Multi–Scale Deformation Studies on a Metastable High Entropy Alloy

Recent studies of FeMnCoCr-based high entropy alloys (HEA) 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 various microstructural parameters including grain size, morphology, and relative orientation etc,. These microstructural parameters are particularly sensitive to post-synthesis thermo-mechanical treatments and various reports indicate the deleterious decomposition of HEA. As HEAs are considered as potential candidates for high temperature applications, decomposition raises concern of resiliency in mechanical performance. Here, we investigated the evolution of hexagonal close pack (HCP) phase on the structure – property relationships of a metastable FeMnCoCr HEA, at varying length scales, after exposure to high temperature. The reference HEA system with predefined chemical composition was synthesized in a vacuum arc furnace, thermo-mechanically processed and exposed to 1200 ºC for various hours. In depth microstructural investigations showed the processed HEA to retain the characteristic austenite/martensite features, with parent face-centered cubic grains (FCC) segmented by HCP laths and no observable decomposition. Uniaxial mechanical testing reveal an unusual insensitivity of this alloy to grain-growth induced weakening effects. Namely, the yield strengths of HEA samples are observed to remain constant across various heat treatments, despite a monotonic increase in the grain size. In addition, a new non-destructive methodology is developed, utilizing xiv acoustic emission in conjunction with Rockwell and micro-Vicker indentation, to fingerprint various deformation sources including dislocation slip and martensitic transformation. The selective activation of various deformation sources via indenting in individual grains made deconvolution of acoustic signals a fingerprinting dislocation slip and martensitic transformation unambigious. Furthermore, spherical nanoindentation was leveraged to understand the formation of martensite phase, and the associated change in flow behavior from individual grains in a single-phase metastable HEA. The change in indentation hardness with depth clearly showed material resistance to flow in the presence of HCP phase. Ongoing work regarding grain specific deformation flow, i.e., stress-strain analysis, will be a helpful in alloy design via providing intrinsic information about parent grain (i.e., grain size, phase fraction, etc.)