Formation of a hexagonal closed-packed phase in Al 0.5 CoCrFeNi high entropy alloy
- PDF / 737,781 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 44 Downloads / 175 Views
Research Letter
Formation of a hexagonal closed-packed phase in Al0.5CoCrFeNi high entropy alloy J. Wang, Y. Zhang, S.Z. Niu, W.Y. Wang, H.C. Kou, and J.S. Li, State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an Shaanxi 710072, China S.Q. Wang, School of Materials Science and Engineering, Xi’an Shiyou University, 18 Dianzi Road, Xi’an Shaanxi 710065, China E. Beaugnon, University Grenoble Alps, LNCMI, F-38000 Grenoble, France; CNRS, LNCMI, F-38000 Grenoble, France Address all correspondence to J. Wang at [email protected] and J.S. Li at [email protected] (Received 13 July 2017; accepted 26 September 2017)
Abstract We report that a hexagonal closed-packed (HCP) phase with high cobalt content precipitates in Al0.5CoCrFeNi high entropy alloy (HEA) after 650 °C/8 h heat-treatment. The precipitate with the shape of plate is completely located at the interdendritic region. Results of electron diffraction and high resolution transmission electron microscopy show that the HCP phase was transformed from the body-centered cubic phase through a simple shear and the two phase obey an orientation relationship. The thermodynamic stability of Al0.5CoCrFeNi HEA should be carefully reevaluated, especially at the vulnerable temperature.
Introduction High entropy alloys (HEAs) known for unique alloy design concept of multi-component alloys with equiatomic or nearequiatomic compositions,[1,2] have attracted the attention of scientific researcher because of excellent performance, such as high temperature stability, low temperature toughness, superior corrosion resistance and simple structure,[3–6] which impart them great potential of high-temperature structural material. Thus it is a crucial issue for the study of structure stability.[7] HEAs are characterized by face-centered cubic (FCC), body-centered cubic (BCC) or other simple solid solutions rather than intermetallic compounds owing to high configurational entropies, because of which they were thought to be thermodynamically stable.[8] However, with the in-depth study of HEAs, researchers found that some ordered solid solution or intermetallic compounds could precipitate from the matrix.[9,10] For example, CoCrFeMnNi alloy was once widely considered as a stable HEA at any temperature below its melting point, but M23C6 and σ phases were found after prolonged exposures at 700 °C afterwards.[9] Recently researchers reevaluated the stability of CoCrFeMnNi alloy and results show that more phases could precipitate at the grain boundaries after longtime anneals at 500–900 °C.[11] Even the CoCrFeNi HEA was demonstrated to be instable.[12] Actually, it is the competition between the entropy depending on the disordering of elements and enthalpy, which really determines the type of phases,[2,13] but it rarely occurs in metallic solution since different atoms occupying random lattice positions.[14] After solidification, elemental segregation takes place in some HEAs even though the x-ray diffraction (XRD) patterns cannot detect the formation of
any n
Data Loading...
