Accepted Manuscript Not Copyedited

International Journal of Minerals, Metallurgy and Materials https://doi.org/10.1007/s12613-020-2214-x International Journal of Minerals, Metallurgy and Materials Accepted manuscript, https://doi.org/10.1007/s12613-020-2214-x © University of Science and Technology Beijing and Springer-Verlag GmbH Germany, part of Springer Nature 2020 Two Refractory High-entropy Alloys CrHfNbTaTi and CrHfMoTaTi: Phase, Microstructure and Compressive Properties Jiaojiao Yia, Fuyang Cao b, Mingqin Xua, Lin Yangc*, Lu Wangc, Long Zengd aSchool of Mechanical Engineering, Jiangsu University of Technology, 1801 Zhongwu RD, Zhonglou District, Changzhou, 213001, PR China Email: Jiaojiao Yi: [email protected] Mingqin Xu: [email protected] bSchool of Materials Science and Engineering, Jiangsu University, No.301, Xuefu RD, Jingkou District, Zhenjiang, 212013, PR China Email: [email protected] cSchool of Materials Science and Engineering, Jiangsu University of Technology, 1801 Zhongwu RD, Zhonglou District, Changzhou, 213001, PR China Email: Lin Yang*: [email protected] Lu Wang: [email protected] dSchool of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan RD. Minhang District, Shanghai, 200240, PR China Email: [email protected] Abstract Two new refractory high-entropy alloys, CrHfNbTaTi and CrHfMoTaTi, derived from the well-known HfNbTaTiZr alloy by principal element substitutions, were prepared by vacuum arc-melting. Their phase components, microstructures, and compressive properties in the as-cast state were investigated intensively, and the results showed that bothAccepted alloys were mainly Manuscript composed of BCC and Not cubic laves Copyedited phases. In terms of the mechanical properties, the yield strength increased remarkably from 926 MPa for HfNbTaTiZr to 1258 MPa for CrHfNbTaTi, while retaining a promising ductility of around 24.3 % fracture strain for CrHfNbTaTi. The morphology and composition of the network-shaped interdendritic regions were closely related to the improved mechanical properties due to elemental substitution. Dendrites were surrounded by an incompact interdendritic shell after Mo incorporation, which deteriorated the yield strength and increased the brittleness. Keywords: Refractory; High-entropy alloys; Phase structure; Microstructure; Yield strength; Elongation 1. Introduction Due to the development of the aerospace, energy generation, and petrochemical industries, there is a high demand for structural materials that can be applied in high-temperature environments. To cater to the corresponding rigorous operation conditions of these industries, Ni-based superalloys have been proposed, but they have an incipient melting point near 1300 ℃, which greatly limits their applications to those which have temperatures between 1160 ℃ and 1277 ℃. [1-3] In addition to most of conventional refractory metals or alloys, the design of Ni-based superalloys is based on one simple idea: one element with attractive properties is used as the principal element, and minor amounts of other elements are added to improve these properties.[4] This has sometimes been referred to as a mature technology because it offers finitude modification possibilities.[5] To overcome inherent drawbacks, high-entropy alloys (HEAs) have been designed, which consist of at least 5 principal elements with the atomic concentrations of each element between 5 and 35 at.%. [6, 7] Soon after the introduction of HEAs, refractory HEAs, in which the most commonly used elements were Cr, Ti, Zr, Hf, Nb, Ta, Mo, W, and V, were first proposed by Senkov et al.[8, 9] The first two reported refractory HEAs, NbMoTaW and VNbMoTaW, both possessed a simple body-centered cubic (BCC) microstructure and pronounced solution hardening.[8, 9] The two alloys also have high yield strengths of 405 MPa and 477 MPa at 1600 ℃, respectively; however, their drawbacks are noticeable, including a density much higher than 9.0 g/cm3, room temperature (RT) brittleness (fracture strain of about 1.7%), and a low RT yield strength of about 1000 MPa. Subsequently, many new refractory alloys were tried and reported.[10-13] Thereinto, the most well-known refractory alloy, HfNbTaTiZr, with a BCC structure revealed excellent ductility exceeding 50% but much lower strength than the first two refractory HEAs.[1, 10, 13, 14] To improve the strength of refractory HEAs based on HfNbTaTiZr, J.W. Yeh et al. subsequently investigated the mechanical behavior ofAccepted HfNbTaTiZr alloys withManuscript different grain sizes.[15] Not The resultCopyedited showed that the alloy with the smallest grain size of 38 μm possessed the highest yield strength of 958 MPa, which was just slightly higher than the initial value of 929 MPa. Modifying the composition of alloying elements, such as minor element addition and element substitution, is a conventional and effective way to alter material properties. Senkov et al. documented that the partial substitution of Al for Hf in HfNbTaTiZr reduced the density of the alloy by 9% and increased the RT hardness by 29% and the enhanced yield strength by 98%.[16] Chun-Ming Lin et al. investigated the effect of Al addition on the mechanical properties of HfNbTaTiZr and found that the yield strength of 2 equiatomic HfNbTaTiZrAl alloy was enhanced to about 1500 MPa under compression.[1] Inspired by these, L. Wang et al. reported a new refractory HEA, HfMoNbTiZr, in which all Mo replaced by Ta in comparison to HfNbTaTiZr.[4] This new alloy showed a significantly enhanced yield strength up to 1575 MPa as well as excellent ductility.[4] Besides, in a recent report, E ́. Fazakas et al. studied the effect of Cr on the mechanical properties of Ti20Zr20Hf20Nb20Cr20 refractory HEA, and attributed the enhanced strength and hardness to the segregation of Cr-containing laves phases (Cr2Hf, Cr2Nb) during casting.[17] Accordingly, it suggested that Cr tends to form laves phase with Hf, Nb or Ta, which promotes the strength, and Cr is also more cost-efficient than other component elements (Hf, Nb, Ta, Ti, and Zr). Thus, in the present work, a new CrHfNbTaTi refractory HEA was designed by first replacing Zr with Cr in HfNbTaTiZr. Besides, the modulus of Mo was more than three times over Nb, even though they have a similar densities,[18] due to which the substitution between them is expected to greatly increase the mechanical properties. Hence, stemming from the alloy CrHfNbTaTi, another new CrHfMoTaTi was derived as well. The phases, microstructures, and mechanical properties of the new CrHfNbTaTi and CrHfMoTaTi refractory HEAs, at RT were investigated and discussed. 2. Materials and methods CrHfNbTaTi and CrHfMoTaTi alloys were prepared using vacuum arc melting of equimolar mixtures in a Ti-getter high-purity argon atmosphere. The elements of Cr, Hf, Nb, Mo, Ta, and Ti all were in the bulk form slugs with a purity of 99.99%, 99.9%, 99.95%, 99.95%, 99.95%, and 99.9% (at.%), respectively. During melting, ingots were melted for 3 min in a water-cooled copper crucible and turned over once after each melting process with electromagnetic stirring. This was repeated at least four times to ensure that the alloys were in a well-mixed state. The dimension of the solidified button sample was about 8 mm in thickness and 15 mm in diameter. The weight loss of each alloy during arc melting was lower than 0.2 percent, as there were no volatile elements in the alloys. The 10 g ingot piecesAccepted were melted by arc meltingManuscript and then sucked into Not an RT cylinder-shaped Copyedited copper mold with a diameter of 4 mm and a length of 60 mm. Table 1 shows the as-prepared alloy compositions of CrHfNbTaTi and CrHfMoTaTi cylindrical specimens, whose actual compositions were very close to the designed compositions. The crystal structure of the as-cast cylindrical samples was identified via X-ray diffractometry (XRD) using a PANalytical X'Pert Powder diffractometer with CuKα radiation operating at 40 kV/40 mA and a scanning rate of 3 °/min with a 2휃 range of 20 to 120°. The microstructure was analyzed using a field emission scanning electron microscope (FESEM, Zeiss sigma 500). The compressive tests were carried out on a computer-controlled Instron 3 (Instron, Norwood, MA) mechanical testing machine using Φ3.7× 5.6 cylindrical samples at a constant ramp speed of 5.6× 10-3 mm/s and an initial strain rate of 10-3 s−1. Table 1. Nominal and actual compositions (at.%) of the studied alloys in this work Composition Cr Hf Nb Mo Ta Ti Nominal 20 20 20 - 20 20 CrHfNbTaTi Actual 19.4 19.9 21.7 - 18.6 20.3 Nominal 20 20 - 20 20 20 CrHfMoTaTi Actual 20.5 21.6 - 18.2 17.7 22 3. Results and discussion 3.1 Phase analysis Figure 1. XRD patterns of CrHfNbTaTi (a) and CrHfMoTaTi (b) alloys in the as-cast condition together with that of the as-cast HfNbTaTiZr alloy (c) reported in Ref. [7]. Figure 1 shows the XRD patterns of the as-cast cylindrical CrHfNbTaTi and CrHfMoTaTi samples in this work (a, b) and as-solidifiedAccepted HfNbTaTiZr alloy Manuscriptextracted from Ref. [19] Not(c). The referenceCopyedited HfNbTaTiZr alloy consists of a single BCC phase (Figure 1(c)). In comparison, the alloy CrHfNbTaTi possesses more complicated phases even though only one principal element, Zr, was replaced by Cr. This implies that Cr plays a vital role in determining the phases of this type of alloys. More specifically, the as-cast CrHfNbTaTi alloy contained three phases, two BCC (BCC1 and BCC2) and one cubic laves phase (Figure 1(a)). The much weaker peaks of BCC2 phase than the BCC1 phase implies that BCC1 is the dominant constituent of the BCC phases. Further, if all peaks are considered, it can be concluded that the BCC phases are the major phases in the CrHfNbTaTi refractory HEA. 4 In addition, according to the JCPDS cards, the BCC2 phase was enriched with Ti and Cr, and depleted by Hf, Nb, and Ta, which matches well with TiCr.

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