Phase stability, transformations, and mechanical properties of FCC-based medium/high-entropy alloys

Abstract

Medium/high-entropy alloys (M/HEAs), also known as multi-principal element alloys (MPEAs), are newly emerging metallic materials with excellent mechanical properties, which make them promising for structural applications, such as automotive, aerospace, and energy industries. FCC-structured HEAs attract considerable attention due to their attractive mechanical properties, such as excellent ductility and fracture toughness at cryogenic temperatures, which enable them serving as an excellent base for designing novel alloys with a high strength-ductility synergy. Precipitation hardening has been proven to be an effective way to strengthen FCC-based M/HEAs. Due to the complicated elemental partitioning and sublattice occupancies in the multicomponent phases compared with the conventional alloys, the research on the nucleation, growth, and coarsening behaviors of second phases becomes more complicated. There are several critical issues that have not been completely understood regarding the FCC-based M/HEAs, including the phase stability and transformations, precipitation mechanisms, and the correlation between the precipitate microstructure and mechanical properties. First, this research investigates the effects of alloying additions and aging temperatures on the phase relations, precipitate microstructure, and mechanical properties of L1₂-strengthened CoCrNi MEAs at intermediate temperatures. In this study, the precipitate type, morphology, and distribution of (CoCrNi)100−2(AlTi) ( = 3, 5, and 7 at.%) MEAs at 600-900 °C were systematically investigated through a combination of scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, electron backscatter diffraction, and thermodynamic calculations. The results reveal that the Al and Ti additions promote the destabilization of supersaturated FCC into L1₂ and σ phases, and the dominating phases of the MEAs change from FCC + L1₂ to FCC + L1₂ + σ and to L1₂ + σ + L2₁ phases as the Al and Ti concentrations increase. In addition, increasing the temperature leads to a change of precipitate morphology from lamellar to granular microstructures. The effects of alloying additions and aging temperature on the phase stability, precipitation behavior, and mechanical properties of the MEAs were discussed from the thermodynamic and kinetic points of view. Second, this study probes the mechanism of the competition between continuous precipitation (CP) and discontinuous precipitation (DP) of L1₂-strengthened HEAs. In this study, the effects of aging temperature, aging time, and grain size on the CP and DP behaviors of L1₂-strengthened HEAs were systematically investigated. The results reveal that low temperatures favor the DP reaction while high temperatures facilitate the CP reaction, which is related to the nucleation site and diffusion path of the two precipitation modes. At intermediate temperatures, both the CP and DP occur simultaneously and compete with each other, and the solute concentration difference across the boundaries between the two precipitation regions serves as the driving force for the sweeping of the CP regions by the DP reaction. In addition, grain size refinement can promote the DP reaction by providing more nucleation sites and more fast diffusion paths, leading to the formation of the DP-dominant microstructure. Third, this research explores how the pre-strain affects the CP and DP behaviors and associated mechanical properties of the L1₂-strengthened HEAs. The effects of pre-strain on the CP and DP as well as the mechanical properties of L1₂-strengthened HEAs were thoroughly studied. It is found that the pre-strain has a dual effect on the L1₂ precipitation behavior depending on the pre-strain level. At low pre-strains, the plastic deformation increases the dislocation density without significantly affecting the grain structure, which accelerates the CP reaction by promoting the CP nucleation and growth, leading to the CP-dominant microstructure. At high pre-strains, the severe plastic deformation induces the formation of a high density of deformation bands and subgrains, the boundaries of which provide preferred nucleation sites for the DP reaction. In addition, the high stored energy induced by the cold deformation enhances the grain boundary migration, which promotes the DP growth and concurrent recrystallization. Mechanical tests further reveal that the pre-strain substantially improves the strength of the alloys without significantly deteriorating the ductility. The contributions of dislocations, grain boundaries, and precipitates to the strengthening of the pre-strained and aged alloys were quantitatively evaluated. Lastly, this research aims to figure out the transformation pathways of hierarchical-structured FCC-based MEAs. A hierarchical microstructure consisting of FCC, L1₂, and BCC nano lamellae was formed in a (FeCoNi)90Al₅Ti₅ MEA, which exhibits a compressive yield strength of more than 2000 MPa and a decent ductility of 14.5%. It was found that this microstructure results from a eutectoid transformation accompanied by a hierarchical precipitation. The phase transformation is initiated by the eutectoid reaction, which results in the formation of a lamellar microstructure consisting of alternating BCC and L1₂ phases. The L1₂ lamellae are supersaturated in Fe and Co at intermediate temperatures, which drives the precipitation of FCC phases in the L1₂ matrix. The phase transformation pathway of this nano-lamellar microstructure was discussed in terms of morphological and compositional evolution. In summary, this thesis systematically investigated the phase relation and precipitates microstructure, CP and DP behaviors, phase transformation pathways, and mechanical properties of the FCC-based M/HEAs. The findings not only shed light on the fundamental understanding of phase stability, transformation behavior and strengthening mechanisms of FCC-based M/HEAs but also provide useful guidelines for the design of advanced M/HEAs for technological applications.