A study on nano-deformation behavior of rheo-formed Al-Si alloy based on depth-sensing indentation with three-dimensional surface analysis

A nanoindentation study of semi-solid processed Al-Si-Mg alloy was performed as part of the research on surface crack behavior in automotive applications. The microstructure of semi-solid processed Al-Si alloy usually consists of an Al region (with a α-Al globular structure) and a eutectic region (with Al/Si eutectic phases). The region of interest in the failure process is the eutectic region in which crack initiation begins with initial fracture of the eutectic silicon particles and inside other intermetallic phases. In this study, the deformation behaviors (surface morphology after indentation and pile-up) and mechanical properties (indentation hardness and elastic modulus) in the primary Al region, the eutectic region, and the primary Al-eutectic boundary were investigated through nanoindentation experiments and atomic force microscopy (AFM) observation. The pile-up was observed around indents in both the primary Al region and the eutectic region. The indent in the primary Al region showed a clean surface and apexes. Contrarily, distorted valleys were mostly observed in the indent in the eutectic region. For the primary Al region, almost constant values for the elastic modulus (E) and the hardness (H) were obtained under the different test conditions. The hardness and elastic modulus of the eutectic region were more irregular than that of the primary Al region due to the inhomogenity of the eutectic structure. It was also found that the measured mechanical properties of some regions were strongly affected by those of the substrate region. In addition, the primary Al region showed higher pile-up and less elastic recovery than the eutectic region because it had higher values of E/H.