Phase transformation and viscoplasticity coupling in polycrystalline NiTiHf high-temperature shape memory alloys

Ni-Ti-Hf based alloys are becoming the choice of high-temperature shape memory alloys (HTSMAs) to be scaled for fabrication and used as actuators. They show potential as energy-dense actuators and have generated applications in rock splitters and torque tubes. This has been possible due to the extensive study done on their actuation life. However, the coupling between phase transformation and viscoplasticity, and its effect on the actuation life, is not examined yet in any Ni-Ti-Hf HTSMA. Therefore, investigating the coupling was the objective of the present study, in a Ti-rich Ni-Ti-20Hf (at.%) HTSMA. To investigate uniaxial constant force thermal cycling (UCFTC) tests were conducted (up till failure) at 1, 10, and 50 ^oC/min, to vary the amount of viscoplastic strain. These experimental results were used to investigate the evolution of transformation temperatures, hysteresis, and transformation strains with cycling. The macroscopic irrecoverable strains were dissociated into their viscoplastic and transformation-induced plasticity (TRIP) components. In addition, the contribution of retained martensite to TRIP was estimated. The results revealed each component to be rate-dependent. Furthermore, a test involving alternating isothermal creep + UCFTC at 10 ^oC/min was conducted to investigate the effect of viscoplasticity at a relatively fast rate. The test also revealed an effect of phase transformation over viscoplasticity. A two-way coupling was, therefore, established between the two phenomena bringing out the importance of understanding the effect of viscoplasticity and thermal cycling rate on phase-transformation.