Martensite Transformation And Strain Recovery Characteristics Of Constrained Aged Ni-Ti-Hf High Temperature Shape Memory Alloys

In this paper,constraint aged was used to control the microstructure of Ni-rich Ni-Ti-Hf alloy and improved its phase transformation behavior and shape memory effect.Using differential scanning calorimeter,X-ray diffractometer,transmission electron microscope,dynamic thermomechanical analysis and other means to systematically analyzed the influence of constraint aged on the phase transformation behavior,microstructure and shape memory properties of Ni-Ti-Hf alloy.Studies had shown that constraint aged would cause the precipitation of H phase in Ni-Ti-Hf alloy.With the increase of constraint temperature or constraint time,the size of H phase became larger,and the shape of precipitated phase grew from granular to elliptical and the amount of second phase increased.In addition,constraint aged made the second phase in the alloy preferentially arranged along the constraint direction,and the H phase was only in one direction.The phase composition of the Ni-Ti-Hf alloy after constraint aged at room temperature was B19'martensite phase at room temperature.With the increase of the constraint time,the morphology of martensite gradually changed from the spear-like and mosaic-like morphologies to belt-like morphology.The martensite variants were(011)type?twin related.Comparing the DSC curves of Ni-Ti-Hf alloys with different compositions,it could be seen that constraint aged had no obvious effect on the order of phase transformation of the alloy,all of which were B2?B19'one-step martensitic transformation.When the constraint stress and aged time were constant,the phase transformation temperature increased with the increase of the constraint temperature.When the aged temperature is 550?,the phase transformation temperature of the alloy increased with the increase of the constraint time and constraint stress.Therefore,the phase transformation temperature of the alloy can be effectively controlled by adjusting the constraint process.By adjusting the constraint stress,aged temperature and aged time in the constraint aged process,the precipitate in the constraint aged alloy was oriented,which could significantly improve the shape memory effect of Ni-Ti-Hf alloy.For Ni_50.4Ti_34.6Hf₁₅,when the constraint temperature was 550?/100MPa/2h,the recoverable strain was 7.5%;for Ni_50.5Ti_34.5Hf₁₅,when the constraint temperature was 450?/200MPa/2h,.the recoverable strain was 4.6%.