Microstructure Evolution and Phase Formation of Fe 25 Ni 25 Co x Mo y Multi-principal-Component Alloys
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rom the traditional alloy design concept, multi-principal-component alloys (MPCAs), also called high-entropy alloys, are not based on one or two main elements but, rather, contain at least five principal components in the range of 5 to 35 at. pct.[1,2] Possessing impressive properties, MPCAs have attracted considerable attention in recent years.[3–16] They can be defined by entropy and composition. According to the entropy-based definition, MPCAs have a configurational entropy in a random state larger than 1.5R, where R is the gas constant, regardless of whether they are single phase or multiphase at room temperature.[17] It has been confirmed that the high mixing entropy is not sufficient enough to stabilize the solid solution phases, and many intermetallic compounds have been found in MPCAs.[1,18–22] According to the composition-based definition, MPCAs are classified as being either first or second generations. First-generation MPCAs are based on at least five principal elements and are equimolar
N. LIU, W. DING, X. J. WANG, C. MU, J.J. DU, and L.X. LIU are with the School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, Jiangsu, P.R. China. Contact e-mails: [email protected], [email protected] Manuscript submitted 25 October 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
alloys with single phases. The second-generation MPCAs are based on at least four principal elements, with dual or complex phases; they are non-equimolar.[23] In recent years, complex ordered r and l phases have been found in many CoCrFeMoNi systems, where the r phase is a simple tetragonal crystal structural being either CoCr, CrFe, or Co2Mo3-typed intermetallic and the l phase is Co7Mo6- or Fe7Mo6-typed intermetallic with a trigonal structure.[24–31] A great trade-off in mechanical properties has been achieved in the CoCrFeNiMo0.3 MPCAs, which suggests that hard r and l intermetallic compounds can be used to manipulate the properties of FCC-type CoCrFeMoNi MPCAs.[31] In the Co-free Ni2CrFeMox MPCAs with medium mixing entropy, single FCC solid solution phase in the Ni2CrFeMo0.1 alloy is stable in both the as-cast and as-annealed state; moreover, the single FCC solid solution is stable at the as-cast state in Ni2CrFeMox (0.4 ‡ x > 0.1) alloys, but the r phase emerges when annealed at different temperatures for a long period of time. FCC is the main constituent part in the Ni2CrFeMo0.5 alloy accompanied by the presence of the r phase, and the volume fraction of r phase increases when the annealing temperature increased from 600 C to 750 C and decreases from 900 C to 1000 C.[32] In this study, a series of Cr-free Fe25Ni25CoxMoy (x + y = 50) MPCAs are designed. In doing so, the
relative contents of both elements are changed from 0 pct to 50 pct to investigate the microstructure evolution and constituent phases, and properties. Significant attention is given to the issue of phase selection in FeNiCoMo MPCAs.
II.
EXPERIMENTAL DETAILS
The designed MPCAs were prepared via vacuum arc furnace in a water-cool
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