In situ silicon particle reinforced ZA27 composites: Part 1 – Microstructures and tensile properties

Abstract

In situ silicon particle reinforced ZA27 composites were fabricated by liquid metallurgy route. The solidification process was specially discussed. The effects of pouring temperature, silicon content and phosphorous modification on microstructures and tensile properties of the composites were investigated. The results indicated that the solidification process included three stages, primary silicon particle crystallisation (the primary phase was α-Al dendrites when silicon content was lower than 1·5 mass%), binary Al–Si eutectic reaction and quaternary Zn–Al–Si–Cu eutectic reaction. The addition of silicon into the ZA27 alloy changed the α dendrites from the original equiaxed shape to a two-dimension (2D) column dendrites and this change was further enhanced with the increase in silicon content. Phosphorous did not only fine the primary silicon particles, but also fined the α dendrites. The sizes of both the silicon particles and α dendrites decreased and the particles distributed more uniformly as the pouring temperature increased. However, the particle size increased and their distribution became more agglomerated as the silicon content increased. The tensile properties increased with increasing pouring temperature while those decreased as the silicon content increased. There were four factors that led the composites to fracture during tensile testing, porosities, fracture of the silicon particles, debonding of the interfaces of silicon particles/matrix and debonding of the interfaces between the 2D column dendrites. The contribution of each mechanism to the fracture varied with the pouring temperature or silicon content.