DSC Cycling Effects on Phase Transformation Temperatures of Micron and Submicron Grain Ni_50.8Ti_49.2 Microwires

The effect of thermal cycling parameters on the phase transformation temperatures of micron and submicron grain size recrystallized Ni–Ti microwires was investigated. The suppression of martensitic transformation by thermal cycling was found to enhance when combined with room temperature aging between the cycles and enhances even more when aged at elevated temperature of 100 °C. While aging at room temperature alone has no clear effect on the martensitic transformation, elevated temperature aging at 100 °C alone suppresses the martensitic transformation. All aforementioned effects were found to be stronger in large grain samples than in small grain samples. Martensitic transformation suppression in all cases was in line with the formation of Ni₄Ti₃ precursors in the form of 〈111〉_B2 Ni clusters as concluded from the observed diffuse intensity in the electron diffraction patterns revealing short-range ordering enhancement. Performing thermal cycling in some different temperature ranges to separate the effect of martensitic transformation and high temperature range of DSC cycling revealed that both high temperature- and martensitic transformation-included cycles enhance the short-range ordering.