Preparation of Refractory High-Entropy Alloys by Electro-Deoxidation and the Effect of Heat Treatment on Microstructure
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https://doi.org/10.1007/s11837-020-04367-2 Ó 2020 The Minerals, Metals & Materials Society
ELECTROMETALLURGICAL PROCESSING
Preparation of Refractory High-Entropy Alloys by Electro-Deoxidation and the Effect of Heat Treatment on Microstructure and Hardness JAGADEESH SURE ,1,2,3,4,5 D. SRI MAHA VISHNU and CARSTEN SCHWANDT 1,2
,1,2
1.—Department of Materials Science and Metallurgy, University of Nizwa, Birkat Al Mouz, 616 Nizwa, Oman. 2.—Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK. 3.—Present address: Energy Frontier Research Center, Molten Salts in Extreme Environments, Chemistry Division, Brookhaven National Laboratory, Upton, NY 11973-5000, USA. 4.—e-mail: [email protected]. 5.—e-mail: [email protected]
Equiatomic refractory high-entropy alloys TiNbTaZr and TiNbTaZrHf have been prepared by electro-deoxidizing pre-mixed and sintered solid metal oxide disks through cathodic polarization in a molten CaCl2 electrolyte. The sintering temperature employed in the preparation of the oxide disks prior to electro-deoxidation was varied between 1173 K and 1573 K, and it was found that this has a noticeable effect on the microstructure and porosity of the reduced alloys. A post-processing heat treatment at 1473 K in vacuum for 24 h was applied to the reduced alloys, and it was observed that the dual-phase bcc structure of TiNbTaZr changes into single-phase bcc while the single-phase bcc structure of TiNbTaZrHf remains unchanged. Notably, the microhardness of both alloys increased substantially, approximately doubling for TiNbTaZrHf. Overall, molten salt electro-deoxidation is a straightforward method for the preparation of refractory high-entropy alloys, and suitably selected heat treatments enable significant further optimization of their properties.
INTRODUCTION High-entropy alloys (HEAs) are defined as multicomponent alloys with four, five or more principal metallic elements, each with a share between 5 at.% and 35 at.%, in which the configurational entropy contributes substantially to the thermodynamic stability.1–7 HEAs are of interest because of the large compositional design space that allows for the adjustment of many properties, which is in direct contrast to conventional alloys that are based on one major metallic element. Two generations of HEAs are now commonly distinguished.8 Initially, research focused mainly on equiatomic and near-
(Received April 24, 2020; accepted August 28, 2020)
equiatomic single-phase solid-solution systems while, more recently, research has increasingly been expanded towards non-equiatomic systems.9 HEAs can be classified according to the elements they are composed of. The first category includes steel-type HEAs based on Fe, Co, Ni, Cr, Mn, Cu, Al and Si; the second category involves refractory HEAs based on Ti, Zr, Hf, V, Nb, Ta, Mo, W and Cr; the third category is rare earth HEAs based on Y, Gd, Tb, Dy and Ho. The refractory HEAs are being developed in particular with a view to their possible applicati
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