Microstructure evolution and related mechanical properties of additively manufactured Ti2AlC-modified Inconel 718 superalloy during long-term thermal exposure

Abstract

Ti₂AlC has been shown to significantly enhance the mechanical properties of Inconel 718 (IN718). For its reliable application in high-temperature environments, understanding its microstructural evolution and mechanical behavior under prolonged thermal exposure is crucial but remains underexplored. This study investigates the microstructural evolution and mechanical properties of laser powder bed fusion (LPBF) fabricated Ti₂AlC-modified IN718 during long-term thermal exposure at 760 °C. The results reveal that (Ti, Nb)C carbides formed from Ti₂AlC decomposition coarsened according to the Lifshitz-Slyozov-Wagner (LSW) model, while σ phases nucleated and grew along grain boundaries adjacent to carbides. The thermal stability of γ″ and γ′ in co-precipitates inhibited the transformation of metastable γ″ to δ phases, contributing to microstructural stability. Cellular structures were stabilized by the pinning effects of (Ti, Nb)C carbides, with boundaries covered by elongated γ″ phases during thermal exposure. This led to Nb depletion within sub-grains, limiting γ″ growth in co-precipitates and driving a stacking sequence evolution from γ″/γ′/γ″ to γ′/γ″/γ′. The coarsening of elongated γ″ precipitates triggered a transition in deformation mechanisms from dislocation shearing to micro-twinning. In regions where cellular structures were annihilated, γ′ and γ″ coarsened slowly, with some γ″/γ′/γ″ evolving into γ′/γ″ duplets, maintaining shearing as the dominant deformation mechanism. Tensile tests at 650 °C demonstrated a continuous decrease in yield strength (YS), primarily attributed to the coarsening of σ phases and the reduction in γ″/γ′/γ″ triplets. Conversely, ductility improved significantly from 9.6 % to 21.1 %, driven by the activation of micro-twinning and enhanced dislocation motion facilitated by coarsened precipitates. These findings highlight the importance of Ti₂AlC in stabilizing the microstructure and optimizing the high-temperature performance of IN718 alloys under prolonged thermal exposure.