Microstructure and Oxidation Behavior of a CrMoNbTaW Refractory High Entropy Alloy

This work reports on the microstructure and oxidation behavior of a refractory CrMoNbTaW high entropy alloy (HEA) produced by vacuum arc melting. Microstructure and mechanical properties of both the as-synthesized alloy and that annealed in air for 6 h in the temperature range 600-900 degrees C were evaluated in detail. The phase identification and microstructure analyses were performed using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD) while microhardness was measured using Vickers hardness tests. The as-fabricated alloy consists of a multiphase microstructure consisting of three phases: a W-rich phase, a Mo-rich phase, and a solid solution phase of BCC structure. As-synthesized HEA for this composition demonstrates excellent microhardness values, of 787 +/- 83.6 HV, which was retained after annealing. The oxidation resistance in air was evaluated from 600 to 1400 degrees C for both 12 and 24 h of exposure time. The alloy exhibits a parabolic mass gain until 1100 degrees C, for which it begins to exhibit a decreasing mass gain at 1200 degrees C and, subsequently, a mass loss at 1300 and 1400 degrees C. The surface oxidation products were investigated, for which no protective oxides were identified. The surface oxide scale consisted of Cr, Nb, Ta, and trace amounts of W. The Mo content at the surface decreased as temperature increased, with no Mo content identified after 1100 degrees C.