NUCLEATION OF AN AL-ALLOY IN A FIBER-REINFORCED AL-ALLOY METAL-MATRIXCOMPOSITE (MMC)

Solidification is a phase-transformation process from liquid to solid. A small solid called a 'nucleus' first forms within the liquid and th e transformation process then continues on the nucleus, The formation of the nuclei can be heterogeneous or homogeneous, but in practice, du e to impurities, nucleation normally takes place heterogeneously. In c ommercial Al-alloy industries, some grain refiners are added to the mo lten Al in order to achieve a high heterogeneous nucleation rate and a fine grain size. Metal-matrix composites (MMCs) are newly developed m aterials. Much work has been done on fabrication and mechanical-proper ties testing. However, little work has been done on the solidification of Al alloys in the presence of fibres. In the present work, the nucl eation mechanism is examined in an Al-6.0 wt% Cu alloy/alumina fibre m etal-matrix composite (MMC) during solidification. It is discovered th at nucleation takes place away from the fibre, i.e. the fibre surface does not act as the nucleation substrate. Due to an uneven distributio n of the fibres within the Al matrix, it is also found that nucleation takes place within a large interstice first and it then occurs in a s mall interstice: the reasons for this are discussed in detail.