Design, processing, and characterization of high strength precipitation-hardened CrCoNiAlTi high entropy alloys

Abstract

High Entropy Alloys (HEAs) have been attracting considerable interest in literature. While early studies focused on producing single-phase HEAs, more recent investigations have expanded to multi-phase compositions to take advantage of precipitation hardening or other benefits of having multiple phases in the microstructure. In this work, two approaches were used to design precipitation-hardened HEAs with an FCC matrix and L12 precipitates. In the first approach, the focus was to introduce L12 precipitates into a highly concentrated Cr-Co-Ni matrix. The Cr29.7Co29.7Ni35.4Al4.0Ti1.2 (at. %) alloy was designed using the CALPHAD method by replacing some Cr, Co, and Ni with Al and Ti, so that an FCC+L12 field was stable at high temperatures. This alloy was produced, processed, and characterized. The results showed that the precipitates were effective in increasing the yield stress of the alloy by about ~55% compared to its homogenized counterpart. Moreover, this approach yielded insights for designing new precipitation-hardened HEAs with optimized strength. In this context, a second approach was proposed to effectively explore the large compositional landscape typical of these multi-component systems and design strong HEAs with an FCC matrix and L12 precipitates. Specifically, thermodynamic calculations using the CALPHAD method were used to screen a series of Cr-Co-Ni-Al-Ti alloys. A total of 11235 compositions was analyzed. After applying specific filtering criteria, the remaining alloys had their solid solution hardening and maximum precipitation hardening contributions to yield strength estimated. To assess the effectiveness of the proposed methodology, three alloys were selected, processed, and characterized using various microstructural and mechanical characterization techniques. The good qualitative agreement between the results and predictions suggests that the approach taken in this study has the potential to significantly expedite the identification and development of new precipitation-hardened alloys with optimized mechanical properties, making it a promising pathway for future research.