Phase stability study of high entropy alloys (HEAs)

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Phase stability study of high entropy alloys (HEAs)

 

Author: Ng, Chun
Title: Phase stability study of high entropy alloys (HEAs)
Degree: Ph.D.
Year: 2014
Subject: Metallic composites.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Mechanical Engineering
Pages: xxiii, 152 leaves : ill. (some col.) ; 30 cm.
InnoPac Record: http://library.polyu.edu.hk/record=b2696085
URI: http://theses.lib.polyu.edu.hk/handle/200/7419
Abstract: High entropy alloys (HEAs) have been emerging as a new frontier in the metallic materials field. The phase stability of HEAs is of critical significance, but progress in understanding this process has been difficult due to the slow diffusion kinetics, which prevent the completion of diffusion assisted phase transformations toward the equilibrium state. We used a series of thermo-mechanical treatments to investigate the stability of solid solution phases in a high entropy Al0.5CoCrCuFeNi alloy. The solid solution phases were found to be stable, against the intermetallic compounds, at high temperatures > 850℃ and at low temperatures < 300℃. At intermediate temperatures, however, the intermetallic σ-phase co-existed with the solid solution phases. Experimental observations were verified by thermodynamic calculation results. The mechanisms of phase stability were investigated, both for the equilibrium state and after quenching-equivalent annealing treatments. The roles of high entropy and slow diffusion kinetics were highlighted. Phase stability is vitally important for HEAs, but understanding of this phenomenon is very limited. The ability to predict phase stability from the fundamental properties of the constituent elements would greatly benefit our capability for alloy design. We systematically studied the relationship between phase stability and the physicochemical/thermodynamic properties of the alloying components in HEAs. The mixing enthalpy was found to be a key factor in controlling the formation of solid solutions or compounds. The stability of face-centered cubic (fcc) and body-centered cubic (bcc) solid solutions was well delineated by the valence electron concentration (VEC). Revealing the effect of the VEC on phase stability is vitally important for alloy design and for controlling the mechanical behavior of HEAs. By applying discoveries concerning the role of VEC, a series of cost-effective Co-free AlxCrCuFeNi₂ HEAs were developed. The solidification microstructure in these alloys showed some anomalies that are rarely seen in other HEAs or multi-component alloys. Specifically, submicron rod-like microstructures were found to exist in the eutectic alloy, in spite of the high volume percentage of the rod-like phases. Sunflower-like microstructures existed in the hyper-eutectic alloys, with 200 nm-sized spinodal-decomposed particles densely distributed in the disk florets. We used a combination of thermo-mechanical treatments and thermodynamic calculations to reveal the phase stability of these HEAs, as exemplified by the newly developed Al0.5CrCuFeNi₂ alloy. The metastable nature of the solid solution phases in HEAs was uncovered through thermo-mechanical treatments and induced phase transformations. These results were verified by thermodynamic calculations. In addition, a systematic study of Al0.5CrCuFeNi2 tensile properties was carried out, and the results were correlated with the alloy's indentation behavior. The tension-compression asymmetry on plasticity was demonstrated, together with their deviations from Tabor's rule on the relationship between hardness and strength.

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