Influence of processing route on the alloying behavior, microstructural evolution and thermal stability of CrMoNbTiW ref
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Influence of processing route on the alloying behavior, microstructural evolution and thermal stability of CrMoNbTiW refractory high-entropy alloy Lavanya Raman1,a) , G. Karthick1, K. Guruvidyathri1, Daniel Fabijanic2, S. V. S. Narayana Murty3, B. S. Murty1,b) , Ravi S. Kottada1,c 1
Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India Institute for Frontier Materials, Deakin University, Geelong, VIC 3220, Australia 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]; [email protected] c)e-mail: [email protected]; [email protected] 2
Received: 22 January 2020; accepted: 11 May 2020
Two different processing routes of mechanical alloying followed by the spark plasma sintering (powder metallurgy) and vacuum arc melting (casting route) were employed to understand the role of processing routes on the phase and microstructural evolution in an equiatomic CrMoNbTiW refractory high-entropy alloy. Besides a major BCC solid solution, a small fraction of carbide, σ phase, nitride, and oxide phases were observed in the alloys prepared by the powder metallurgy route in contrast to a single-phase BCC solid solution in the casting route. The milling atmosphere (dry milling in air and Ar) has significantly influenced the phase and microstructural evolution, illustrating the substantial role of contaminants. Good thermal stability of microstructure at high homologous temperatures was shown based on the long-term heat treatment at 1300 °C for 240 h. The phase evolution predictions via Calphad studies were found to be in reasonable agreement with the experimental observations, albeit with some limitations.
Introduction Recently, there has been a renewed interest in the field of hightemperature materials to push the limits of elevated temperature operations and increase engine efficiency. The discovery of more promising compositions for high-temperature applications resulted in a paradigm shift in the alloy design strategy, which gave birth to refractory high-entropy alloys (RHEAs) [1]. The RHEAs show considerable potential as ultra-high temperature materials due to their very high melting point (>2000 °C) and the unique property of strength retention at elevated temperature [2, 3]. An overview of the current understanding of RHEAs with multiple phases is summarized and presented in Supplementary Table SI. The majority of RHEAs were synthesized using vacuum arc melting (VAM) besides a few studies by the mechanical alloying (MA) + spark plasma sintering (SPS) route (Table SI). The MA is a unique method to synthesize alloys from the elements having a high melting point and vast difference in vapor
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pressure [4, 5]. Through MA, grain refinement, improved homogenization, and alloying can be achieved due to diffusion in the nanocrystalline state caused by repeated cold welding and fractur
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