Effect of annealing treatment on the microstructure of matrix in SiC whisker reinforced aluminum composite

Abstract

Residual stress is an intrinsic feature of SiCw/Al composites due to the difference of coefficients of thermal expansion between the ceramic whiskers and the aluminum matrix [1–3]. When composites cool down from higher temperature, the residual stress increases with temperature changing (1T ), if1T is sufficiently high, the residual stress can exceed the yield strength of the matrix of composite [4], which results in residual stress relaxation, plastic deformation of the matrix and a high density of dislocations in the matrix. Certainly, the residual stress relaxation and microstructure of matrix have an important influence on the properties of composites. In the present study, the effect of annealing treatment on the matrix microstructure of SiCw/Al composites was investigated. The composites used were SiC whisker with a volume fraction of 20% reinforced pure aluminum (SiCw/p-Al) and 2024Al (SiCw/2024Al), the composites were fabricated by squeeze casting. The annealing techniques are list in Table I. The dislocation state was examined by transmission electron microscopy (TEM), and the specimens for TEM observation were prepared by ion milling. The dislocation states of pure aluminum matrix composites are shown in Fig. 1. The dislocation density in the matrix of the as-cast composite was very high, and dislocation tangles can be seen in the matrix (see Fig. 1a). The high density of dislocations in the matrix suggests that heavier plastic deformation had been induced in the matrix during the quenching process, due to residual stress relaxation. As shown in Fig. 1b, a low dislocation density can be found in the matrix of SiCw/p-Al composite annealed at 200 ◦C for 4 h then cooled with a low cooling rate, and some sub-grain boundaries can also be found. Because the high residual stress in the matrix may reduce the recovery and recrystallization temperature of the