Microstructure, mechanical properties and irradiation response of CrMnFeCoNi-(Y) high-entropy alloys
Keywords: irradiation response
mechanical properties
microstructure
high-entropy alloys

CrMnFeCoNi, also known as the Cantor alloy, exhibits attractive mechanical properties at cryogenic temperatures and encouraging irradiation resistance. This thesis uses electron backscatter diffraction (EBSD), (scanning) transmission electron microscopy ((S)TEM), transmission Kikuchi diffraction (TKD), and heavy ion irradiation as well as nanoindentation to investigate the microstructure, mechanical properties and irradiation response of bulk spray-formed CrMnFeCoNi-(Y), where Y is added in low concentration to create a fine dispersion of Y-based oxides.
As-sprayed bulk 15 kg CrMnFeCoNi (CSP) and Y-doped CrMnFeCoNi (YCSP) were swaged and then annealed to form as-swaged and as-annealed CrMnFeCoNi (CSW) and Y-doped CrMnFeCoNi (YCSW). The alloys comprised a single face-centred cubic phase of equiaxed, polygonal grains with a high-level of elemental homogeneity. The average grain size for CSP, YCSP, CSW and YCSW was 48.5, 22.3, 8.2 and 5.4 µm. Y reacted with internal oxygen to form Y-based oxides with size of 425±17 nm and volume fraction of 0.307±0.02% as well as Y2O3 dispersoids of a few nanometers and other Y-based oxides of varying compositions, which significantly refined the grains.
The alloys showed comparable bulk tensile properties at -196℃ and 23℃ to other Cantor alloys produced by different (usually small-scale) methods. Unlike the limited ductility of other ODS-HEAs, YCSW showed balanced strength and ductility, similar to those of conventional austensitic ODS steels. Grain boundary strengthening was the main strengthening mechanism with only limited precipitation strengthening due to the non-optimum Y-based oxide dispersion. Using EBSD, (S)TEM and TKD, extensive deformation nano-twinning in fractured CSW and YCSW specimens at -196℃ showed a strong crystallographic orientation dependence. Multiple twin variants were activated even in grains where twinning was unfavoured, causing twin-twin intersections. Deformation twinning and profuse nano-stacking faults contributed to higher storage capacity for plastic deformation.
Using 2 MeV V+ and Ni+ ion irradiation at 500℃ , at low (0.51 and 0.63 dpa) and high dose (2.33 and 2.84 dpa), radiation-induced segregation (RIS) was observed in grain boundaries only at high dose. Cr-rich, MnNi-rich and FeCo-rich precipitates occurred at grain boundaries, most likely due to thermal annealing occurred during ion irradiation. Irradiated CSW and YCSW contained 1/3<111> partial loops and 1/2<110> perfect loops. YCSW showed better resistance than CSW in terms of its smaller average size, lower number density and slower growth rate of loops, likely due to oxides acting as sinks for point defects. CSW and YCSW showed no obvious improvement in irradiation resistance, over conventional alloys such as 316SS. Irradiation-induced hardening was likely caused by a synergistic effect of defect clusters, RIS, and precipitates. Using TKD, deformation twinning, usually rarely observed in unirradiated CSW, occurred after irradiation, likely due to the stress concentration effect of irradiation-induced defect clusters.