Enhanced Relative Slip Distance in Gas-Tungsten-Arc-Welded Al 0.5 CoCrFeNi High-Entropy Alloy
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entropy alloy (HEA), a novel alloy system, is composed of five or more principal metallic elements with at least 5 to 35 at. pct of each. HEA forms simple solid solution structures (fcc, bcc, hcp, or mixed) instead of forming brittle intermetallic phases owing to its high entropy of mixing.[1,2] The difference in chemical configuration of the surrounding atoms and lattice distortion introduced due to the atomic size difference slows the diffusion kinetics and also enhances the mechanical, electrochemical, and oxidation properties. Because of their sluggish defect formation kinetics under heavy radiation doses, HEAs can replace the currently available structural materials in nuclear and high efficiency thermal power plants in the future.[3–8] One of the most prominent fabrication processes to produce permanent joints is welding. The grain refinement in the range of 0.35 to 15 lm from several millimeters and 5 to 100 nm from 3 to 4 lm was achieved by the friction stir
R. SOKKALINGAM, SOURAV MISHRA, SRINIVASA RAKESH CHEETHIRALA, V. MUTHUPANDI, and K. SIVAPRASAD are with the Advanced Materials Processing Laboratory, Department of Metallurgical and Materials Engineering, National Institute of Technology, Tiruchirappalli, Tamil Nadu, 620015, India. Contact email: [email protected] Manuscript submitted January 27, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
processing of HEAs.[9,10] Hardness and microstructural investigations on spot gas-tungsten-arc-welded (GTAW) CoCrFeNiAl–HEA have shown grain-size refinement and enhanced hardness values.[11] In the electron-beam-welded (EBW) AlxCoCrFeNi–HEA (x = 0.6 and 0.8) system, HEAs having high Al content are found to be susceptible to hot cracking.[12] Single-phase equiatomic CoCrFeMnNi–HEA has also been tested for its weldability with electron-beam welding.[13] However, to date, the published information on joining of the AlxCoCrFeNi and HEA by GTAW is scanty. Al0.5CoCrFeNi–HEA ingot was prepared using 99.9 pct purity raw elemental metals by vacuum arc melting in an argon atmosphere (remelted 5 times). Then the ingot was forged to a plate and cut into coupons of size 30 9 30 9 2.5 mm. These coupons were homogenized at 1423 K (1150 °C) for 24 hours followed by furnace cooling. An autogenous melt run was made on the homogenized coupon using a mechanized GTAW setup with 2-mm arc gap, 40-A current, and 12-V voltage at 80-mm/min weld speed. Microstructural evaluation and elemental mapping were performed using scanning electron microscopy (SEM). The phases formed in the homogenized base metal (BM) and GTAW Al0.5CoCr FeNi–HEA (TW) were confirmed by X-ray diffraction (XRD) patterns. The volumetric fractions of phase and lattice constants were also calculated from XRD. A microhardness survey was conducted across the weld, applying 500-g load. The tensile properties of the BM and TW were evaluated at a strain rate of 103 s1, with the tensile specimens having gage length, width, and thickness of 6, 4, and 0.5 mm, respectively. The fractured surfaces were observed under SEM, and the fractographs we
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