The Study On Deformation-induced α’martensite In Ti-30Zr-5Mo Alloy

Ti-30Zr-5Mo alloy is a novel metastable β-type titanium alloy,which exhibits deformation-induced α’ martensite transformation during deformation.The deformationinduced α’ martensite transformation greatly affects the deformation behavior,work hardening behavior and mechanical properties of Ti-30Zr-5Mo alloy.Understanding the influence of various factors on the deformation-induced α’ martensite transformation is beneficial to adjust the overall properties of Ti-30Zr-5Mo alloy.Therefore,this study explored the effects of strain rate,β-phase grain size and deformation temperature on the deformation-induced α’ martensite transformation,as well as the mechanical properties and work hardening behavior of the alloy.The results show that the Ti-30Zr-5Mo alloy after solution treatment is composed of the metastable β phase,trace adiabatic α’ martensite and adiabatic ω phase.During the tensile test at strain rate in Range I(3×10-5～3×10-1 s-1)and Range Ⅱ(101～103 s-1),Ti-30Zr-5Mo alloy exhibited deformation-induced α’ martensite transformation.The deformation-induced α’martensite transformation significantly improves the work hardening ability of the alloy,and the peak work hardening rate can reach 8.7 GPa.With the increase of strain rate,the trigger stress of the deformation-induced α’ martensite transformation increases,the amount of α’martensite phase transformation decreases,leading to the weaker work hardening ability of the alloy,in addition,the yield strength,tensile strength and elongation of the alloy fluctuate.In the deformed structure subjected to the 3×10-1 s-1 tensile test,the adiabatic ω phase disappeared,and {1011｝α’,internal twins were found in some α’ martensite.Deformation-induced α’ martensite transformation and its reverse phase transformation(SIMRT)was used to refine the grain size of Ti-30Zr-5Mo alloy.The samples with the grain sizes of 300,30,15 and 6μm were successfully prepared.The results show that the specimens of Ti-30Zr-5Mo alloy with different grain sizes exhibited deformation-induced α’ martensite transformation during cold rolling.The smaller the grain size,the lower the α’martensite phase content in the Ti-30Zr-5Mo alloy after cold rolling with the same reduction.When the grain size reduced to 15 μm and 6μm,the alloy exhibited the reverse transformation of deformationinduced α’ martensite transformation during cold rolling.During the tensile test,with the grain size decreases,the yield strength and tensile strength of the alloy increases monotonously,from 617 MPa to 838 MPa,and from 790 MPa to 988 MPa respectively,the elongation do not change significantly,and the trigger stress of deformation-induced α’ martensite transformation fluctuates between 211 MPa and 389 MPa.When the deformation temperature is 298,173 and 77K,Ti-30Zr-5Mo alloy exhibited deformation-induced α’ martensite transformation during tensile test.With the decrease of deformation temperature,the trigger stress of deformation-induced α’martensitic transformation in Ti-30Zr-5Mo alloy increases,the amount of α’ martensite phase transformation decreases,and the work hardening ability of the alloy due to the deformation-induced α’martensitic transformation diminishes.Compared with 298K and 173K,when the deformation temperature is 77K,the laths of α’martensite in the deformed structure are coarser and the density is lower.Otherwise,the closer to the fracture,the number of primary and secondary α’martensite increases,in addition,there is a broadening trend for these α’martensite laths.As the deformation temperature decreases,the yield strength and tensile strength of the alloy increase,from 617 MPa to 1145 MPa and from 790 MPa to 1418 MPa,respectively,and the elongation decreases from 23.8%to 9.6%.