Masters Theses

Date of Award


Degree Type


Degree Name

Master of Science


Materials Science and Engineering

Major Professor

Peter K. Liaw

Committee Members

Carl D. Lundin, Carl J. McHargue


Recently, high-entropy alloys (HEAs) have attracted increasing attentions because of their unique compositions, microstructures, and adjustable properties. In this work, microstructure and phase composition of the AlCoCrFeNi high-entropy alloy (HEA) were studied in as-cast and heat-treated conditions. Using a combination of Electron Backscatter Diffraction (EBSD), X-ray energy spectroscopy analysis, and synchrotron X-ray diffraction techniques, two nonequilibrium phases were identified in the as-cast condition: BCC_A2 and B2. Long-term heat treatment transformed these nonequilibrium phases into four equilibrium phases: FCC_A1, BCC_A2, B2, and σ [sigma] phase. The electron microscopic results were consistent with thermodynamic simulations of the equilibrium and nonequilibrium conditions.

Tensile properties of AlCoCrFeNi high entropy alloy (HEA) in two conditions, (i) as-cast and (ii) heat-treated, were also reported. The heat treatment consisted of hot isostatic pressing at 1,373 K, 207 MPa for 1 hours followed by annealing at 1,423 K for 50 hours. A noticeable increase in the tensile ductility occurred after heat treatment. During deformation at 973 K, elongation of the heat-treated alloy was 11 %, while the as-cast alloy showed elongation of only ~ 0.9 %. The ultimate tensile strength was almost unaffected by heat treatment, and it was 407 ± 5 MPa at 973 K. The properties of the alloy were correlated to its as-cast and heat-treated microstructures. The fracture mechanisms were discussed in detail by analyzing the features on and below the fracture surface. The formation of FCC_A1 phase, less casting pores, and residual stresses removed, may contribute to the significant improvement of tensile behavior.

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