In Situ Optical Microscope Examinations of the Thermally Induced Martensitic Transformations in Cu-Based Shape Memory Alloys

The martensitic transformations are in the centre of interest since they had been first observed due to their unique characteristics and effect on the alloys’ properties. The desire to understand the detailed characteristics of these transformations became more important when the shape memory effect was explored and different types of the martensitic transformations were distinguished. There are many different examination methods in practice to describe the martensitic transformations. Besides the common examination methods, in situ optical microscopy is an effective technique to examine the microstructural changes during the martensitic phase transformations. The aim of this work is to provide additional information that can not be achieved by other methods about the nucleation locations, growth of the individual martensite variants and the propagation of the martensite/austenite interface during thermally induced forward and reverse martensitic transformations in CuAlNiMn, CuAlNiMnTi and CuAlNiMnFe shape memory alloys using in situ optical microscope technique with Differential Interference Contrast (DIC) imaging and high frame per second frequency. The effects of thermal cycling and isothermal martensite ageing on these characteristics of the thermoelastic martensitic transformations were also examined. It was previously reported that bainitic reactions occur in all of the examined alloys that strongly affect the shape memory behaviour. Another aim is to examine the growth mechanism of the bainitic reactions in the these alloys.