Microstructure and mechanical properties of CrFeNi high-entropy alloy reinforced by Al and Ti elements

Developing high entropy alloys (HEAs) that possess both superior mechanical properties and low cost is a classic challenge in the design of alloys used in engineering applications. In this work, a series of Co-free Al x Ti (9– x ) (CrFeNi) 91 ( x = 0, 3, 6, and 9 at%, denoted as Al x Ti(9- x ) hereafter) HEAs were designed to investigate the influence of Al and Ti elements on the microstructure and mechanical properties of the FCC-structured CrFeNi alloy and pursue a considerable strength-plasticity balance. Massive precipitates with a nano-scale but different morphology and/or structure formed in the inter-dendritic region of alloys. The morphology of precipitates was strongly linked with the variation in the relative content of Al and Ti elements, which varied from a granular shape (ordered BCC structure) in Al9 and Al6Ti3 alloys to a strip shape (HCP structure) in Al3Ti6 alloy. With an increase (decrease) in Ti (Al) content, the yield strength of alloy significantly improved, which benefited from the combined effect of solution strengthening and precipitation strengthening, but the elongation dropped sharply. Compared to the same amount of Al element, Ti element contributed to the improvement of strength but deteriorated the plasticity of alloy. Thereinto, Al6Ti3 alloy presented a relatively favorable strength-plasticity balance with a yield strength of 768 MPa and an elongation of 10.2%. Apparently, only the moderate amount of Al and Ti elements helps to the favorable configuration of strength and plasticity. The findings in this work provide a distinct insight into the design and optimization of Co-free HEAs as the structural applications.