MICROSTRUCTURE AND MECHANICAL PROPERTIES OF SICP/AZ91 MAGNESIUM MATRIX COMPOSITES PROCESSED BY STIR CASTING

Abstract

Magnesium alloy (AZ91) matrix composites reinforced with various volume fractions of SiC-particulates were processed. A Stir casting process has been employed under an inert atmosphere to produce magnesium matrix composites. This process leads to a complete wetting of SiC particles in the molten magnesium. Microstructural feature, such as SiC particles distribution, and the interface between the magnesium matrix and SiC was characterized by scanning electron microscopy (SEM) and energy dispersion X-ray analysis (EDX). The results show a refined matrix structure of the composite compared to the matrix alloy, a uniform SiC distribution in the matrix and that no reaction takes place during the synthesis of the magnesium alloy matrix composites between the oxidized SiC and the molten magnesium. However, AZ91 matrix composite showed a massive formation of ternary intermetallic compound of Mg17 (ZnAl)12 at the SiC interface. The mechanical properties of AZ91 and AZ91-SiC composites have been evaluated and the results are compared with the unreinforced AZ91 alloy. Results of the mechanical properties revealed an increase in hardness value, maximum tensile strength and 0.2 % yielding strength. It was proposed that the strength increase due to SiC addition to AZ91 alloy was a result of a change in the matrix strength, i.e. an increase in dislocation density and a reduction of matrix grain size. However, it is also evident that the strain to failure significantly decreased as the volume fraction of the particulate increased. Examinations of the fracture surface show good bonding between SiC/matrix, extent of fine dimples, matrix cracking and particle cracking.