Experimental and theoretical study of the relation between phase and structural deformations in shape memory alloys
The structure of the oriented martensite, which determines the macroscopic deformation of shape memory alloys (SMA), can be formed in two ways: directly from the austenitic phase as a result of direct phase transformation under load, and also from the chaotic martensite under its isothermal structural transformation. The deformation produced by the first method is called the phase deformation, the deformation produced by the second method is called the structural deformation, but the difference in the terms used reflects only the difference in the mechanisms of their initiation, whereas the final product - the oriented martensite - is the same for both types of deformation. Therefore, some SMA phenomenological models take into account the uniformity of these two deformation components by determining their interrelation through the direct transformation diagrams F 1 (for phase deformation) and the diagrams of martensitic inelasticity F 2 (for structural deformation). The theoretical framework of these models is based on the hypothesis that the process of further deformation of the oriented martensite does not depend on the mechanism of its formation and involves three material functions: F 1, F 2, and also the function of their interrelation f. The experimental study of wire samples of TiNi, described in this paper, was performed with the aim to substantiate the developed hypothesis and establish three material functions used in the suggested theoretical description. A new method for determining the function f, which can be used as a verifying experiment, is proposed. The range of validity of the hypothesis has been determined both for the succeeding isothermal deformation of the samples with initial phase and structural deformations, and for the processes associated with their subsequent heating. The experiments demonstrated that the further-orientation diagrams for such samples coincide, which is indicative of the cross- hardening effect.