UC Irvine

As the field of high entropy alloy research has matured, investigation of phase stability in these alloys, particularly those with complex compositions, has become paramount if they are to be considered for structural engineering applications. The objective of this dissertation research is to study phase stability in targeted high entropy alloy (HEA) compositions as it pertains to the mechanical properties and processability of the material. While the studies presented here implement similar methodology of comparing experimental analysis of different HEAs to computational modeling of the alloys by the CALculated PHAse Diagram (CALPHAD) approach, each study aims to elucidate a unique aspect of phase stability and structural properties in HEAs. The first study describes the phase decomposition in a CoCuFeMnNi HEA and how the reported secondary phases influence mechanical behavior to establish these behaviors for the typical starting composition for an HEA system: equiatomic. Thermomechanical processing, followed by systematic annealing treatments, revealed the formation of two distinct secondary phases within the equiatomic face-centered cubic (FCC) matrix: Fe-Co rich ordered B2 precipitates that contributed precipitation hardening and Cu segregation, due to its immiscibility with the other constituents, eventually forms a Cu-rich chemically disordered FCC phase. The thermal stability and chemistry of these phases are compared to those predicted on the basis of CALPHAD analyses. In the second study, a novel non-equiatomic composition within the CoCuFeMnNi HEA system is developed to reduce the stability of the Cu-rich FCC and determine the role of constituent concentration on the mechanical behavior of alloys in the CoCuFeMnNi system. The equiatomic and non-equiatomic compositions were processed by high pressure torsion (HPT) and the mechanical behavior of the single FCC phase alloys before and after high temperature heat treatments are compared to determine the role of Mn and Cu in solid solution strengthening for these alloys. The third study assesses the phase stability in the nanocrystalline non-equiatomic composition Co26Cu10Fe27Mn10Ni27 at.% HEA and the implications of secondary phase formation on the mechanical behavior. After processing the material by HPT to achieve a nanocrystalline FCC matrix, thermal analyses and microstructural characterization of heat treated samples track the precipitation and dissolution behavior of Fe-Co rich B2 precipitates. The CALPHAD approach correctly predicted the composition and volume fraction of each phase after heat treatment. The presence of the B2 phase increased the stiffness and strength of the nanocrystalline HEA while causing embrittlement. The final study of the dissertation continues to focus on the phase stability but transitions to the refractory HEA system AlMoNbTiZr. The addition of Al to the equiatomic MoNbTiZr base, is predicted and validated to increase the stability of multiple BCC solid solutions. Al additions below 8 at.% enhanced the formation of a secondary BCC upon solidification while maintaining a single-phase BCC region at elevated temperatures. The hardness of the alloys increased with the increase of Al and deformation behavior in single-crystal micropillar samples are markedly different with and without Al. Together, these studies present an effective approach to assess phase stability in HEAs and design them for structural applications.