Cr-rich structure evolution and enhanced mechanical properties of CoCrFeNi high entropy alloys by mechanical alloying

Abstract

Mechanically-alloyed CoCrFeNi high entropy alloys (MA-HEAs) show better mechanical properties than as-cast HEAs. However, the limited amount of powder milled in one-milling cycle creates a challenge in larger-scale applications. The low ball-to-powder ratio (BPR) milling used to increase powder yield resulted in the formation of Cr-rich second phases in the face-centered cubic matrix. To overcome the above issues, in this work, with a low BPR of 5:1, four different processes were analyzed: quicker milling intervals, higher milling speed, and the pre-alloying of Cr and Ni. All parameters mentioned before were used and tested together in the fourth sample. Samples were consolidated by spark plasma sintering technique. Microstructural characterization was made on a Scanning Electron Microscope equipped with EDS and EBSD detectors and a Transmission Electron Microscope. X-ray diffraction was employed to develop the structural evolution of powders sintered and annealed samples. The global hardness was measured using microhardness tests, while the nanohardness tests helped us correlate the evolution of secondary phases with their mechanical properties. The results show that pre-alloying elements with the lowest diffusion coefficient before the main milling process might be an attractive strategy to improve productivity and optimize microstructure without limiting the efficiency of low BPR MA-HEAs. The results described below demonstrate the promising perspective for the implementation of powder metallurgy techniques in a mass scale production as low BPR is essential for numerous applications like aerospace industry, car engines and medicine, where improved productivity of powder mixing is mandatory.

Keywords

• High entropy alloys
• Mechanical alloying
• SEM
• X-ray diffraction
• Nanoindentation
• Chromium carbides