Microstructure And Mechanical Properties Of Ti_p/Mg-6Zn-0.5Ca Magnesium Matrix Composite After Hot Deformation

In this paper,Mg-6Zn-0.5Ca（ZX60）alloy was used as the matrix,and micro-sized pure Ti particles was used as the reinforcement.Ti_p/Mg-6Zn-0.5Ca magnesium matrix composites with different particle sizes（20,60μm）and particle contents（0,5,10,15 vol.%）were prepared by semi-solid stirring casting process and followed by hot extrusion.The effects of deformable Ti_p on the microstructure and properties of as-cast and as-extruded ZX60 magnesium matrix was systematically studied.The results show that the microstructure of as-cast ZX60 alloy composed of coarse grains and a large amount of Ca₂Mg₆Zn₃ phases distribute in a semi-continuous network along the grain boundaries.After adding 20μm 5 vol.%Ti_p,grain size is significantly reduced.Ca₂Mg₆Zn₃ phases nucleate preferentially around Ti_p,and the morphology change to flocculent.With the increase of Ti_p content,grain size decreases gradually,and the number of Ca₂Mg₆Zn₃phases increases.When the particle content is kept at 10 vol.%and the particle size increases from 20μm to 60μm,the heterogeneous nucleation sites provided by Ti_p decreases,then grain size increases.The YS and UTS of Ti_p/ZX60 composites increases gradually with increasing particle content and decreasing particle size.The comprehensive mechanical properties of 20μm 10 vol.%Ti_p/ZX60 composite are the best,with YS,UTS and EL of～122.6 MPa,～234.1MP and～3.5%,respectively.After hot extrusion,Ti_p in 20μm 10 vol.%Ti_p/ZX60 composite is plastically deformed along the extrusion direction,and the average aspect ratio increases significantly.Dynamic recrystallization（DRX）grains occur in the matrix of Ti_p/ZX60 composite,and grain size is significantly reduced.Besides,lots of Mg Zn₂phases precipitate in DRX region.With the increase of extrusion temperature,the aspect ratio of Ti_p decreases,and the volume fraction and grain size of DRX increase.The size of Mg Zn₂ precipitates increases,but the volume fraction decreases.When the particle size remains 20μm and the particle content increases from 0 vol.%to 15 vol%,grain size of Ti_p/ZX60 composites decreases,and DRX volume fraction increases;the volume fraction of Mg Zn₂ precipitates increases at first,then decreases.When particle size increases from 20μm to 60μm,the aspect ratio of Ti_p increased greatly,and the coordinated deformation ability between the particles and the matrix is enhanced.Moreover,with the increase of particle size,grain size increases,the volume fraction of DRX and the volume fraction of Mg Zn₂ precipitates both decreases.Compared with ZX60 alloy,YS and UTS of as-extruded Ti_p/ZX60 composites are improved.After being extruding at 240℃,20μm 10 vol.%Ti_p/ZX60 composite has a higher strengthening and toughening efficiency,and its YS,UTS and EL are～401.2 MPa,～437.4 MPa and～7.6%,respectively.The strengthening mechanisms of Ti_p/ZX60 composites include:grain boundary strengthening,load transfer effect,thermal mismatch strengthening and Orowan strengthening.Among them,the effect of grain refinement strengthening is the most significant,followed by load transfer.Based on tensile tests and cyclic stress relaxation tests,the strain hardening behavior and softening behavior of ZX60 alloy,20μm 10 vol.%Ti_p/ZX60 and 60μm 10 vol.%Ti_p/ZX60composites are investigated,respectively.The results show that the deformable Ti_p leads to an increment in the strain hardening rate and strain hardening index of the material,which is due to that fine grains and precipitates,as well as high dislocation density in as-extruded Ti_p/ZX60composite can hinder the movement of dislocations and promote the multiplication of dislocations.At the late stage of tensile tests,Ti_p promotes the cross-slip of dislocations,so it is easier for Ti_p/ZX60 composite to soften.The stress relaxation curves are fitted by the stress relaxation dynamic model.The activation volumes of ZX60 alloy and Ti_p/ZX60 composites decrease with the increase of effective stress,and their value are varied between 30～600b³.The softening behavior during stress relaxation is dominated by dislocation motion.At the initial stage of stress relaxation,the activation volume of 20μm 10 vol.%Ti_p/ZX60 composite is the smallest,ranging from 25～35b³.Ti_p reduces activation volumes of ZX60 matrix and promotes stress relaxation of Ti_p/ZX60 composite.It is due to the small grain size of the Ti_p/ZX60composite,which promotes the recovery of dislocations.Besides,the plastic deformation of Ti_prelieves the stress concentration in the composite,which contributes to soften during stress relaxation tests.