The role of phase compatibility in martensite

Abstract

Shape memory alloys inherit their macroscopic properties from their mesoscale microstructure originated from the martensitic phase transformation. In a cubic to orthorhombic transition, a single variant of martensite can have a compatible (exact) interface with the austenite for some special lattice parameters in contrast to conventional austenite/twinned martensite interface with a transition layer. Experimentally, the phase compatibility results in a dramatic drop in thermal hysteresis and gives rise to very stable functional properties over cycling. Here, we investigate the microstructures observed in Ti50Ni50−xPdx alloys that undergo a cubic to orthorhombic martensitic transformation using a three-dimensional phase field approach. We will show that the simulation results are in very good agreement with transmission electron microscopy observations. However, the understanding of the drop in thermal hysteresis requires the coupling of phase transformation with plastic activity. We will discuss this point within the framework of thermoelasticity, which is a generic feature of the martensitic transformation.

On the asymmetry of the forward and reverse martensitic transformations in shape memory alloys

“…Let us start from the picture depicted e.g. in [20] (see also the Appendix): the storage/release of the elastic energy during the forward and reverse transformation itself is not an irreversible process, whereas the presence of local free energy barriers leads to irreversibility and intermittent dynamics (e.g. noises).…”

“…In general the storage/release of the elastic energy during the forward and reverse transformation itself is not an irreversible process, whereas the presence of local free energy barriers (related to friction on local external defects and to the relaxations of the elastic energy) leads to irreversibility and intermittent dynamics (e.g. noises) [20]. Let us consider the heat measurable in a DSC run.…”

Landau model of the microstructure and shape memory properties of cubic–monoclinic-II martensite

“…GL or phase field, has been widely applied to represent the microstructural evolution in a wide variety of martensitic transformations [3][4][5][6][7][8][9][10][11][12][13][14][15] at the mesoscale. It is clear that, despite the great success of these approaches, many essential features of SMAs are not captured by these models, such as constant transformation strain, and weakly temperature dependent or constant stress hysteresis, principally related to imposing inappropriate potentials.…”

Acoustic Emissions during Structural Changes in Shape Memory Alloys

“…In the denominator of the expression for βh, Et−Erc appears because this difference is the actual elastic energy stored during cooling and part of it can be relaxed by AE during heating. Thus, from (7) and (8), one obtains It is clear that the storage/release of the elastic energy during the forward and reverse transformation itself is not an irreversible process, whereas the presence of local free-energy barriers leads to irreversible and intermittent dynamics (noises) [35]. Consider that during cooling and heating, three types of AE sources (local free-energy barriers) are operative (see, e.g., [29,35,36]):…”