|M.Sc Student||Bronstein Emil|
|Subject||Thermodynamics and Microstructure of Austenite-Martensite|
Interfaces and Phase Transformations in Shape
|Department||Department of Mechanical Engineering||Supervisors||Professor Doron Shilo|
|Dr. Eilon Faran|
|Full Thesis text|
Martensitic phase transformations in shape memory alloys (SMA) are the processes that responsible for the unique shape memory and pseudoelasticity effects in these materials. The forward and reverse phase transformations can be described by stress-temperature phase diagram, which provides fundamental scientific and applications-related information on this important process. For example, phase diagrams of SMA are used as a design tool for SMA based actuators and are implemented in macroscopic phenomenological models under quasistatic conditions. Currently, phase diagrams of martensitic phase transformation are typically obtained from a set of discrete mechanical loading experiments, which are both time consuming and provide limited non-continuous data.
The martensitic phase transformations proceed by the propagation of austenite-martensite phase boundaries and the accompanied martensite microstructure, which consists of periodically alternating twinning lamellas. The latter is formed in order to account for the fundamental mismatch between the crystallographic structures of the two phases. At equilibrium, the twinned microstructure at the austenite-martensite interface is formed in a way that obeys energy minimization criteria. Generally, three contributions to the overall energy can be recognized: 1) the elastic strain energy that is associated with the local strain incompatibility between austenite and martensite, 2) the energy per unit area of a twin boundary, and 3) the line energy of topological dislocation-like defects that exist on the twin boundaries and are called twinning disconnections. While the first two energies have been considered in previous works that evaluated the total interface energy, the contribution of twinning disconnections’ energy was not.
In this work, we present a new experimental method that enables direct measurement of the complete stress-temperature phase diagram of SMA within a single experiment. The experimental setup is placed under an optical microscope and allows complementary visualization of the microstructure evolution throughout the phase transformation. The method is demonstrated on a SMA Ni-Mn-Ga single crystal, and the values of the Clausius-Clapeyron relation along with the latent heat of the transformation are extracted.
Furthermore, we investigate the microstructure of a Ni-Mn-Ga single crystal during the phase transformation based on optical images that were captured by the aforementioned developed setup. In this study, we formulate analytical expressions for all three energy terms and minimize the total energy with respect to the periodicity of the twinned microstructure. This procedure provides expressions for the twin periodicity as a function of the distance from the phase boundary. Simultaneously, the periodicity is measured using microscopy image analysis. Combination of the analytical models together with the measurements allows extraction and numerical evaluation of the energies of a twin boundary and twinning disconnection. These atomistic scale properties play a meaningful role in phase transformation and their values cannot be obtained by direct measurements. The presented combination of analytical modeling and microscopy image analysis leads to a direct link between the material’s microstructure and its atomistic scale properties.