LAUSR

The relationship between the nucleation process and thermal hysteresis width in reversible thermoelastic martensitic transformations remains unclear, particularly as the volume of transforming material decreases. Understanding the number density and nature of defects which serve as nucleation sites in this class of materials requires a quantitative analysis of nucleation site potency distributions in different classes of materials with different intrinsic barriers to nucleation. Here, we investigate the size dependence of hysteresis in microscale ${\mathrm{Ni}}_{43}{\mathrm{Co}}_{7}{\mathrm{Mn}}_{39}{\mathrm{Sn}}_{11}$ alloy particles (radius $4.4\text{--}19.0\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$) during reversible martensitic transformations by collecting temperature-dependent magnetization of 126 individual alloy particles. Size-dependent hysteresis is quantified by a power law model and attributed to friction-induced energy dissipation. In samples with ideal nucleation-limited transformations, martensitic transformation temperatures on cooling decreased with decreasing particle volume due to the low probability of including relatively sparse high-energy nucleation sites. Nucleation site potency distributions are quantified as a function of thermodynamic driving force and compared against potency distributions for thermoelastic martensitic transformations in other classes of materials and for burst martensitic transformations. Across different classes of materials, as the energy barrier associated with the martensitic transformation increases, number densities of defects with sufficient potency to nucleate the transformation decrease dramatically. This finding suggests that very different kinds of defects may be responsible for nucleation of martensitic phase transformations in different material systems.