Aging behavior of squeeze cast SiCw/AZ91 magnesium matrix composite

Abstract

The aging behavior of SiC whisker reinforced AZ91 magnesium matrix composites was investigated with Vickers hardness measurement, differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The precipitation process observed in the monolithic alloy was not altered by the addition of SiC whiskers. The composite exhibited an accelerated hardening response compared with the unreinforced matrix alloy. However, the addition of SiC whiskers altered the distribution of the Mg₁₇Al₁₂ precipitates. Mg₁₇Al₁₂ precipitated preferentially at the SiCw–AZ91 interface, with a definite orientation relationship with SiC whisker: (1̄10)_Mg17Al12//(111)_SiCw, [111]_Mg17Al12//[01$\text{1}\text{̄}$]_SiCw. The preferential interfacial precipitate decreased the Al content in the matrix, resulting in a decrease in the amount of continuous precipitates. As a consequence, the age-hardening efficiency in the composite was lower.

Introduction

Metal matrix composites reinforced with discontinuous reinforcement (short fiber, whisker or particle) are attractive for applications requiring higher stiffness and strength than traditional alloys. Unlike continuously reinforced composites, where the properties are mainly influenced by fibers, the properties of the discontinuously reinforced composites seem to be influenced more by matrix properties. Most of the discontinuously reinforced composites are based on age-hardenable light alloys, so that aging treatments can be applied to develop the optimum properties of the composites. The aging behavior of discontinuously reinforced composites has been a subject of great interest both from scientific and technological view points. Extensive researches have been carried out on the aging behavior of aluminum matrix composites. These researches indicate that the aging kinetics and aging hardening efficiency during aging of these composites depend on variety of factors, such as: the size, shape and volume fraction of reinforcement [1], [2]; fabrication method of the composites [3]; secondary processing [4]; temperature of aging [5] and the nature of matrix-reinforcement interface [6].

Through these extensive investigations, a clear understanding of aging behavior in discontinuously reinforced aluminum matrix composites has begun to be achieved. However, there is little information available on the aging behavior of discontinuously reinforced magnesium matrix composites, although more and more attention is being attracted to these composites because of their low density, superior specific properties and high damping capacity [7], [8], [9], [10], [11], [12], [13]. Chaudhury [14], [15] found that the precipitation characteristics in the SiCp/Mg–6Zn composites were similar to those of SiC/Al composites, significant hardening being achieved by artificial aging of SiCp/Mg–6Zn. The mechanism of precipitation at different temperatures was different. However, Gu [16] observed a different precipitation process in the B₄Cp+SiCw/ZK60 composite compared with that in the SiCp/Mg–6Zn composite. The aging hardening was accelerated and the aging hardening efficiency of the B₄Cp+SiCw/ZK60 composite was the same as that of the monolithic alloy. Badini [17] investigated the aging characteristics of PM B₄C/AZ80 composite by optical microscopy and XRD. The addition of B₄C particle increased the aging rate of the composite. However, the hardness increase due to the artificial aging was slightly lower in the B₄C/AZ80 composites than in the monolithic alloy, because of the preferential formation of Mg₁₇Al₁₂ close to the B₄Cp–AZ80 interface. Kiehn [18] studied the electrical resistivity changes due to precipitation during isochronal annealing up to 300 °C in alumina fiber reinforced AZ91D composite fabricated by squeeze casting. These results show no accelerated age-hardening due to the addition of alumina fiber, and the poor age-hardening response of the unreinforced alloy remained unaltered in the composite.

AZ91 magnesium alloy is the most common magnesium casting alloy. It has good combination of castability, mechanical strength and ductility. Additionally, AZ91 alloy is an age hardenable alloy, cellular discontinuous precipitation at the grain boundary occurring simultaneously and competitively with the continuous precipitation within the grain over a wide temperature range [19], [20], [21]. Whether Mg₁₇Al₁₂ precipitates continuously or discontinuously, no GP Zone or other transition phases has been observed. The continuous precipitates are responsible for age-hardening, whereas the discontinuous precipitates are detrimental to age-hardening. Although the maximum hardness achievable in AZ91 alloy is substantially lower than that in the precipitation-hardened aluminum alloys, the hardening response in the alloy is such as to have practical significance.

The aim of this study is to investigate the effect of SiC whisker on the aging hardening kinetics and age-hardening efficiency of squeeze cast SiCw/AZ91 magnesium matrix composite. The aging behavior has been examined using Vickers hardness and differential scanning calorimetry (DSC) analysis, combined with microstructure observation developed during heat treatment by optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM).