Coexistence of five domains at single propagating interface in single-crystal Ni-Mn-Ga shape memory alloy

Coexistence of both austenite and martensite during phase transformation is a common feature of all Shape Memory Alloys (SMAs). The martensite has different variants featuring characteristic deformations rotationally linked to each other due to the symmetries of the austenite parent phase, and the martensite variants can form twins with different mean characteristic deformations. Multiple-domain microstructures (consisting of austenite, martensite twins and individual martensite variants) evolve collectively within an SMA sample during the phase transformation, contributing thus to the material's macroscopic response (e.g., its global deformation). Particularly, the multiple domains can exist at the diffuse austenite-martensite interface nucleating and propagating in a single crystal in certain conditions. This implies an energy barrier for this interfacial structure, influencing the interface kinetics and the driving force (energy dissipation) of the phase transformation. In this paper, we report an experimentally observed interface consisting of five domains (austenite, one martensite variant and three twins) in a Ni-Mn-Ga single-crystal initially consisting of one martensite variant and subjected to a uniaxial thermal gradient. The compatibility analysis (performed from the characteristic strains of the three martensite variants having approximately a tetragonal symmetry) reveals that the fivedomain interface is not a perfectly compatible pattern like the basic habit plane (consisting of only one twin compatible with austenite). However, its level of non-compatibility is similar to that of the quite common X-interface (four-domain coexistence) which is observed in many SMAs. Further, the significant effects of the thermal loading path and the material initial state (the initial martensite variant) on the domain pattern formation are demonstrated and analyzed. The experimental observation and the theoretical analysis of the domain patterns can provide hints to better understand diffuse interface kinetics and phase transformation hysteresis.