Influence of Deep Cryogenic Treatment on the Pseudoelastic Behavior of the Ni₅₇Ti₄₃ Alloy

The Ni57Ti43 alloy is a strategy material mainly due to its pseudoelasticity and shape memory properties. In this work, the influence of deep cryogenic treatment on the pseudoelastic behavior of Ni57Ti43 alloy under cyclic thermomechanical loading is investigated. The samples with a uniform gauge section format were initially heat treated by annealing at 500 °C for 10 min. Furthermore, a group of them were subsequently cryogenically treated by immersion in liquid nitrogen (approx. -196 °C), for 12 h. All samples was submitted to uniaxial cyclic tensile test at room temperature, with controlled applied force to result in a stress of 500 MPa and 750 MPa under the frequency of 0.5 Hz, until the stabilization of the stress–strain curve was reached. A reduction in phase transformation start stresses was observed around 17% for direct transformation (austenite to martensite) and 35% for inverse transformation (martensite to austenite). A reduction was observed both in the maximum recoverable deformation and the residual deformation, estimated to be around 40% and 45% lower, respectively. Finally, a decrease of 83.4% in damping was identified. The microstructure analysis showed that the non-treated samples accumulated more martensite than cryogenically treated ones when subjected to cyclic loading.