Ultra-high fatigue life of NiTi cylinders for compression-based elastocaloric cooling

Abstract

We investigated the fatigue behavior of superelastic polycrystalline NiTi cylinders during reversible partial transformation under cyclic adiabatic compression at 20 Hz with a maximum stress of 800 MPa. A fatigue life of ∼70 million cycles is recorded for the cylinder with a reliable load carrying capacity until ultimate failure. Surface morphology and fractograph observations show that such an ultrahigh fatigue life is realized by the extremely low average fatigue crack growth rates (0.15 nm/cycle for the longitudinal main crack and 0.03 nm/cycle for the transverse main crack) under cyclic compression. The intersection of the longitudinal and transverse cracks causes a fatigue failure mode of spalling. The cylinders under adiabatic partial transformation with a temperature drop of 9.9 K have a coefficient of performance of the material (COPmat) of 18.8 which is about 78% of the Carnot cycle COP. The extraordinary fatigue and cooling performances of the superelastic NiTi cylinder pave the way for future compression-based solid-state green refrigeration.