Date of Award

12-2012

Degree Type

Thesis

Degree Name

Master of Science

Major

Materials Science and Engineering

Major Professor

Peter K. Liaw

Committee Members

Carl D. Lundin, Carl J. McHargue

Abstract

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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