Microstructural Developments Through Marforming in a Ni-Ti-Fe Shape Memory Alloy

A hot-rolled Ni-Ti-Fe alloy was subjected to marforming: approximately 8 pct reduction (in thickness) in the martensite phase by laboratory rolling. Before the next marforming step, the sample was annealed to bring back calorimetric signatures of reversible austenite-martensite phase transformation. Significant differences in microstructure could be achieved by combinations of marforming and annealing. Such differences, on the other hand, originated from the marformed microstructure: two distinct regions of remarkably different substructures. The difference was mainly through the relative presence of defects: micro-twins and dislocations. The regions of lower defect densities got textured gradually, with marforming steps, to γ (ND//〈111〉) fiber. The regions with high defect densities, on the other hand, provided non- γ fine clustered grains after annealing. Though debates may continue on the exact nature and origin of micro-twins, the present study brought out their dominant role in determining almost all aspects of microstructural developments.