Phase transformation and viscoplasticity coupling in polycrystalline nickel-titanium-hafnium high-temperature shape memory alloys

The objective of this study was to investigate the interactions between phase transformation and viscoplasticity during uniaxial constant force thermal cycling (UCFTC) of a Ti-rich Ni-Ti-20Hf (at.%) high-temperature shape memory alloy (HTSMA). These tests were conducted (up till failure) at 1, 10 and 50 ${}^{\circ }\text{C}/\text{min}$, to vary the duration of exposure to high temperatures, viz. the amount of viscoplasticity, and to examine the rate-dependency of actuation. The macroscopic results from the tests were used to investigate the evolution of transformation temperatures, hysteresis, transformation and irrecoverable strains for the cycles in which the effect of potential damage mechanisms could be assumed to be negligible. The phenomena that affected the behavior were: viscoplasticity at 1 ${}^{\circ }\text{C}/\text{min}$, transformation-induced plasticity (TRIP) at 10 and 50 ${}^{\circ }\text{C}/\text{min}$, and accumulation of retained martensite at all the three rates. More interestingly, the response at 1${}^{\circ }\text{C}/\text{min}$ indicated a unique interplay between the effect of viscoplasticity over phase transformation and static recovery. The retained martensite was identified through a series of DSC and XRD analyses, and its contribution to TRIP strain was quantified through a UCFTC test. Furthermore, a test involving alternating isothermal creep and UCFTC at 10 ${}^{\circ }\text{C}/\text{min}$ was conducted to investigate an effect of viscoplasticity produced by creep on the behavior, while reducing the viscoplasticity during thermal cycling. The alternating test revealed an effect of phase transformation over the viscoplastic strain rate. The experimental investigations demonstrated a rate-dependent phase transformation behavior, and a two-way coupling between phase transformation and viscoplasticity, bringing out the importance of understanding viscoplastic deformations in phase-transforming materials.