Regulation of strengthening and deformation mechanisms of Fe-Co-Cr-Ni-Mo-C medium-entropy alloys via control of composition and microstructure

Abstract

The advent of high-entropy alloys (HEAs) has greatly expanded the scope of alloy design. The multi-component feature of the HEAs has provided a much larger window of possible compositions. The trend of development has been moving from focusing on single phase solid solution in equiatomic compositions to exploiting non-equiatomic compositions and multiple phases in search of improved properties. Ferrous medium-entropy alloys (MEAs) are examples that show such a trend, which contain high amount of Fe contents (over 50 at%) and harness synergy of strengthening mechanisms in steels such as precipitation strengthening and transformation-induced plasticity (TRIP) effect with the inherent massive substitutional solid solution strengthening of HEAs. In this thesis, strengthening and deformation mechanisms of Fe-Co-Cr-Ni- Mo-C ferrous MEAs were modulated via control of chemical compositions and thermomechanical processing in pursuit of optimized strength and ductility. The addition of C in the Fe-Co-Cr-Ni-Mo led to formation of carbide precipitates, which involved deviation from bulk composition and destabilization of FCC matrix. This enabled modulating deformation-induced martensitic transformation (DIMT) into BCC and associated TRIP effect at cryogenic temperature with C addition. As a result of the precipitation-driven metastability engineering, ultrahigh tensile strength of ~2 GPa with uniform elongation of ~50% was achieved in Fe55Co17.5Cr12.5Ni10Mo3C2 (at%) MEA at liquid nitrogen temperature. Different thermomechanical processing routes led to different microstructures in the Fe-Co-Cr-Ni-Mo-C MEA, such as different recrystallization fractions, precipitation behaviors, and grain sizes. The role of each microstructural feature in determining the kinetics of DIMT and release of TRIP effect was investigated. While the formation of precipitates and their volume fractions largely affected the FCC stability of the matrix, nucleation sites for BCC martensite also played an important role in dictating the phase transformation kinetics. The non- recrystallized grains act as the primary nucleation sites due to profuse shear bands and deformation twins retained from cold rolling. Otherwise, grain boundaries offer the main nucleation sites in fine-grained samples, and the coarse-grained samples exhibited DIMT initiation mainly at the newly formed shear bands and their intersections. By controlling the initial microstructures, the optimal conditions for the release of TRIP effect that dictate strength and ductility were presented at room and liquid nitrogen temperatures. The final part of the thesis concerns the development of a maraging MEA. By excluding Cr and C in the system to avoid formation of undesired phases, a novel ferrous MEA with a chemical composition of Fe60Co25Ni10Mo5 (at%) was designed by utilizing the strengthening strategy of maraging steels. By a single-step aging of density of (Fe, Co, Ni)7Mo6-type nanoprecipitates in lath martensite structure and reverted FCC phase, which led to an ultrahigh yield strength higher than 2 GPa. Additionally, the MEA exhibited a high ultimate tensile strength of ~2.2 GPa and uniform ductility of ~6% by harnessing DIMT from the reverted FCC to BCC martensite, which has hardly been exploited in conventional maraging steels.