Effect of slow shot speed on externally solidified crystal,porosity and tensile property in a newly developed high-pressure die-cast Al-Si alloy

The effect of slow shot speed on externally solidified crystal(ESC),porosity and tensile property in a newly developed high-pressure die-cast Al-Si alloy was investigated by optical microscopy(OM),scanning electron microscopy(SEM)and laboratory computed tomography(CT).Results showed that the newly developed AlSi9MnMoV alloy exhibited improved mechanical properties when compared to the AlSi10MnMg alloy.The AlSi9MnMoV alloy,which was designed with trace multicomponent additions,displays a notable grain refining effect in comparison to the AlSi10MnMg alloy.Refining elements Ti,Zr,V,Nb,B promote heterogeneous nucleation and reduce the grain size of primaryα-Al.At a lower slow shot speed,the large ESCs are easier to form and gather,developing into the dendrite net and net-shrinkage.With an increase in slow shot speed,the size and number of ESCs and porosities significantly reduce.In addition,the distribution of ESCs is more dispersed and the net-shrinkage disappears.The tensile property is greatly improved by adopting a higher slow shot speed.The ultimate tensile strength is enhanced from 260.31 MPa to 290.31 MPa(increased by 11.52%),and the elongation is enhanced from 3.72%to 6.34%(increased by 70.52%).

Effects of nanostructuring on mechanical and tribological behaviors of FeCoNi medium-entropy alloy

The effects of nanostructuring on the mechanical and dry-sliding wear behaviors of a FeCoNi medium-entropy alloy(MEA)were systematically investigated through nano-indentation and ball-on-disc wear tests.The results show that reducing the grain size down into the nano-meter regime,on the one hand,significantly elevates the hardness of the FeCoNi alloy,and on the other hand,facilitates the formation of a surface oxide layer.As a result,the wear rate of the nanocrystalline(NC)FeCoNi alloy is one order of magnitude lower than its coarse-grained counterpart.The NC FeCoNi alloy also exhibits obviously enhanced wear resistance compared with conventional NC Ni and Ni-based alloys in terms of both lower wear rate and friction coefficient.Such enhancement in tribological properties mainly stems from the improved strain hardening ability,owing to the inevitable concentration heterogeneity in MEA that imposes extra resistance to dislocation motion.