On the strengthening effect of Al-Cr-Si dispersoid in an Al-Si-Mg-Cu casting alloy with Cr addition

Abstract

In the present study, chromium (Cr) was added to an Al-Si-Mg-Cu casting alloy to investigate the strengthening effect of face-centered cubic structured Al-Cr-Si dispersoid precipitated in the intra-dendritic region. The Al-Si-Mg-Cu alloy with Cr addition shows a stronger hardening response to solution treatment than the Al-Si-Mg-Cu alloy without Cr addition, which was ascribed to the precipitation of Al-Cr-Si dispersoid occurred during solution treatment. Detailed characterization on Al-Cr-Si dispersoid via scanning transmission electron microscopy (STEM) indicates that there is a preferential orientation relationship between Al-Cr-Si dispersoid and Al matrix. This is in a good agreement with the prediction by the edge-to-edge model. In addition, the present study unveils a contradiction between the castability of Al-Si-Mg-Cu-Cr alloy and strengthening effect of Al-Cr-Si dispersoid. This contradiction was discussed based on thermodynamic calculations. Tensile properties of the Cr-free and Cr-containing Al-Si-Mg-Cu alloys after ageing treatment have been characterized. Unfortunately, strengthening effect of Al-Cr-Si dispersoid was not retained in the Cr-containing alloy after ageing treatment, owing to the strong interaction between Al-Cr-Si dispersoid and age strengthening elements during quenching. Quench-induced particles were extensively nucleated on the dispersoid, which may result from the synergic effects of dispersoid/matrix interphase energy and Si enrichment in the dispersoid. The undesirable quench-induced particles attached to Al-Cr-Si dispersoid consumed age strengthening elements in the Al matrix and undermined the thermal stability of Al-Cr-Si dispersoid.