On interfacial energy of macroscopic domains in polycrystalline NiTi shape memory alloys

Under tension or tension/torsion combined loading, the formation and movement of macroscopic interfaces (domain fronts which separate the austenite and martensite regions) in the phase transition of a bulk fine-grained polycrystalline NiTi shape memory alloy (SMA) bar or tube specimen are well-observed phenomena. The interfacial energy of the macroscopic domain front, i.e., the energy per unit area of the front (interface), competes with the bulk energy of the system and plays an important role in understanding and modeling both equilibrium and non-equilibrium domain patterns. This paper investigates the physical origin of the interfacial energy of a planar interface and its orientation and length scale dependences in a bar structure. The energetics of a martensite domain with such a planar interface is quantified by using an elastic inclusion model. We show that the strain energy due to the presence of the martensite domain consists of two parts, the bulk energy (such as from bending) due to the constraints at the bar ends and the localized energy due to the mismatch between the spontaneous transformation strain of the domain and the surrounding austenite. The latter is stored around the domain front and is named as the interfacial energy (domain front energy). It is proved that the interface or the domain front of the bar prefers the orientation that minimizes the system total energy. This is supported by comparison with the experimental observation. The dependences of the interfacial energy on the bar geometry (bar thickness and width), the transformation strain and the interface orientation are quantified.