Date of Award
Master of Science (MS)
Garrett J Pataky
CoCrFeMnNi high entropy alloy (HEA) serves as a model material that researchers can utilize to gain better understanding of HEA-specific characteristics. When considering CoCrFeMnNi and other HEAs for load-bearing applications, it is necessary to understand possible phase decomposition and its potential influence on mechanical behavior. The aim of this study is to describe phase decomposition in coarse-grained equiatomic CoCrFeMnNi HEA after aging 360 h at 610 °C and 720 h at 700 °C, and show how the reported secondary phases influenced its mechanical behavior via Vickers microhardness and quasi-static tensile experimentation. A Cr-rich phase and MnS inclusion were observed in CoCrFeMnNi aged for 360 h at 610 °C. The magnitude of hardness for this aged material was statistically similar to that of the unaged material, indicating that the Cr-rich phase and MnS precipitates were too small to significantly affect the material hardness. Formation of Cr-rich phase resulted in precipitation strengthening where the yield strength increased by 9% with no significant loss in ductility. Precipitation of Cr-rich, FeCo-rich, and FeCr-rich phases and the MnS inclusion were observed after aging 720 h at 700 °C, resulting in a substantial decrease in yield strength from 291 MPa to 130 MPa. The ultimate tensile strength exhibited a shallower decline due in part to strength preservation via the extensive work hardening ability of CoCrFeMnNi. The significant decrease in strength was accompanied by reduction in ductility due to the overall brittle nature of the secondary phases and severe softening resulting from segregation of Ni. The effects of phase decomposition on the mechanical properties of CoCrFeMnNi after aging 720 h at 700 °C are consistent with reports concerning phase decomposed austenitic stainless steel, namely degradation of the material’s strength and ductility.
Monroe, Fredrick Christopher, "Effects of Phase Decomposition on the Strength Of CoCrFeMnNi High Entropy Alloy" (2020). All Theses. 3471.