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Free keywords:
Annealing; Chromium alloys; Cobalt alloys; Crystal structure; Entropy; Grain refinement; Heat treatment; High pressure engineering; High resolution transmission electron microscopy; Mechanical properties; Microstructure; Nanocrystals; Scanning electron microscopy; Tensile testing; Ternary alloys; Thermodynamic stability; Torsional stress; Transmission electron microscopy; X ray diffraction, High entropy alloys; High pressure torsions; Nanocrystallines; Phase decompositions; Severe plastic deformations, Nanocrystalline alloys
Abstract:
An equiatomic CrCoNi medium-entropy alloy was subjected to high-pressure torsion. This process led to a refinement of the microstructure to a grain size of about 50 nm, combined with a strong increase in the materials hardness. Subsequently, the thermodynamic stability of the medium entropy alloy was evaluated by isothermal and isochronal heat treatments. Annealed samples were investigated by scanning and transmission electron microscopy as well as X-ray diffraction, and were subjected to tensile tests to establish microstructure-property relationships. Furthermore, a comparison of mechanical properties with a grade 316L stainless steel was performed in order to evaluate if the CrCoNi alloy is competitive with commercially available structural materials in the nanocrystalline state. A minority phase embedded in the face-centered cubic matrix of the CrCoNi alloy could be observed in multiple annealed states, as well as the as-received and high-pressure torsion processed material. For 200 h of annealing at 500 °C, it was determined that the minority phase has a hexagonal-closed-packed crystal structure. A possible explanation for the formation of the phase is a preferential segregation of Co to stacking faults. © 2018 by the authors.
