In Situ Optical Microscope Study of the Thermally Induced Displacive Transformations in Cualni-Based Shape-Memory Alloys

In situ optical microscope examinations were carried out on the thermally induced thermoelastic martensitic transformation in an untrained CuAlNi alloy and on the bainitic reaction in a CuAlNiMnFe alloy. It was found that a different martensite variant structure formed after every thermal cycle and the transformation was not accompanied with observable plastic deformation in the CuAlNi alloy. Elastic deformations were observed as the martensite plates reached grain boundaries. The growth of new martensite plates was initiated at these locations. The result are discussed and compared to results of other alloys found in the literature. The bainitic reaction was found to occur under isothermal conditions in the CuAlNiMnFe alloys. The reaction was accompanied with a relief formation much finer than that of the thermoelastic martensitic transformation.