Phase evolution of refractory high-entropy alloy CrMoNbTiW during mechanical alloying and spark plasma sintering
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NANOCRYSTALLINE HIGH ENTROPY MATERIALS: PROCESSING CHALLENGES AND PROPERTIES
Phase evolution of refractory high-entropy alloy CrMoNbTiW during mechanical alloying and spark plasma sintering Lavanya Raman1,a) , K. Guruvidyathri2, Geeta Kumari1, S.V.S. Narayana Murty3, Ravi Sankar Kottada1,b), B.S. Murty1,c) 1
Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India; High Entropy Materials Center, Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan 3 Materials and Metallurgy Group, Vikram Sarabhai Space Center, Trivandrum 695022, India a) Address all correspondence to these authors. e-mail: [email protected] b) e-mail: [email protected] c) e-mail: [email protected] 2
Received: 15 September 2018; accepted: 30 November 2018
In the present study, the phase evolution and microstructure of CrMoNbTiW, a new equi-atomic refractory highentropy alloy, are studied. The alloy was synthesized through mechanical alloying (MA) followed by consolidation using spark plasma sintering. After MA, a major BCC solid solution along with residual Cr and Nb were observed. However, secondary phases such as Laves and carbides were also observed in addition to the major BCC solid solution after sintering. Unsolicited contamination from the milling media is found to be one of the reasons for the formation of secondary phases. The high hardness of 8.9 GPa after sintering was attributed to the presence of secondary phases along with the nanocrystalline nature of the alloy. To understand the phase evolution, calculation of phase diagram was carried out using CALPHAD. Further, binary phase diagram inspection and simple empirical parameters were also used to assess their effectiveness in predicting phases.
Introduction Alloys with optimum density and superior high-temperature properties are in great demand in the aero engines. Recently, a new class of materials with multiple elements of equiatomic or near-equi-atomic proportion are known to form simple crystal structures (FCC/BCC) and exhibit better properties. These are called high-entropy alloys (HEAs) or multicomponent alloys (MCAs) or complex concentrated alloys (CCAs) [1, 2]. Recently, refractory high-entropy alloys (RHEAs) have been considered as promising materials for high-temperature applications [3]. A generic way of defining the refractory alloys is as those which comprise elements whose melting point is above 2000 °C. Those alloys which fulfill this criterion are called the first-generation refractory alloys. The major drawback of the first-generation RHEAs is their high density, poor oxidation, and corrosion resistance. Thus, in the second-generation RHEAs, some of the heavier elements are replaced with Cr, Ti, Zr, and Al to reduce the
ª Materials Research Society 2019
density and to improve the corrosion and/or oxidation resistance [4]. Thes
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