Effect of aging temperature on microstructure and mechanical properties of laser melted and hot isostatically pressed (CoCrFeMnNi)96(TiAl)4 alloy

In this study, a selectively laser melted (SLM) and hot isostatically pressed (HIPed) high entropy alloy (HEA), (CoCrFeMnNi)96(TiAl)4, was aged at different temperatures to investigate the microstructural evolution and mechanical property development with the aim of identifying the optimum aging condition that can produce the best combination of tensile properties for the alloy. It was found that the SLM-HIPed HEA contains a small density of nano-sized Al2O3 and Sigma particles. Upon aging at relatively low temperatures (650 degrees C and 700 degrees C), a high density of nano-sized long-range ordered (LRO) L12 domains formed from the matrix. Heusler and B2 particles also started to form from the matrix. With increased aging temperature, both the area fractions and sizes of Heusler, B2 and Sigma particles increased continuously. Aging at 750 degrees C gave rise to no LRO L12 domains but gamma ' particles while aging at 800 degrees C led to disappearance of both LRO domains and gamma '. The formation of Heusler and B2 precipitates was found to be associated with the diffusion of Al, Ti and Ni while that of Sigma is linked to the diffusion of Cr. Increased precipitation of Heusler and B2 with increased aging temperature was accompanied by the disappearance of LRO L12 domains, suggesting that the former may have competed with the latter in terms of absorption of Ni, Ti and Al. The samples con-taining LRO L12 domains and fine precipitates show the highest 0.2% yield strength (YS > 610 MPa) and ultimate tensile strength (UTS > 960 MPa) while maintaining a high elongation (EL > 25%). Those that are free of LRO L12 domains and contain large Heusler, B2 and Sigma particles show much lower YS (similar to 490 MPa) and UTS (760-850 MPa) as well as reduced ductility. The results indicate that the formation of LRO L12 domains is crucial for acquisition of enhanced YS in the HEA. In the early stage of plastic deformation, paired dis-locations were found to shear through the LRO L12 domains and with increased strains, a number of stacking faults formed with many of them intersecting with each other to form Lomer-Cottrell locks which favoured strain hardening. Dislocations tended to loop around Al2O3, Heusler and Sigma precipitates, indicating that they can act as effective dislocation motion obstacles.(c) 2022 Elsevier B.V. All rights reserved.

Automated summary

A selectively laser melted and hot isostatically pressed high entropy alloy was aged at different temperatures to study its microstructural evolution. The formation of nano-sized ordered structures and precipitates was observed at lower temperatures, while higher temperatures led to increased particle fractions and sizes with the disappearance of ordered structures. Samples with ordered structures and fine precipitates exhibited higher yield strength, ultimate tensile strength, and ductility compared to those without ordered structures and with larger precipitates.