Role of applied stress level on the actuation fatigue behavior of NiTiHf high temperature shape memory alloys

This study presents the actuation fatigue response of nano-precipitation hardened Ni_50.3Ti_29.7Hf₂₀ high temperature shape memory alloy (HTSMA) undergoing thermal cycling between martensite and austenite under various tensile stress levels up to 500 MPa. Changes in fatigue life, and actuation and irrecoverable strains were monitored as a function of the number of cycles to failure. The experimental results revealed a consistent increase in actuation strain concomitant with the applied load at the expense of fatigue life. Significantly high number of cycles to failure were observed for this class of materials: specimens tested under 200 MPa achieved ∼21,000 cycles with the average actuation strain of ∼2.15% while those tested under 500 MPa experienced ∼2,100 cycles to failure with the average actuation strain of 3.22%. Fracture surface and crack density analyses revealed notable crack formation prior to failure at all stress levels. However, the rate of crack formation during repeated transformation increased with the applied stress. It was also demonstrated that the actuation fatigue lives of the present HTSMAs exhibit an almost perfect power law correlation with average actuation work output. The same work-based power law was shown to successfully capture actuation fatigue lives of several low temperature SMAs. Remarkably, the power law exponents for many SMAs were shown here to be either ∼ −0.5 or ∼ −0.8, which points out the likelihood of the existence of a universal empirical rule for actuation fatigue response of SMAs. The current findings constitute the first report on the applied stress - actuation fatigue interrelationship in NiTiHf HTSMAs.