Microstructural evolution, segregation and fracture behavior of A390 alloy prepared by combined Rheo-HPDC processing and Sr-modifier

Abstract

Abstract The microstructural evolution, segregation behavior and fracture mechanism of the hypereutectic A390 alloy prepared by Rheo-HPDC processing with Sr-modifier were investigated by morphology analysis and electron probe microanalysis technique (EPMA). The results indicate that the Rheo-HPDC processing with Sr-modifier can simultaneously refine the primary Si particles (PSPs) and modify the eutectic Si. Strontium reduces the formation tendency of α-Al dendritic halo around the PSPs by narrowing the constitutional undercooled zone in the solid-liquid interface front, which leads to the nucleation of eutectic Si on the primary Si. The liquid segregation causes the PSPs to concentrate toward the center of the sample and other elements to segregate toward the edge of the sample. The trace amounts of Sr intensify the degree of element segregation by increasing the melt viscosity. With the increase in Sr content, the melt viscosity gradually decreases. The fracture mechanisms of the A390 alloy without and with the addition of Sr-modifier are quasi-cleavage fracture. Strontium atom increases the internal stress within the PSPs to promote its cracking.