Microstructural transitions of aluminum-silicon alloys during eutectic solidification

The microstructural transitions due to the variation of growth rate and/or impurity additions during the eutectic solidification of the Al-Si alloys (modification phenomena) were investigated in detail. A transparent organic analog of the non-faceted/faceted eutectic (Succinonitrile-Borneol) in the presence of impurities was also examined on a temperature gradient stage. The experimental results and kinetic analysis indicate that the solidification patterns for both the normal and Na modified alloys are essentially the same. The growth kinetics of the Si rather than the nucleation is responsible for the microstructural transitions. In the normal alloy, the Si phase appears as flakes with a {dollar}{lcub}111{rcub}{dollar} habit and a {dollar}langle 211rangle{dollar} preferred growth direction. The flakes branch by a small angle mechanism as well as by a large angle mechanism. A low {dollar}{lcub}111{rcub}{dollar} twin density was observed and the TPRE mechanism (twin plane reentrant edge) for atomic attachment is incidental to the Si growth. The intrinsic steps on the Si interface provide adequate sites for attachment, leading to a small kinetic requirement. The Al phase exists as low angle subgrains of the order of 1 {dollar}mu{dollar}m width within rather large eutectic colonies and the misorientations between the subgrains are typically {dollar}<{dollar}2{dollar}spcirc{dollar}. The Si fibers in the quench modified and Na modified alloys are different. The former is generally non-faceted without a preferred growth direction and contains few twins; the latter is multi-faceted with a preferred {dollar}langle 100rangle{dollar} growth direction and contains a very high density of multiple twins. Quenching causes a transition from anisotropic to isotropic growth of the Si crystals and the atomic attachment kinetics are insensitive to internal twins. The Na, which is associated with the Si phase in the modified alloy, inhibits intrinsic step sources for atomic attachment on the growing interface of the Si crystals and so leads to multiple and repetitive twinning with growth depending on the TPRE mechanism. A model of impurity introduced twinning is presented and it is suggested that the microstructural transition for the impurity modified alloy is related to the atomic radii of the impurity additions because of the crystallographic requirement of the impurity introduced twinning. (Abstract shortened with permission of author.)...