Peritectic reaction between two primary Mn-rich phases during solidification of Al-Si-Cu-Mn heat-resistant alloy and the effect of cooling rate on it

Abstract

In order to demonstrate the transformation relation of two primary Al13Mn4Si8 and Al15Mn3Si2 phases during solidification of Al-12Si-4Cu-2Mn (in wt%) alloy, verification experiments through controlling cooling rate of solidification and quenching at different stages during solidification were designed and the microstructure was characterized by optical microscope, scanning electron microscope and transmission electron microscope. The cooling rate during solidification plays a decisive role in the formation of primary Mn-rich phases. Fast cooling rate favors the slender rod-like Al13Mn4Si8 phase and slow cooling rate leads to the formation of dendritic Al15Mn3Si2 phase.A great number of symbiotic complexes, constituted with inner Al13Mn4Si8 and outer Al15Mn3Si2 phases, are found in the transition area of the surface and center of the casting with a middle cooling rate. Microstructure observation of the liquid quenching samples demonstrates that the first phase formed during solidification must be 2D lath-like Al13Mn4Si8 phase whether with high cooling rate or with slow cooling rate. And 3D dendritic Al15Mn3Si2 phase is transformed from 2D Al13Mn4Si8 by a peritectic reaction: L+ Al13Mn4Si8→All5Mn3Si2, which is dependent of the cooling rate during solidification. A schematic model with double nose curves was established to exhibit the influence of cooling rate on the peritectic reaction. With sufficient high cooling rate, this peritectic reaction will be suppressed completely. When the cooling rate is slow enough, the peritectic reaction will proceed completely. However, in case of middle rate, the peritectic reaction will happen but incompletely.