Cooperation Projects of Nanjing,China(No.201818014)。
文摘The retrogression and re-aging(RRA)processes,aimed mainly at tailoring intergranular precipitates,could significantly improve the corrosion resistance(i.e.,stress corrosion cracking resistance)without considerably decreasing the strength,which signifies that an efficient control of the size,distribution and evolution of intergranular and intragranular precipitates becomes critical for the integrated properties of the(mid-)thick high-strength Al alloy plates.Compared to RRA process with retrogression at200℃ (T77),this study investigated the impact of a modified RRA process(MT77)with lower retrogression temperatures(155-175℃ )and first-stage under-aging on the properties of a high-strength AA7050 Al alloy,in combination with detailed precipitate characterization.The study showed that the strength/microhardness of the RRA-treated alloys decreased with raising retrogression temperature and/or prolonging retrogression time,along with the increased electrical conductivity.The rapid responsiveness of microstructure/property typical of retrogression at 200℃ was obviously postponed or decreased by using MT77 process with longer retrogression time that was more suitable for treating the(mid-)thick plates.On the other hand,higher retrogression temperature facilitated more intragranularηprecipitates,coarse intergranular precipitates and wide precipitate free zones,which prominently increased the electrical conductivity alongside a considerable strength loss as compared to the MT77-treated alloys.With the preferred MT77 process,the high strength approaching T6 level as well as good corrosion resistance was achieved.However,though a relatively homogeneous through-thickness strength was obtained,some small discrepancies of properties between the central and surface areas of an 86-mm thick 7050 Al alloy plate were observed,possibly related to the quenching sensitivity.The precipitate evolution and mechanistic connection to the properties were discussed and reviewed for high-strength Al alloys along with suggestions for further RRA optimization.
基金This work was financially supported by the National Natural Science Foundation of China(Nos.51404302 and 51801003)the Natural Science Foundation of Hunan Province(No.2020JJ4732).
文摘In this work,a process of cold rolling with 70%thickness reduction and different annealing temperatures was selected to regulate the microstructure of Ti-3wt%Cu alloy.Microstructural evolution,mechanical properties and antibacterial properties of the Ti-3wt%Cu alloy under different conditions were systematically investigated in terms of X-ray diffraction(XRD),scanning electron microscope(SEM),transmission electron microscope(TEM),tensile and antibacterial test.The results indicated that cold rolling could dramatically increase the ultimate tensile stress(UTS)from 520 to 928 MPa,but reduce the fracture strain from 15.3%to 3.8%.With the annealing temperature increasing from 400 to 800C for 1 h,the UTS decreased from 744 to 506 MPa and the fracture strain increased from12.7%to 24.4%.Moreover,the antibacterial properties of the Ti-3wt%Cu alloy under different conditions showed excellent antibacterial rate(>96.69%).The results also indicated that the excellent combination of strength and ductility of the Ti-3wt%Cu alloy with cold rolling and following annealing could be achieved in a trade-off by tuning the size and distribution of Ti2Cu phase,which could increase the applicability of the alloy in clinical practice.More importantly,the antibacterial properties maintained a good stability for the Ti-3wt%Cu alloy under different conditions.The excellent combination of mechanical properties and antibacterial properties could make the Ti-3wt%Cu alloy a good candidate for long-term orthopaedic implant application.
