Large superelastic recovery and elastocaloric effect in as-deposited additive manufactured Ni50.8Ti49.2 alloy

Abstract

A Ni50.8Ti49.2 alloy is fabricated by the Laser Solid Forming (LSF) technique by using prealloyed powder in this work. Characterization of the microstructure, phase transformation, and shape recovery of the as-deposited alloy are explored. Columnar grains with ⟨110⟩ preferred orientation are well aligned with the building direction, and the martensitic transformation (MT) behavior shows spatial distribution characteristics. With the increase in the deposited height, MT varies from the two-stage B2 →R→ B19′ transformation to the one-stage B2 → B19′ transformation. Shape recovery properties are studied by compression tests. The as-deposited material without any further heat treatment shows a remarkably large superelastic recovery strain of 9.2% at room temperature, which is the largest value in the additive manufactured NiTi alloy. A negative adiabatic temperature change of –18.6 K induced by the elastocaloric effect during the unloading process is obtained. The shape memory effect (SME) of the as-deposited LSF Ni50.8Ti49.2 alloy is not only dependent on the detwinning of martensite but also associated with the partial superelasticity of the parent phase in the present work, demonstrating a unique two-stage yielding behavior. A residual strain up to 6% is recovered while heating, indicating an excellent SME.