Thermally driven reconfiguration of non-equilibrium structures and fracture evolution in LMD Ni2.1CrAlFe high-entropy alloy

This study investigates the regulatory mechanisms of stress-relief by annealing heat treatment on the microstructure and fracture behavior of Ni _2.1 CrAlFe high-entropy alloy fabricated via laser metal deposition (LMD). The microstructural changes induced by heat treatment was examined through characterization techniques including OM, SEM/EDS, XRD, and EBSD, as well as mechanical testing and fracture analysis. The study reveals that heat treatment disrupts the non-equilibrium inherited structural chain dominated by columnar grains, strong texture, and residual stress formed during the LMD process. This leads to grain equiaxialization, randomization of crystallographic orientation, and significant stress relief. XRD and EDS results indicate a structural evolution from metastable A2 phase to an ordered B2 structure, accompanied by a uniform distribution of solute elements. Mechanically, the material exhibits a moderate decrease in yield strength but a notable increase in ductility. The dominant strengthening mechanism shifts from dislocation-based to synergy of grain boundary and solid-solution strengthening. Fractography shows a transition from brittle cleavage fracture to ductile dimple coalescence. Based on these findings, this study proposes a non-equilibrium structural regulation mechanism for additively manufactured high entropy alloys, which explains how heat treatment reconstructs the LMD-induced microstructure through diffusion, ordering, and recovery processes. These findings provide theoretical support for residual stress control and fracture toughness optimization in additively manufactured high entropy alloys.