Negative grain boundary excess entropy in NiTi shape memory alloy

Atomistic calculations of free energy were performed for several symmetric tilt grain boundaries over a wide temperature range in the shape memory alloy NiTi. Contrary to expectation in a non-segregating alloy, it was found that the grain boundary excess free energy either increases with temperature or remains relatively constant, a consequence of a negative grain boundary excess entropy. This anomalous behavior can be associated with the martensitic transformation in NiTi. Some grain boundaries host martensite nuclei at their cores which have a lower vibrational entropy than the surrounding bulk austenite. Other grain boundaries, without apparent martensite cores, also exhibit negative excess entropy, leading to the hypothesis that a larger family of grain boundaries in NiTi can access vibrational modes associated with the martensite structure. Such negative excess entropy grain boundaries could aid in nucleation of the martensitic transformation, making them an important microstructural feature for engineering shape memory alloy properties.