  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 prolonged heat treatment at
high temperature. Microstructures are found to retain their characteristic
austenite/martensite features, with parent face-centered cubic grains
partitioned 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, which segments parent
austenite grains. This crystal partitioning counteracts conventional grain
growth-induced 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 prolonged high
temperature heat treatment.
