Microstructure And Martensitic Transformation Behavior Of NiTiHfScNb High Temperature Shape Memory Alloy

NiTiHf based high temperature shape memory alloy has become one of the most valuable high temperature shape memory alloys due to its good shape memory effect and low cost.However,NiTiHf alloys have low superelasticity,poor cold working performance and poor cycle stability,which limit the engineering application of NiTiHf alloys.In this experiment,Ni_50.3Ti_28.7Hf_20.0Sc_1.0,which has good thermal cycling performance,was used as the matrix to investigate the effects of Nb content and heat treatment process on(Ni_50.3Ti_28.7Hf_20.0Sc_1.0)_100-xNb_x(x=0,5,10,15)alloy.The specific research contents include the observation of microstructure,the analysis of martensitic transformation behavior and the test of mechanical properties.At room temperature,the solid solution NiTiHfSc alloy is mainly composed of martensite matrix and Sc₂O₃ phase.After adding Nb element,the alloy forms?-Nb phase,which forms lamellar eutectic structure with the matrix.The results of TEM show that the martensitic variants of the alloy present(011)type I twin relationship,while the substructure of martensite is(001)composite twin.After aging at 550?,spindle like H phase precipitates in Ni_50.3Ti_28.7Hf_20.0Sc_1.0 alloy,which disperses in the martensite variant and a small amount of H phase segregates around the finer martensite variant.Compared with the solid solution alloy,the martensite morphology of(Ni_50.3Ti_28.7Hf_20.0Sc_1.0)₁₅Nb₅ alloy does not change greatly after aging at 550?.Compared with the solid solution alloy,the martensite morphology of(Ni_50.3Ti_28.7Hf_20.0Sc_1.0)₁₅Nb₅ alloy does not change greatly after aging at 550?,but the content of H phase is much lower than that of the alloy without Nb addition,which may be related to the?-Nb phase.The martensitic transformation temperature of the solid solution alloy increases first and then decreases with the increase of Nb content.The martensitic transformation temperature is the highest when the Nb content is 5 at.%.The phase transformation hysteresis of solid solution alloy increases with the increase of Nb content.With the increase of the number of cycles,the martensitic transformation temperature of the solid solution alloy decreases.After adding Nb,the thermal cycle stability of the alloy becomes worse,and the phase transformation hysteresis also decreases with the increase of the number of thermal cycles.Aging treatment can improve the transformation temperature of the alloy without addition Nb,but has little effect on Nb-containing alloys.With the change of aging temperature,the ability of Nb element to increase the transformation temperature decreases,and transformation temperatures even decrease after aging at 600?.After aging treatment,the phase transition hysteresis of the alloy decreases slightly,and the degree of decrease is related to the heat treatment process which is related to the heat treatment process.With the increase of the number of thermal cycles,the phase transformation temperature and the phase transformation hysteresis of the aged alloy decrease,and the decreasing extent increases after adding Nb,which indicates that the addition of Nb reduces the thermal cycle stability of the alloy.The elastic recovery ability of the solid solution alloy increases with the increase of Nb content,and its shape memory recovery ability does not change significantly.After aging treatment,the elastic recovery ability of the alloy increases,but the shape memory recovery ability decreases slightly.The microhardness of the solid solution alloy increases with the increase of Nb content.The microhardness of the alloy aged at 550?is slightly higher than that of the solid solution alloy,which may be related to the appearance of H phase.With the increase of Nb content,the microhardness of nb10 alloy increases first and then decreases,and the microhardness of nb10 alloy aged at 550?is the highest.