Higher Temperatures Yield Smaller Grains in a Thermally Stable Phase-Transforming Nanocrystalline Alloy

Grains in crystalline materials usually grow with increased thermal exposure. Classical phenomena such
as recrystallization may lead to a purely temporary decrease in the grain size, while recent advances in alloy
design can yield thermally stable nanocrystalline materials in which grain growth stagnates. But grains
never shrink, since there is a lack of interface-generating mechanisms at high temperatures, which are
required to decrease the grain size if such was the system’s thermodynamic tendency. Here we sidestep this
paradigm by designing a nanocrystalline alloy having an allotropic phase transformation—an interfacegenerating
mechanism—such that only the high-temperature phase is stabilized against grain growth. We
demonstrate that for an Fe-Au alloy cycled through the α ↔ γ transformation, the high-temperature phase
(γ-Fe) has a stable fine grain size, smaller than its low-temperature counterpart (α-Fe). The result is an
unusual material in which an increase in temperature leads to finer grains that are stable in size.