Study On Microstructure And Properties Of Deformed Mg-Sm-Ca Alloy

Magnesium(Mg)alloys containing rare-earth(RE)elements have been used as structural materials for application in automotive,electrical and aerospace owing to their distinctive properties such as extraordinary low density,low elastic modulus,etc.However,low strength,poor thermal stability and high cost remain to be the restrictive factors for the development of Mg-RE alloys.Wrought Mg-RE alloys can achieve excellent mechanical properties which cannot be obtained from traditional coarse grain materials.However,there are few reports about the effect of HPT process and rolling process on the microstructure and mechanical properties of low-cost Mg-Sm-Ca alloy,especially in the aspects of aging behavior,thermal stability and strengthening mechanism of nanocrystalline Mg-Sm-Ca alloy.In this paper,the Mg-4.97Sm-0.84Ca(wt.%)alloy was subjected to deformation process and heat treatment,achieving nanocrystalline Mg-Sm-Ca alloy.The microstructure evolution and mechanical properties of the nanocrystalline Mg-Sm-Ca alloy were investigated,and the thermal stability and strengthening mechanism of the Mg-Sm-Ca alloy were revealed.Solution heat-treated Mg-Sm-Ca alloy at 530?for 8 h was processed by HPT for 4revolutions at room temperature.The hardness values of the center and edge position of the sample tend to be consistent,and the hardness value is about 109 HV.There are nano-sized grains with high-angle grain boundaries(GBs)in the microstructure of the alloy,and the average grain size is about 70 nm.The crystal lattice to be distorted and high-density dislocations to be formed in the alloy were attributed to the severe shear strain after HPT processing.The HPT-processed Mg-Sm-Ca alloy for 1 revolution at room temperature reached the peak hardness value of 134 HV at 175?for 1.5 h.The peak hardness value of HPT-processed Mg-Sm-Ca alloy for 4 revolution at room temperature can reach to 145 HV aging at 125?for 8 h.There are dense and fine second phase particles with an average size about 32 nm in the microstructure of the HPT-processed Mg-Sm-Ca alloy for 4 revolution at room temperature after annealing at 200?for 1 h,which plays a vital role in reducing GBs mobility.The thermal stability and the contribution of each strengthening mechanism of the Mg-Sm-Ca alloy was evaluated.At the same time,the activation energy of the Mg-Sm-Ca alloy grain growth was calculated.Three phases,Mg₄₁Sm₅ phase,Mg₃Sm phase and Mg₂Ca phase,appeared in the Mg-Sm-Ca alloy after rolling treatment.HPT deformation transforms Mg₃Sm phase into Mg₄₁Sm₅ phase and leads to the morphological change of Mg₄₁Sm₅ phase from rod-like to kink band-like.The ultra-high hardness of 175 HV can achieve for Mg-Sm-Ca alloys with composite deformed(rolling and HPT deformation)after aging at 150?for 3.5 h,which is mainly attributed to the effect of high density nano-precipitates with low lattice misfit.At the same time,the dense precipitates located in Mg-Sm-Ca alloy can resist dislocation motion effectively.