Onset of Θ' to Θ Phase Transformation in Al-Cu Alloys
37 Pages Posted: 10 Oct 2019
Abstract
The θʹ to θ phase transformation that occurs during overaging in Al-Cu alloys is detrimental for their mechanical properties. Experiments suggest that the onset of this transformation is dependent on the temperature and duration of thermal exposure, but the microstructural conditions which trigger this transformation are still unclear. In this study, the critical conditions associated with the onset of θʹ to θ phase transformation are predicted using thermodynamic analysis. We compare the predicted microstructural conditions with both new and previously reported microstructural observations, and also test the predictions using phase field modeling. This multi-pronged approach suggests that the tendency of a θʹ particle to transform into the θ phase depends primarily on temperature, particle size, and particle aspect ratio. We define a "critical" aspect ratio below which θʹ to θ phase transformation becomes thermodynamically favored and show how this critical aspect ratio increases with both temperature and particle volume. With this approach, the intimate link between phase stability, morphological stability, and coarsening resistance for θʹ precipitates in Al-Cu alloys is illustrated.
Keywords: Al-Cu alloys, θ´-Al2Cu, Phase field simulation, Phase Transformation
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