Isothermal deformation of shape memory alloy in different temperature ranges. uniaxial case | Mekhanika | kompozitsionnykh | materialov i konstruktsii
> Volume 23 > №2 / 2017 / Pages: 263-282

Isothermal deformation of shape memory alloy in different temperature ranges. uniaxial case


Temperature is one of the main factors, which determines the kinetics of martensitic transformation in shape memory alloys (SMA). This is the reason, why the isothermal deformation curves obtained for SMA samples under loading and subsequent unloading conditions at various constant temperatures differ qualitatively and quantitatively. Here, a determining factor is the location of the deformation temperature range relative to the phase transition temperatures. In this study, based on the analysis of the phase diagram for NiTi, we have identified seven temperature ranges, for which the deformation curves are qualitatively different. For each case we have developed a theoretical description based on the phenomenological model recently proposed by the authors of this paper. The geometrical interpretation of the model is that the phase deformation is defined as a relative change in the length of a chain comprising a number of series connected structural elements – spherical austenitic and elongated martensitic ones. The martensitic elements are formed from the austenitic ones during the forward phase transition caused by a drop of temperature or application of load. During the reverse transition they again transform into the austenitic elements. The orientation of the martensitic element (an angle of its inclination to the longitudinal axis of the chain), which is determined by the macroscopic stress acting at the time of the element formation, characterizes the degree of orientation of the martensitic lamels with respect to the direction of this stress. Structural deformation, resulting from the re-orientation of the martensitis lamels due to increase of loads, is considered as a change in the orientation angle of the martensitic element due to a change in the external stress. The proposed approach enables us to define uniquely the phase and structural deformation components and obviates the necessity of differentiating between them during the description of the successive deformation process.