Enhanced mechanical properties of Mg-1Al-12Y alloy containing long period stacking ordered phase

In this study, a systematic investigation on the effect of solution treatment time(2–8 h) at 540℃ on the microstructure and mechanical properties in as-cast Mg-1Al-12Y(AY112, wt.%) alloy was performed. The results showed that the solution treatment did not cause the growth of grains and the change of texture;however, the mechanical properties had been significantly improved, which was mainly attributed to the precipitation of 18R long period ordered stacking(LPSO) phase in the solution-treated alloys. In addition, the dissolution of β-Mg_(24)Y_(5)phase and the diffusion of solute atoms during the solution treatment were both beneficial to the mechanical properties. When the as-cast alloy was solution-treated at 540℃ for 4 h(T4-4h alloy), the mechanical properties of the alloy are optimal. Compared with the as-cast alloy,the ultimate tensile strength(UTS) and elongation of the T4-4h alloy are increased by ~23% and ~179%, respectively. The deformation of the T4-4h alloys was dominated by a combination of basal slip and non-basal slip, and the presence of the LPSO phase effectively inhibited the nucleation of extension twin. Besides, the LPSO phase can also hinder the activation of basal dislocations and the movement of non-basal dislocations. Therefore, the LPSO phase simultaneously improves the strength and plasticity of the alloy.

Reducing the yield asymmetry in Mg-5Li-3Al-2Zn alloy by hot-extrusion and multi-pass rolling

Reducing the yield asymmetry is very important concern for wrought Mg-Li alloys.In this study,Mg-5Li-3Al-2 Zn(LAZ532)alloy was successfully produced by hot-extrusion followed by multi-pass rolling at 573 K.Microstructure evolution,mechanical properties and yield asymmetry reducing of LAZ532 alloys at different rolling passes were studied.By observing microstructure using transmission electron microscopy showed that a small amount of ultra-fine Al Mg_(4)Zn_(11)and nano Li_(3)Al_(2) phases existed in the alloy.With the increasing of rolling passes,the grains of the alloys were obviously refined,and comprehensive mechanical properties were dramatically improved.Meanwhile,it also showed an excellent tension and compression yield symmetry(TYS/CYS was about 1).The results showed that the combined action of the weak{0001}basal lamellar texture,grain refinement and addition of Li element could effectively improve the yield symmetry.Furthermore,based on theoretical analysis,the yield strength in the alloys mainly depended on the strengthening contributions of LAGBs and HAGBs,and strengthening effect of HAGBs most(~50%)to the yield strength improvement.

Influence of nano-diamond content on the microstructure,mechanical and thermal properties of the ZK60 composites

The effect of nano diamond(ND)content on the microstructure,mechanical properties,and thermal conductivity of ZK60+x(x=0,0.05,0.1,0.15,0.2 wt.%)ND composites were investigated.The microstructures of ND/ZK60 composites were observed,which indicated that the nanoscale MgZn_(2) and ND particles distributed evenly in theα-Mg matrix.The tensile yield strength(TYS)and compressive yield strength(CYS)of the composites first increased remarkably and then decreased with further increasing the ND content.Due to the surface area of the matrix-diamond interface increased and the grains sizes of composites decreased with the amount of ND increase,which cause the coefficient of thermal expansion(CTE)of the composites reduced significantly.Meanwhile,the thermal conductivity of the composite material decreases from 129 W·m^(−1)·K^(−1) to 116 W·m^(−1)·K^(−1) with the content of ND increasing from 0.05%to 2.0%.The thermal conductivity of the composites increases to the maximum and then decrease with the increase of temperature(in temperature range of 273-573 K).The ZK60+0.05 ND showed superior mechanical and thermal conductivity property,TYS of 343.97 MPa,CYS of 341.74 MPa,elongation of 15.71%,CTE of 7.3×10^(−6)K^(−1),and thermal conductivity of 129 W·m^(−1)·K^(−1) at room temperature.It is demonstrated that the ND content has an obvious influence on the microstructure,mechanical properties,and thermal conductivity of ND/ZK60 composites.

Twin recrystallization mechanisms in a high strain rate compressed Mg-Zn alloy

Static recrystallization of a high strain rate compressed Mg-1 Zn(wt.%)alloy was investigated using electron backscattered diffraction(EBSD).A novel 53°1010 structure was observed in the as-deformed alloy,which showed a{1012}-{1012}double twin relationship with the matrix.When the as-deformed alloy was annealed at 200°C,the{1011}compression twins and{1011}-{1012}double twins showed a higher priority to recrystallize.In addition,the coarse{1012}tension twins and their inner double twins were preferentially to recrystallize,while the lenticular tension twins had little impact on the recrystallization.Therefore,obtaining more compression twins or coarse twins instead of lenticular tension twins can be an effective approach to manipulate recrystallization process in deformed Mg alloys.

Hot Deformation Behavior and Microstructures Evolution of GNP-Reinforced Fine-Grained Mg Composites

Graphene nanoplates(GNPs)-reinforced magnesium matrix composites have been attracted great attention.However,knowledge is lack for the hot deformation behavior of GNP-reinforced magnesium(GNPs/Mg)composite.In this study,the fine-grained GNPs/Mg composite was fabricated by powder metallurgy process followed by extrusion.The hot deformation behavior,microstructure evolution and dynamic recrystallization(DRX)mechanism of fine-grained GNPs/Mg composite were investigated by hot compression test and electron back-scatter diffraction(EBSD).The hot compression tests of the composite were conducted at temperatures between 423 and 573 K with the strain rates from 0.001 to 1 s^(-1).The strain compensated power law equation was established to describe the hot deformation behavior of the composites.The stress exponent and activation energy of the composite are 7.76 and 83.23 kJ/mol,respectively,suggesting that the deformation mechanism is grain boundary slip controlled dislocation climb creep.The abnormally high stress exponent and activation energy are unattainable in the composite due to the fine grain size of the composites and the absence of Zener pinning and Orowan effects of GNPs reinforcement.The grain size increases with the decrease in Zener-Hollomn(Z)parameter,which can be well fitted by power-law relationship.With the increase in grain size and decrease in Z parameter,the geometrically necessary dislocation density decreases,which shows the approximately power-law relationship.A random and weak texture was formed after hot compression.The discontinuous dynamic recrystallization and continuous dynamic recrystallization mechanism dominated the DRX behavior at 473 K/0.001 s^(-1) and 573 K/0.001 s^(-1),respectively.

Precious trimetallic single-cluster catalysts for oxygen and hydrogen electrocatalytic reactions:Theoretical considerations

Single cluster catalysts(SCCs),which exhibit remarkable catalytic performance due to their high metal loading and synergy effect between metal atoms,have attracted great attention in research.Herein,by means of density functional theory calculations,the oxygen reduction reaction(ORR),oxygen evolution reaction(OER),hydrogen evolution reaction(HER)performances of precious metal(Pt,Pd,Rh,Ir)trimetallic single-cluster electrocatalyst(U_(x)V_(y)W_(z)-NG)are investigated.The calculation results show that Pt,Pd,Ir have significant effect on ORR,OER,HER,respectively,all the calculated U_(x)V_(y)W_(z)-NG structures are thermodynamically stable due to the negative formation energies and binding energies.The Pt_(3)-NG,Pd_(3)-NG,Ir_(3)-NG show the lowest ORR,OER,HER overpotentials of 0.63,0.77,−0.02 V,respectively,among all combinations of U_(x)V_(y)W_(z)-NG.These overpotentials are lower than that of precious metal single atom catalysts(SACs),which indicate better activities of precious trimetallic SCCs than those of SACs.The electronic structure reveals that the O-2p orbital shows strong hybridization strength with Pt-3d orbitals in the system of OH adsorbed Pt_(3)-NG and thus facilitates the electrocatalytic reactions.The results are helpful for the rational design of high-performance triatomic catalysts.

Effect of long-period ordered stacking on twinning behavior and mechanical properties of Mg-Al-Y alloy during uniaxial hot compression

Two alloys with(T4-4h)and without(T4-8h)long-period stacking ordered(LPSO)phase were obtained by solution treatment of as-cast Mg-1Al-12Y alloy at 540℃ for 4 h and 8 h,respectively.The compressive tests(300-450℃,ε=0.01s^(-1))showed that the strengthening effect of LPSO in T4-4h alloy gradually disappeared with temperature increase.In addition,the corresponding compression deformation behavior of the two alloys was also investigated by electron backscatter diffraction(EBSD).It was found that the twinning of the two alloys was dominated by{1012}extension twins,and the large amount of lamellar LPSO in T4-4 h alloy inhibited the activation and growth of twins.The[0001]//compression direction(CD)texture formed after deformation is mostly attributed to the activation and growth of extension twins.By analyzing the activated twin variants,it was found that the activation of twin variants mainly depended on Schmid factor(SF).For twin variants with smaller SF,they may be activated due to local stress concentration.Based on the analysis results of in-grain misorientation axis(IGMA)and SF,the deformation mechanism of the alloys at elevated temperature is dominated by basal slip and non-basal slip.The stronger mechanical properties of T4-4h alloy than those of T4-8h are attributed to the obstruction of non-basal slip movement by LPSO.

Investigation of Portevin–Le Chatelier effect in rolled α-phase Mg-Li alloy during tensile and compressive deformation

Avoiding the Portevin-Le Chatelier(PLC)effect is very important concern for wrought Mg-Li alloys.In this study,the special PLC effect was found in rolled Mg-5Li-3Al-2Zn(LAZ532)alloy during tensile and compressive deformation.By observing microstructure evolution of the alloy during tensile and compressive deformation,it was found that prismaticand pyramidalslips were activated during tensile deformation,resulting in plenty of dislocation accumulation.In the deformation process after compressive yielding,the deformations in coarse grains and fine grains were dominated by{1012}extension twinning and grain boundary slip,respectively.Based on experimental result analysis,the sudden appearance of PLC effect in the later stage of axial tensile deformation(along rolled direction)was caused by interaction between solute atoms and dislocations.In the process of axial compressive deformation(along rolled direction),PLC effect presented the complex and changeable phenomenon of appeared-disappeared-appeared,which was mainly caused by the continuous nucleation of twin in the material,the activation of grain boundary slip and the shear deformation of twin,respectively.

Hot Deformation Behavior and Workability of(SiC_p + Mg_2B_2O_(5w))/6061 Al Hybrid and SiC_p/6061 Al Composites

The hot deformation behavior of （3 vol% SiCp -4- 3 vol% Mg2B2Osw）/6061 A1 （W3P3） hybrid composite and 6 vol% SiCp/6061 A1 （P6） composite have been characterized in the temperature range of 300--450 ℃ and strain rate range of 0.0001-0.1 s-1 using isothermal constant true strain rate tests. The flow behavior and processing maps have been investigated using the corrected data to eliminate the effect of friction. Under the same deformation conditions, the compressive resistance of the singular composite remains superior to that of the hybrid composites. The processing map of W3P3 hybrid composite exhibits a single hot working domain at the temperature between 350 and 450 ℃ with strain rate between 0.0001 and 0.003 s-1 （domain A）. Two hot working domains exist for P6 composite： （i） 300-400 ℃/ 0.0001-0.003 s-1 （domain B1）; （ii） 380-450 ℃/0.01-0.1 s-1 （domain B2）. The processing maps also reveal the flow instability of the two composites, which is associated with whiskers breakage, whisker/matrix interfacial debonding, SiCp/ matrix interfacial decohesion, adiabatic shear bands or flow localization, and wedge cracking in the corresponding regions. The estimated apparent activation energies are about 224 kJ mo1-1 in domain A for W3P3 hybrid composite, 177 kJ mo1-1 in domain B1 and 263 kJ mo1-1 in domain B2 for P6 composite, respectively. These values are higher than that for self-diffusion in A1 （142 kJ mol-1）, suggesting that there is a significant contribution from the back stress caused by the presence of particles and/or whiskers in the matrix. The deformation mechanisms corresponding to domain B 1 and domain B2 are dislocation climb controlled creep and cross-slip for P6 composite, respectively. For W3P3 hybrid composite, the deformation mechanisms contain dislocation climb controlled creep and grain boundary sliding caused by DRX in domain A.

The effect of basal dislocation on {11■1} twin boundary evolution in a Mg-Gd-Y-Zr alloy

This study aims to understand the features of{11■1}twin boundaries in a high strain rate compressed Mg-10 Gd-3Y-0.4Zr using electron backscatter diffraction(EBSD)and transmission electron microscopy(TEM).Based on the Schmid factor calculations,the basal dislocations are easily activated within the{11■1}twin and suppressed in the surrounding matrix.The consequent basal dislocation-twin interaction during deformation can lead to the misorentiation deviation of{11■1}twin boundaries,accompanying with the formation of a step at the twin boundary.Such a{11■1}twin boundary evolution mechanism provides a new vision of twinning behavior in Mg alloys.

Dislocation behavior in a polycrystalline Mg-Y alloy using multi-scale characterization and VPSC simulation

In this study,the dislocation behavior of a polycrystalline Mg-5Y alloy during tensile deformation was quantitatively studied by an in-situ tensile test,visco-plastic self-consistent(VPSC)modeling,and transmission electron microscopy(TEM).The results of the in-situ tensile test show that<a>dislocations contribute to most of the deformation,while a small fraction of<c+a>dislocations are also activated near grain boundaries(GBs).The critical resolved shear stresses(CRSSs)of different dislocation slip systems were estimated.The CRSS ratio between prismatic and basal<a>dislocation slip in the Mg-Y alloy(~13)is lower than that of pure Mg(~80),which is considered as a major reason for the high ductility of the alloy.TEM study shows that the<c+a>dislocations in the alloy have high mobility,which also helps to accommodate the deformation near GBs.