Single Crystal Growth and Chemical Disorder Trapping of Refractory MoNbReTaW High-Entropy Alloy Solidified Under Electro
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TRODUCTION
THE phenomenon of supercooling, which entraps a liquid state well below its freezing temperature, is a common property of all liquids regardless of the nature of their chemical bonding.[1,2] With regard to the metallic liquid, a sufficient supercooling will induce topologically structural disorder to the formation of metallic glass[2] and chemical disorder to the transformation from chemically ordered intermetallics to disordered solid solutions.[3,4] The accompanying disorder trapping effect on account of the high crystallization speed makes the composition of solid at the liquid/ crystal interface approach that of liquid and then engenders this transformation.[4] The formation of disordered solid solution is quite meaningful in reducing defects and improving performance especially for the multi-principal-element alloy, which is also termed as high-entropy alloy (HEA).[5,6] The basic strategy for
L. HU, L. WANG, M.J. LIN, and B. WEI are with the Department of Applied Physics, Northwestern Polytechnical University, Xi’an, 710072, P.R. China; [email protected] Manuscript submitted February 16, 2020; accepted September 28, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
designing a stable disordered solid solution lies in the thermodynamic equation DGmix = DHmix TDSmix, which is a high configurational entropy of mixing or low attractive energy between constituent elements.[7,8] But, the high mixing entropy effect will be weakened and then results in the chemical ordering or even evolves in the precipitation of stoichiometric compound with the decrease of temperature.[9] When a chemically ordered phase is more inclined to form, the disorder trapping induced by supercooling may provide a new insight to realize the transformation to disordered solid solution. To this end, the refractory high-entropy alloy (RHEA), which is extremely difficult to be melted and supercooled, was chosen to verify this thought. The innovation of RHEA formed from body-centered cubic (BCC) structured refractory metals W, Ta, Mo, Nb, Hf and V was firstly implemented by Senkov et al.[10,11] to obtain better strength at high temperature than conventional HEAs based on Al, Co, Cr, Cu, Fe and Ni. Here, we design a new equiatomic RHEA, which is MoNbReTaW, which includes the hexagonal closepacked (HCP) structured element Re. What is unique for this RHEA is that a large negative attractive energy exists between Re and any one of other four elements, which leads to the formation of highly ordered Laves phase r and inhibits the stable solid solution. The rest four elements except for Re possess an ability of complete
miscibility. Moreover, because both two largest atoms Ta and Nb have a very strong binding force with the third largest atom Re, the crystal lattice is liable to be distorted under the equilibrium solidification process. In light of the empirical criteria for a stable solid solution formation[7,8] and the fact that Re has no BCC/HCP transformation,[12] we reckon that this RHEA will form a solid solution featured with a conspicuous l
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