A hexagonal close-packed high-entropy alloy: The effect of entropy
Graphical abstract
Introduction
Recently, a new alloying concept, termed as high-entropy alloys (HEAs) by Yeh et al. [1], and also defined as equiatomic multi-component alloys by Cantor et al. [2], has been proposed in the physical metallurgy community. This concept represents a radical departure from the traditional alloy design that is based on one or two principal elements. One definition for HEAs by Yeh et al. is that the alloys contain at least five principal elements, each with the concentrations between 5 and 35 at.% [1]. Many attractive properties have been found in HEAs, such as soft magnetism [3], high strength [4], [5], [6], [7], high hardness [8], [9], high wear resistance [10], [11], [12], excellent corrosion resistance [13], and good thermal stability [14], [15]. These excellent properties of HEAs have contributed to the increasing interest in research and development in this new exciting field. For recent comprehensive reviews on HEAs, readers are referred to [10], [16].
As pointed by Zhang et al. [10] and Gao et al. [17], the vast majority of the HEAs studied in literature typically form solid solutions in face-centered cubic (fcc) structures, body-centered cubic (bcc) structures, or their mixtures, and to date, few HEAs in hexagonal close-packed (hcp) structures have been reported. Chen and his co-workers [18] prepared the equi-molar BeCoMgTi and BeCoMgTiZn alloys whose constituent elements are all stable in the hcp structure by mechanical alloying, but only amorphous phases rather than solid solution phases formed. Gao and Alman [19] predicted a CoOsReRu alloy with the hcp structure by combining phase diagram inspection and computational modeling. Zhang et al. [10] first proposed that it is likely to form HEAs with the hcp structure comprising of rare earth elements, because they all have similar atomic sizes and crystal structures, and all can form isomorphous solid solutions based on their binary phase diagrams. In fact, formation of the rare-earth-elements hcp HEAs was confirmed later on in DyGdHoTbY by Feuerbacher et al. [20] and in DyGdLuTbY and DyGdLuTbTm by Takeuchi et al. [21].
Recently, a comprehensive study on the hcp HEA formations comprising rare earth metals or transition metals is carried out by Gao et al. [17]. Based on phase diagram inspection, CALPHAD modeling, and ab initio molecular dynamics simulations, they proposed a variety of new hcp HEAs, and formation of a single hcp solid solution was confirmed in the as-cast CoFeReRu alloy. In particular, they proposed that single hcp HEAs could be formed at equal molar ratios in the decadal Dy–Er–Gd–Ho–Lu–Sc–Sm–Tb–Tm–Y system and its sub-systems.
Following the ideas of the hcp HEA formation proposed by Zhang et al. [10] and Gao et al. [17], the present study aims to study the microstructure and mechanical properties of the GdHoLaTbY HEA. One objective is to examine whether a single hcp solid solution forms. The answer is not obvious since a hexagonal compound (prototype LaY, Pearson symbol hR9, space group ) forms in Gd–La, Ho–La, Tb–La, and Y–La systems [22]. The other objective is to examine the effect of entropy and thus lattice distortion on the mechanical properties. Since rare earth elements differ little in atomic size, electronegativity, and electronic structure, it is intuitively expected that the mechanical properties of GdHoLaTbY may obey the rule of mixture, and the present experiment was designed to verify this hypothesis.
Section snippets
Experimental procedures
Alloy ingot with a nominal composition of GdHoLaTbY (in atomic proportion) was prepared by arc-melting mixtures of pure metals (weight purity ≥ 99.9%) in a Ti-gettered high-purity argon atmosphere. The ingot was remelted at least four times to improve the chemical homogeneity of the alloy. The cylindrical rods with a diameter of 3 mm and a length of about 60 mm were then prepared by suction casting into a water-cooled copper mold. The rods of its constituent elements with the same size were also
Results
Fig. 1 shows the X-ray diffraction (XRD) pattern of the as-cast alloy. From the XRD analysis, all the diffraction peaks can be identified to be a single hcp phase. Table 1 lists the lattice constants (a and c) and the c/a ratios of the hcp phase for the alloy and all constituent elements. The lattice constants of the solid solution phase are estimated to be a = 0.3658 nm and c = 0.5812 nm, and the c/a ratio is 1.5888. It is noteworthy that the c/a ratio of the alloy is very close to that of the
Rule of mixture
According to the mechanical properties listed in Table 3, it is noticeable that the mechanical properties (including the yield strength, fracture strength, and hardness) of the GdHoLaTbY HEA obey the rule of mixture, or the “additivity law” [24]. The formula is expressed as follows:where ci and pi are the atomic percentage and the mechanical property of each constituent element, respectively. The present study indicates that the entropy effect plays little role in the mechanical
Conclusions
In summary, microstructure characterization and mechanical property tests on the as-cast GdHoLaTbY HEA produced by arc-melting were carried out in the present study. The alloy forms a hcp solid solution based on XRD, SEM, and DTA results. The compression yield strength, fracture strength, and plastic strain are 108 MPa, 880 MPa, and 21.8%, respectively. The hardness of the alloy is 96 HV. The mechanical properties and Vickers hardness obey the rule of mixture, and the alloy does not behave the
Acknowledgments
The authors would like to acknowledge the financial support of National Natural Science Foundation of China (Nos. 51371122 and 51501123), the Program for the Innovative Talents of Higher Learning Institutions of Shanxi (2013), the Youth Natural Science Foundation of Shanxi Province, China (Nos. 2015021005, 2015021006, and 2014021017-3), and the financial support from State Key Lab of Advanced Metals and Materials (No. 2015-Z07). M.C.G. acknowledges financial support by the Cross-Cutting
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