Insight into annealing-induced hardening and softening behaviors in a laser powder-bed fusion printed in-situ composite eutectic high-entropy alloy

Laser powder-bed fusion (LPBF) printed AlCoCrFeNi_2.1 eutectic high-entropy alloy (EHEA) typically exhibits an in-situ composite structure with an out-of-equilibrium character. Its distinctive hierarchical microstructure implies the potential challenges and opportunities in engineering advanced materials with high performances. Here, an informative overview of the microstructure evolution across various scales and its effect on tensile properties in LPBFed AlCoCrFeNi_2.1 subjected to annealing treatments over a wide temperature range is discussed. Microstructural observations indicate that spinodal decomposition within the B2 lamellae becomes evident after annealing at 500 °C, marked by a pronounced compositional fluctuation of Cr. Annealing the sample at 600 °C leads to the precipitation of BCC nanoparticles through structural disordering of Cr-rich regions within the modulated nanostructure. Tensile tests and strengthening models reveal that the annealing-induced hardening responses are largely associated with precipitation strengthening resulting from the ordering strengthening of L1₂ nanoprecipitates and the coherency and modulus strengthening of BCC nanoprecipitates, as well as the spinodal hardening from modulated nanostructure. Notably, BCC precipitation strengthening plays a crucial role in enhancing the strength of the annealed alloys. A strong precipitation strengthening effect is realized due to the coarsening of the BCC and L1₂ nanoprecipitates at 600 °C, and the strengthening effect from BCC precipitates gradually weakens as their dissolution at 650 °C, resulting in abnormal hardening behavior. Upon annealing at higher temperatures (700–1000 °C), phase decomposition, precipitation dissolution, even recrystallization occur sequentially, resulting in varying degrees of softening behaviors.