Thermal evolution and strengthening mechanisms in LPBF CoCrFeMnNi alloy: New insights on correlation between microstructure and mechanics with LAGBs

The complex microstructure with multi-scale features in CoCrFeMnNi alloy, prepared by laser powder bed fusion (LPBF), significantly improves strength while maintaining sufficient ductility, but it theoretically has thermal instability. The thermal stability of these multi-scale microstructure features and their impact on mechanical properties are systematically analyzed by means of annealing. The multi-scale microstructure features are characterized by asynchronous changes as temperature increases, leading to non-uniform changes in mechanical properties, notably at 400 °C and 1000 °C. The entangled dislocations and segregated elements at cellular substructure boundaries exhibit limited thermal stability, beginning to disintegrate at 400 °C and complete by 1000 °C. In contrast, both low-angle and high-angle grain boundaries maintain their effectiveness up to 1000 °C. The strengthening effect of low-angle grain boundaries (LAGBs), distinct from dislocation strengthening, has been quantitatively assessed through experimental methods, revealing that LAGBs contribute 17.13% to the overall strengthening, effective up to 1000 °C. However, dislocation strengthening contributes only 25.87%，marking a notable deviation from the traditionally estimated 50% dislocation contribution. This study provides a new perspective on understanding the relationship between the microstructure and mechanical properties of alloys prepared by LPBF.