Mechanism of carbide transformation and its impact on the mechanical properties of electron beam freeform fabricated TiC-γ'/Ni-Fe-Cr composite during aging treatment

Abstract

The inherent instability of the microstructure in nickel matrix composites fabricated via additive manufacturing limits their application in high-temperature environments. This research proposed and prepared a Ni-Fe-Cr matrix composite reinforced with in-situ micron-size TiC and nano-size γ' phases using electron beam freeform fabrication (EBF3) with nickel-based powder-cored wire. To attain a stable microstructure at high temperature, the deposited composite was subjected to aging treatment for different durations. The mechanism of carbide transformation from TiC to Cr23C6 and its influence on the mechanical properties were thoroughly investigated. After aging heat treatment, the γ' particles coarsened from around 10 nm to over 100 nm. TiC phase partially decomposed and transformed into Cr23C6 carbides through the TiC+γ→Cr23C6+γ′ reaction. A distinct Cr23C6/γ'/γ transition layer was formed around the TiC phase. The ideal work of adhesion of the phase interfaces was calculated by first-principles calculations, and the γ' (200)/Cr23C6 (200) interface exhibits the lowest value. With prolonged aging time, the ongoing reaction leads to an increased proportion of mixed carbides, exhibiting a continuous aggregated distribution along grain boundaries. This phenomenon adversely affects the toughness due to weak bonding at the γ'/Cr23C6 interface. As a result, the ultimate tensile strength of the aging treated composites initially increased and then decreased, while the elongation continuously decreased with the extension of aging time.