  Recent studies of FeMnCoCr-based high entropy alloys have demonstrated
uncommon deformation behaviors such as transformation-induced plasticity, which
were largely believed to be restricted to select families of steels. Coupled
with the potential for entropy stabilization of high symmetry phases at high
temperatures, this system represents a promising class of materials for
structural applications in extreme environments. Yet, transformation-induced
plasticity mechanisms are notably sensitive to microstructure parameters and
the literature offers examples of deleterious decomposition of high entropy
alloys under heat treatment, which raises concerns of resiliency in mechanical
performance. Here, we evaluate the evolution of microstructure and mechanical
properties of a FeMnCoCr high entropy alloy after aggressive heat treatments.
Microstructures are found to retain their characteristic austenite/martensite
features, with parent face-centered cubic grains segmented by hexagonal
close-packed laths after heat treatment at 1200 C for up to 48 hours. Results
of mechanical testing reveal an unusual insensitivity of this alloy to grain
growth-induced weakening effects. Namely, the yield strengths of FeMnCoCr
samples are observed to remain constant across all heat treatment conditions,
despite a near four-fold increase in the grain size. Close examination of
post-heat treatment microstructures reveals a dramatic decrease in the
inter-lath spacing at longer durations. These microstructure evolution
phenomena lead to significant intracrystalline refinement that counteract
conventional Hall-Petch weakening by introducing additional barriers for
dislocation pile-up. These results offer new insights into the mechanical
resiliency of this transformation-induced plasticity high entropy alloy under
aggressive heat treatments.
