Evolution of ductile L12 phase in (FeCoNi)86-Al7Ti7 high-entropy alloy aging at various temperatures and its strengthening mechanism

Ductile participation with small lattice misfit against matrix has been a long-sought-after character in toughening alloys, and recently multicomponent intermetallic nanoparticle L1 ₂ phase was explored by compositional modification in FeCoNi-based alloys and reported its benefits on strength and ductility through 780 °C for 4 h aging treatment. However, the L1 ₂ (A ₃B: Ni ₃Al) evolution in (FeCoNi) ₈₆-Al ₇Ti ₇ at a wide range of aging temperatures still lacks of comprehensive investigations. Herein, a series of aging temperatures (580 °C, 650 °C, 690 °C, 720 °C, 780 °C, and 820 °C) were carried out based on the synchrotron in-situ variable temperature XRD of the alloy. Results showed that both the composition and morphology of the L1 ₂ phase are dramatically dependent on the aging temperatures. Specifically, with aging temperature increased from 580 °C to 820 °C, A sites preferentially incorporated by more Co and Fe gradually turn into B sites partially substituted by Fe and Ti in the L1 ₂ phase, together with its morphology transforming from spherical to cuboidal. Meanwhile, the hierarchical microstructure induced by the precipitates of the tiny L1 ₂ phase at the aging temperature of 780 °C compensated the size departure of the primary L1 ₂ phase against the critical size to enhance the strength by enhancing its dislocation storage capacity. These hierarchical L1 ₂ phases strengthen the strong pair-coupling mechanism, ultimately illustrating its excellent strength-ductility balance aging at 780 °C against other aging temperatures.