Effects Of β Phase Stability On Microstructure Evolution,Mechanical Properties And Deformation Mechanisms Of Ti-3Al-5Mo-4.5V Alloy

In the present study,β phase stability alteration is achieved through quenching temperature variation,ranging from dual α+β field to single β field.Combining with optical microscope,x-ray diffraction,scanning electron microscope,electron probe micro-analysis,and transmission electron microscope,the effects of β phase stability on the microstructure evolution,mechanical properties and deformation mechanisms have been systematically investigated,which provides theoretical basis for future design of α+β titanium alloys.With the increase of quenching temperature,concentration of β-stabilizing Mo decreases from 8.727(wt.%)to 4.904 and V from 6.659 to 4.281,whereas the α-stabilizing Al increases from 2.043 to 2.857,which leads to varied β phase stability.The phase composition of transformed β phase is found to be greatly influenced by β phase stability.When quenched from dual α+β field(700℃,750℃,800℃)to single β field(880℃),phase composition of TC16 alloy are α+β,α+β+ω),α+β+ω+α" and α" respectively.The volume fraction of primary αdecrease from 51.6%to 0 rapidly with an increase of quenching temperature,meanwhile the morphology of primary α phase evolves from the mixture of needle-like and lath-like into globular shape.Detailed microstructure analysis shows that with decreasing β phase stability,athermal α" martensite was formed in the β matrix when quenching at 800℃ and 880℃,which has the following orientation relationships:[111]β//[110]α",[100]β//[100]α" and[-110]β//[00-1]α" with respect to the β matrix;The β→ω phase transformation does occur when quenching at 750℃ and 800℃.It is also demonstrated that co-reflections change from incommensurate ω-structures(750℃)to ideal co-spots(800℃)as the β-stability of the alloy decreases.Detailed discussion about the relationship between deformation mechanisms and β phase stability has been outlined.With an increase of quenching temperature,the β phase stability decreases and the deformation mode changes from stress induced martensite to slip.When quenched from 750℃,{332}<113>deformation twinning,{112}<111>deformation twinning and stress induced martensite are confirmed to be operative in the alloy with intermediate β phase stability.It is believed that the difference in mechanical properties is closely related to deformation mechanisms.Tensile tests performed at ambient temperature show that apparent yield strength of the alloy experiences an abrupt decrease followed by a significant increase.Work hardening behavior is characterized by transition from the initial two-regime feature to the three-stage outlook.Concurrently,the maximum working hardening rate drops from 14000 MPa to 3000 MPa,which is concurrent with the shrinking volume fraction of primary α phase.