Insensitivity of nucleation rate to order-disorder interfaces in reversible thermoelastic martensitic transformations

Reversible martensitic transformations nucleate from sparse defects that lower the transformation's nucleation energy barrier. However, most defects do not serve as potent nucleation sites and, instead, can pin boundary motion and impede phase growth. Identifying potent defects from the general defect population remains an open challenge and has important implications for engineering reversible alloys. This study considers the influence of mesoscale order-disorder domains and the phase boundaries between the $L{2}₁$ and $B2$ phases on nucleation kinetics. We use solution heat treatment and secondary annealing in ${\mathrm{Ni}}₄₅{\mathrm{Co}}₅{\mathrm{Mn}}_36.7{\mathrm{In}}_13.3$ microparticles to compare an average $L{2}₁$ domain interfacial area density of $590.6\ensuremath{-}50.6\phantom{\rule{0.16em}{0ex}}\textmu{}{\mathrm{m}}^{\ensuremath{-}1}$, measured from transmission electron microscopy micrographs using the intercept method in ASTM standard E112-13. A total of 131 single particles with radii between 3.8 and 20.7 \ensuremath{\mu}m were individually characterized magnetically to measure their transformation temperatures and the transformation behavior. Overall, the undercooling in each particle ranged from 11.3 to 59.4 K, with the smallest volumes having the largest magnitude and variance. The nucleation site potency distributions between the two domain sizes were statistically alike, suggesting that $L{2}₁$ domain size is not a critical factor in initiating nucleation at the length scales of this experiment. The implications for microlevel devices include opportunities to heat treat materials to high operational temperatures (e.g., 773 K) without impacting nucleation behavior.