Monocrystalline elastic constants of fcc-CrMnFeCoNi high entropy alloy
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Monocrystalline elastic constants of fcc-CrMnFeCoNi high entropy alloy Katsushi Tanaka1, Takeshi Teramoto1 and Ryo Ito1 1 Department of Mechanical Engineering, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe, 657-8501, Japan. ABSTRACT Mono-crystalline elastic constants of equiatomic quinary Cr-Mn-Fe-Co-Ni high entropy alloy with the fcc structure have experimentally been determined by a resonance ultrasound spectroscopy at room temperature. The values of the bulk modulus of the high entropy alloy experimentally determined are similar to other conventional fcc metals when the values are normalized by the melting points. This indicates that the entropy change at melting is similar to that of conventional metals. The values of Pough's index and the Cauchy pressure are determined as 1.79 and -11.6 GPa, respectively. When the ductility of the alloy is judged from the indices, the ductility of the high entropy alloy is limited. In order to explain the negative Cauchy pressure of the high entropy alloy, it is required to assume that relatively strong directional interatomic bondings like intermetallic compounds exist in the alloy though the crystal is disordered solid solution. INTRODUCTION High entropy alloys are characterized as a single phase solid solution containing at least five major elements whose concentration ranges are 5 to 35 at.% [1]. This new class of multicomponent alloy system has been discovered for the structures of body-centered cubic (BCC) [2-6], face-centered cubic (FCC) [7-10], hexagonal closed packed (HCP) [11] and orthorhombic [12]. The term of "high entropy" comes from the large entropy of configuration of constituent atoms that is thought as the origin of stability of the solid solution state in the alloys. Since the alloys have the unique stabilization mechanism of the structure, extensive efforts have been accumulated to search other alloy systems, deformation behaviors and other mechanical properties. Elastic properties are one of the important mechanical properties, which directly reflect the interatomic bonding nature of crystals. There are some reports of experimentally determined polycrystalline elastic constants and their temperature dependence [13-15], and theoretically calculated monocrystalline elastic constants of the high entropy alloy [16]. However, experimentally determined monocrystalline elastic constants of the alloys have not been reported yet. Monocrystalline elastic constants involve the elastic anisotropy factor of shear deformations, which is one of the important parameter representing the stability of the crystal lattice. Another parameter derived from monocrystalline elastic constants is so called "Cauchy pressure" that is defined as C12-C44, whose sign is an index for ductility of the crystals; positive and negative values indicate the crystal being ductile and brittle, respectively. According to the results of theoretical calculation reported, both of the sign of the Cauchy pressure is derived by different calculation methods [16]. In this paper, experimentally determin
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