Papers by Author: K. Yang

Abstract: Cross-sectional microhardness maps of cast-to-shape flat tensile specimens have been obtained for a binary Mg-3.44 mass% La alloy. Higher microhardness numbers were generally found near the casting surface, at the corners and along the segregation band. The higher hardness values were ascribed to the finer solidification microstructure near the surface and to localized positive macro segregation. The majority of lower hardness numbers was found at the core region. Lower hardness values were ascribed to the coarser grain size prevalent at the core and to dispersed microporosity. The non uniformity of the harder surface layer in both depth and hardness appeared related to local homogeneities in the grain size distribution caused by the scattered presence of large externally solidified grains.

Abstract: Cross sectional microhardness maps of cast-to-shape tensile specimens of rectangular and circular cross-sections have been obtained for a Mg-9 mass% Al alloy. The hardness is generally higher near the surface and at the corners of the cross-section in comparison with that at the centre. The lower hardness values at the casting core are accounted for by the coarser solidification microstructure and the concentration of porosity. The cross sectional mapping shows that the harder surface layer is generally uneven, questioning the concept of a well defined, uniform and continuous skin. Physical reasons for these features of the casting’s skin are discussed in terms of the grain microstructure.