The effect of heat treatment on pseudoelastic behavior of spark plasma sintered NiTi

Abstract

Porous NiTi shape memory alloys are considered as artificial bio-materials for possible use in implant applications. Their extraordinary pseudoelastic response under uniaxial loading through phase change needs further analysis. The objective of this work is to show the effect of heat treatment on the pseudoelastic behavior of NiTi SMA produced by spark plasma sintering (SPS). NiTi samples were prepared from pre-alloyed Ti-50.7 at.% Ni powders of 100-150 μm average diameter, and subjected to a wide range of homogenization and/or aging at different temperatures and duration. Transformation characteristics of heat treated samples were obtained from DSC, their phase composition was identified using X-ray diffraction (XRD), and checked with scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). Instrumented micro-indentation was carried out to measure the hardness altered by aging. Uniaxial compression tests were performed on heat treated samples, and their pseudoelastic behavior was shown on the stress–strain diagrams; the results showed a variation in pseudoelasticity. In general, Ni concentration of the matrix resulted in partial-pseudoelastic behavior of samples. Homogenization prior to aging improved the pseudoelasticity of the SPSed NiTi. The pseudoelastic response of the samples that have not shown the expected full pseudoelastic behavior was enhanced by applying a considerably long period of homogenization and aging.