Microstructure And Transformation Behavior Of Aged TiNiNb Shape Memory Alloys

TiNiNb alloys which is from addition of Nb element to the Ni rich TiNi memory alloys have the advantages of excellent shape memory effect,good ductility and low phase transformation temperature.So,TiNiNb alloys has wide application prospects in industry.In present work,the effects of aging treatment on the microstructure,transformation behavior and mechanical behavior of Ti₄₈Ni₅₂?(Ti₄₈Ni₅₂)₉₄Nb₆ and(Ti₄₈Ni₅₂)₉₀Nb₁₀ alloys were studied by means of optical metallographic microscope,X-ray diffraction,scanning electron microscope,transmission electron microscope,differential scanning calorimetry,tensile deformation tests and bending tests.The influencing mechanism was analyzed.At room temperature,the solution-treated Ti₄₈Ni₅₂ alloy consists of B2 parent phase?A?and second phase Ti₂Ni.Ti₃Ni₄,Ti₃Ni₂ and Ti₃Ni precipitates formed after aging treatment between 350?⁴50?.The solution-treated(Ti₄₈Ni₅₂)₉₄Nb₆ and(Ti₄₈Ni₅₂)₉₀Nb₁₀ alloys consist of B2 parent phase,?-Nb phase and?Ti,Nb?₂Ni phase.Nanoscale precipitates formed in these alloys after aging treatment,which were coherent with parent phaes.With the aging temperature increasing,the nanoscale precipitates became larger in size.When test temperature range was-120?⁷0?,martensitic transformation was not observed in the solution-treated Ti₄₈Ni₅₂ alloy.After Ti₄₈Ni₅₂ alloy was aged at different temperatures for 3h,during the cooling process,the A?R?R phase?transformation was observed in the samples aged at 350?and 400?;the A?R?M?martensite?transformation occured with the aging temperature increasing;the A?M transformation was observed when the aging temperature was higher than 500?.During the heating process,the R?A transformation was observed in the samples aged at 350?;the M?R?A transformation occured with the aging temperature increasing;when the aging temperature was higher than450?,the M?A transformation was observed.The martensitic transformation temperature firstly decreased and then increased with the aging temperature increasing.After Ti₄₈Ni₅₂ alloy was aged at 450?for different time,during the cooling process,the A?R transformation was observed in the samples aged for 0.5 h;with the aging time increasing,the two-step transformation A?R?M occurred;when the aging time was longer than 3h,the A?M transformation was observed.During the heating process,the R?A transformation was observed in the samples aged for 0.5 h;when the aging time was longer than 0.5h,the M?R?A transformation occured.The martensitic transformation temperature firstly increased and then decreased with the aging time increasing.The phase transformation behavior of the aged Ti₄₈Ni₅₂ alloy is closely related to Ti₃Ni₄ phase.The A?M transformation in the solution-treated and aged(Ti₄₈Ni₅₂)₉₄Nb₆ and(Ti₄₈Ni₅₂)₉₀Nb₁₀ alloys during cooling and heating process was observed.The martensitic transformation temperature firstly increased and then decreased with the aging temperature or time increasing.The phase transformation temperature of the two aged alloys is closely related to the nanoscale precipitates.The martensitic transformation temperature of(Ti₄₈Ni₅₂)₉₀Nb₁₀alloy is lower than that of(Ti₄₈Ni₅₂)₉₄Nb₆ alloy after the same aging treatment process.The aging treatment showed a significant effect on the shape memory effect of(Ti₄₈Ni₅₂)₉₄Nb₆ and(Ti₄₈Ni₅₂)₉₀Nb₁₀ alloys.When deformation was less than 3%,the shape recovery rate of alloys was about 100%.The shape recovery rate decreased with the deformation or aging temperature or content of Nb increasing.