Microstructure and functional behaviour of shape memory annealed Ni24.7Ti50.3Pd25 and Ni24.7Ti49.3Pd25Sc1 alloys

Ni24.7Ti50.3Pd25 and Ni24.7Ti49.3Pd25Sc1 high-temperature shape memory alloys were thermo-mechanically processed and characterized in the present study. The microstructural characteristics of cold-worked alloys showed the presence of shear/deformation bands and defects. TEM observation of cold-worked and annealed (400 to 500 degrees C) microstructures revealed the emergence of small fraction of newly formed martensite twins at 400 degrees C. With the increase in the annealing temperature from 400 to 500 degrees C, the fraction of the martensite twins in the microstructure was found to increase. The Ni24.7Ti50.3Pd25 and Ni24.7Ti49.3Pd25Sc1 alloys showed martensite finish temperature of similar to 180 degrees C and similar to 136 degrees C with an hysteresis of similar to 10 degrees C and similar to 9 degrees C, respectively. Ni24.7Ti50.3Pd25 and Ni24.7Ti49.3Pd25Sc1 alloys showed a recovery strain of 1.5% and 1.7% at a stress of 175 MPa. During cycling under load, the residual strain for the Ni24.7Ti49.3Pd25Sc1 alloy was found to be negligible as compared to similar to 0.2% in the Ni24.7Ti50.3Pd25 alloy. The results of the study indicate that these alloys have potential for use as high temperature thermal actuator materials.