Comparison of the transformation behavior of cold rolling with aging and hot extrusion with aging processed Ni50.3Ti29.7Hf20 high temperature shape memory alloy

Among NiTiHf high temperature shape memory alloys, nickel rich ternary Ni50.3Ti29.7Hf20 (at%) alloy has been studied extensively and found to be promising for high temperature applications especially in aerospace industry. NiTiHf alloys have very high strength and transformation temperatures (TTs) proportional to their hafnium percentage. Therefore, these alloys are accepted as a hard to deform material. Hot extrusion at 900 °C and solutionizing at 1050 °C-1100°C treatments have been generally used in literature for the homogenization of the cast microstructure and chemistry and thus, the strength of the material eventually increases. In this study, one set of the as cast Ni50.3Ti29.7Hf20 alloy was hot extruded at 900 °C and then aged at 550 °C for 3 h and the other set was solutionized at 1050 °C for 2 h, cold rolled for 10% at room temperature and then aged at 550 °C for 3 h. The TTs of the hot extruded, hot extruded and aged, solutionized, solutionized-cold rolled and aged samples were measured using DSC and heating-cooling experiments under increasing stress magnitudes starting from 100 MPa and increasing up to 600 MPa were conducted on all thermal and thermo-mechanically treated samples in order to compare the shape memory characteristics such as actuation strain, irrecoverable strain and TTs. The samples which were hot extruded and aged and solutionized-cold rolled and aged showed very high dimensional stability with no irrecoverable strain values up to 500 MPa. However, the actuation strain magnitudes of the cold rolled with aging processed sample were half of the actuation strain values of the sample which was hot extruded and aging treated. Additionally, higher undercooling and overheating were necessary to achieve full transformation in the cold worked samples. These results might be due to the very high dislocation density and texture formation which was induced during cold rolling process.