Experimental and microstructural characterization of simultaneous creep, plasticity and phase transformation in Ti50Pd40Ni10Ti50Pd40Ni10 high-temperature shape memory alloy

The influence of viscoplastic phenomena on the transformation behavior of a Ti50Pd40Ni10Ti50Pd40Ni10 high-temperature shape memory alloy (HTSMA) is investigated by thermomechanical testing followed by transmission electron microscopy (TEM). An ingot of the alloy is fabricated by vacuum arc melting and processed by hot rolling. Cylindrical specimens for compression testing are cut from the rolled bar and standard creep tests and thermally induced phase transformation tests are conducted using a custom high-temperature setup. Thin sections are prepared from the specimens before and after the thermomechanical tests and their microstructure is studied using TEM. The thermomechanical test results show that irrecoverable strains generated due to creep and transformation-induced plasticity impact the actuation performance of the HTSMA. The results from the microstructural study indicate different microstructural changes due to the two irrecoverable strains (plastic and viscoplastic) generated during the course of transformation. Both these mechanisms in turn affect the actuation behavior in different ways. The results from the thermomechanical testing and the microstructural observations indicate that the irrecoverable strain generated due to creep does not seem to affect the transformation behavior or the rate-independent irrecoverable strain (generated in part due to retained martensite).