The Microstructure Stability of Precipitation Strengthened Medium to High Entropy Superalloys
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HIGH entropy alloys (HEA) with elevated solution contents have been a subject under intensive investigations recently.[1–3] Although several studies have shown interesting strength and toughness properties in HEA,[4–7] recent studies have also pointed out that a single FCC phase HEA is not strong enough for practical applications.[8] So, additional strengthening mechanisms are required to achieve desirable mechanical strength. Precipitation strengthening can be one of the promising methods and has already been applied to several HEA systems. Liu et al.[9] and He et al.[10] have shown that CoCrFeNi HEA can be strengthened by Nb-rich precipitates. In other studies, AlxCoCrFeNi can be strengthened by nano-sized B2 precipitates to increase both the yield stress and ultimate tensile stress[11,12]; however, this increase in strength often corresponds with a decrease in plasticity. According to the physical metallurgy of superalloys, uniformly distributed c¢ particles are coherent with the FCC c matrix and can render excellent high-temperature strength.[13,14] Based on the microstructure of superalloys, several c¢-bearing HEA systems have been studied and reported.[15–27] Gwalani et al.[15] investigated Al0.3CoCrFeNi and Al0.3CrCuFeNi2 alloys. The precipitation of L12 c¢ were observed, but the c¢ precipitates in Al0.3CoCrFeNi can only be stable up to 823 K (550 C) and then be de-stabilized, replaced by B2 phase after annealing at 973 K (700 C); the thermal stability of c¢ in Al0.3CrCuFeNi2 is slightly higher. Borkar et al. [16] reported that by increasing Al from 0.0 to 1.5 in mole fraction for AlxCrCuFeNi2, the predominant microstructure would
TE-KANG TSAO and AN-CHOU YEH are with the Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, 30013, Taiwan, ROC. Contact e-mail: [email protected]. edu.tw HIDEYUKI MURAKAMI is with the National Institute for Materials Science, Sengen 1-2-1, Tsukuba, Ibaraki 305-0047, Japan. Manuscript submitted October 18, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
evolve from FCC solid-solution, to FCC + L12, to mixed FCC + L12/BCC + B2 and finally to BCC + B2.[16] Several studies on Al0.5CoCrCuFeNi by Pickering et al.[17] Xu et al.[18] and Jones et al.[19] revealed the nano-scale phase separation of microstructure, which is consisted of one FCC phase rich in Cr, Co, Fe, Ni, one FCC rich in Cu, and a L12 phase. Daoud et al.[20,21] has further simplified the microstructure to be mainly FCC solid-solution phase as the matrix and with uniformly distributed nano-size L12 precipitates, e.g., the Al8Co17Cr17Cu8Fe17Ni33 and Al10Co25Cr8Fe15Ni36Ti6 alloys containing 20 to 40 pct of c¢; comparing to those of commercial alloys such as Inconel 617 and Alloy 800H, the higher tensile strength can be achieved. Similarly, He et al.[22] reported that with minor additions of Al and Ti in CoCrFeNi, L12 coherent precipitates can be formed in the alloy matrix, and well-balanced room temperature tensile properties have been achieved;[22] however, the excessive addition
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