A Comparative Study on Deformation Behaviour of Superelastic NiTi with Traditional Elastic–Plastic Alloys in Sub-micron Scale

A nanoindentation-based study is conducted to analyse the small-scale deformation behaviour of superelastic NiTi-based shape memory alloy. The mechanism is compared with respect to that for traditional elastic–plastic ferrous and non-ferrous alloys: ferritic stainless steel of grade 409 and Al. To develop a comprehensive insight into the deformation behaviour, various experiments including nanoindentation, optical microscopy, X-ray diffraction, differential scanning calorimetry are performed on all alloy systems. This detailed study demonstrates that structural properties are primarily controlled by the crystal structures of the phases present in the respective alloy systems. Among these three studied alloys, the highest and lowest nano-hardness is realized for the steel and Al, respectively. Intermediate hardness is noted for the NiTi alloy. Nevertheless, the localized deformation characteristics of the superelastic alloy appeared to be entirely different in relation to that for the ferrous and non-ferrous materials. Most importantly, a fourfold increment in the depth recovery is realized for the NiTi alloy in contrast to the traditional metallic system. To understand this exceptional recovery, detailed analysis is performed on the unloading response of all the alloys. The crucial role of reversible stress-induced martensitic transformation in contributing towards the depth recoverability for superelastic NiTi alloy is thereby revealed.