Microstructure Evolution And Martensitic Transformation Behavior Of TiNiNb Alloy Processed By Equal Channel Angular Pressing

In present work,microstructure,martensitic transformation behavior and mechanical properties of Ti44Ni47Nb9 alloy processed by equal channel angular pressing(ECAP)with different processing parameters were studied by means of X-ray diffraction(XRD)analysis,scanning electron microscope(SEM),transmission electron microscope(TEM),microhardness testing,mechanical testing,differential scanning calorimetry(DSC)and mechanical testing.The influencing mechanism was analyzed.Microstructure of Ti44Ni47Nb9 alloys consists of B2 matrix and P-Nb phase at room temperature.ECAP processing can significantly influence the microstructure of as-ECAP processed alloys.After ECAP,the eutectic structure with grid shape of initial alloy is destroyed.With the increase of pass number,the distribution of β-Nb phase gradually changes to streamline features,and microstructure becomes fine and uniform.When route Bc and route BA are used,microstructure of matrix is characterized by equaxied grain.When route A is used,the grain of as-ECAP processed alloy is elongated.By using route BC,route A and route BA,respectively,the average grain size of Ti44Ni47Nb9 alloy is reduced to 295nm,279nm and 358nm after 8 passes,respectively.Martensitic transformation temperaturesare of as-ECAP processed alloy are significantly lower than those of the initial alloy.This can be attributed to grain refinement and the dislocations introduced during ECAP.As pass number increases,martensitic transformation temperatures first decrease sharply.After two passes,the tendency of decrease of transformation temperatures become ssaturated.The effect of pressing route on martensitic transformation temperature is negligible.With the increase of thermal cycling number,martensitic transformation temperatures decrease.The thermal cycling stability of martensitic transformation can be significantly improved by ECAP.After deformation at Ms+30℃,the reverse transformation temperatures can be significantly increased.This is related to the release of elastic strain energy and the cross of martensite interface.The transformation hysteresis of the as-ECAP processed alloy is larger than the initial counterpart and increases with increasing pre-strain.ECAP can significantly enhance the yield strength of Ti44Ni47Nb9 alloy.With the increase of pass number,the yield strength increase.After 8 passes through the route Bc,the alloy has good mechanical properties.The critical stress to induce martensitic transformation of Ti44Ni47Nb9 alloy with different states(σ)linearly increases with increasing deformation temperature.As compared to the initial alloy,the dσ/dT of as-ECAP processed alloys decreases.The value of dσ/dT is almost same between the samples processed by different routes.