Characterization of Martensitic Transformation Morphology in Wide Hysteresis Shape Memory Alloys

NiTi (aka Nitinol) shape memory alloys can be alloyed with Nb in order facilitate a wider thermal hysteresis. The ternary alloy thermal hysteresis is nearly 100 °C, which is considerably higher than binary Nitinol. The wider hysteresis for NiTiNb makes the alloy suitable for the civil engineering operating temperature ranges. Thus the alloys are promising for integration into structures and can lead to the development of SMA hybrid composite smart structures. The wider hysteresis is attributed to the presence of second phases within the microstructure. The microstructure consists of a matrix, which undergoes the martensitic transformation and thus gives rise to shape memory behavior. A eutectic-like microstructure constituent is evident, with a substantial volume fraction of nano-precipitates dispersed throughout, and neither takes part in the transformation. Consequences of tailoring the microstructure via thermo-mechanical treatments are explored. In this work, the interaction of the non-transforming microstructural constituents with the martensitic transformation morphology is characterized using a fundamental thermo-mechanical framework. The bulk scale deformation measurement are reported as well as the meso-/micro-structure scale full-field and local deformation analyses. The technique used in this work is in-situ Digital Image Correlation (DIC). With this, the current work assesses the surface strain morphology in order to elucidate the influence of microstructure on the bulk scale shape memory responses.