Grain refinement tailoring the kinetics of phase transformation in nanograined Fe alloy: Smaller is slower

The conventional belief for diffusive phase transformations (PTs), based on the role of grain boundaries (GBs) in promoting heterogeneous nucleation, has been that a reduction in grain size at the micrometer scale tends to accelerate the overall transformation kinetics, i.e., a classical phenomenon known as smaller is faster. Here, by investigating the ferrite (α) to austenite (γ) transformation upon heating in nanograined Fe-Ni-based alloys, interestingly, we demonstrate that refining the grain size at the nanoscale leads to slower kinetics, that is, smaller is slower. Based on experimental analysis and phase-field crystal simulations, such slower kinetics in nanograined materials is attributed to the inhibiting effect of GBs on new phase growth being stronger than their facilitating effect on nucleation. This new finding reshapes our view on the mechanism of the size dependence of PTs and provides useful guidance for designing PTs by grain refinement.