Microstructure – Property correlations for additively manufactured NiTi based shape memory alloys

Abstract

The research focuses on systematically studying the effect of processing parameters in controlling the microstructure and mechanical behavior including pseudoelasticity of a NiTi alloy. The alloy is prepared by laser engineered net shaping (LENS) based additive manufacturing technique. Laser energy densities of the manufacturing method are modified by altering laser scan speed and power. Corresponding effects on microstructure and phase evolution of NiTi alloy are evaluated thoroughly. Subsequently, mechanical properties are assessed implementing macro-, micro- and nano-indentations with load levels varying over five orders of magnitude. Layered microstructures, as a signature of the manufacturing process, are evident from the front and side planes of the products. Interestingly, existence of trace volume fraction of precipitates and martensite phase are noted in the alloy manufactured with highest laser energy density. These influence hardness, elastic modulus and indentation size effect of the alloy. Most importantly, pseudoelastic recovery of NiTi gets adversely affected. Spherical-nanoindentation is performed to precisely assess pseudoelasticity of NiTi alloys. Indentation-stress–strain graphs are generated from the spherical-nanoindentation. This is a unique and state-of-the-art way to assess the localized mechanical performance of materials. Considering that additive manufacturing is associated with microstructural inhomogeneity thereby leading to property variation along different regions, investigation is corroborated with bulk-scale tensile testing of the alloys, revealing similar results. Subsequent cyclic tensile tests complemented with digital image correlation are performed to appreciate their strain-recoverabilities. This study provides a scope to optimize the parameters of LENS to manufacture NiTi with the best combination of microstructure, phase stability and pseudoelasticity.