Hybrid Effect on Whisker Orientation Dependence of Composite Strength of Aluminum Cast Alloy Reinforced by Al2O3 Whiskers and SiC Particles

Abstract

The hybrid effect on the orientation dependence of the composite strength of an aluminum cast alloy reinforced by Al 2 O 3 whiskers and SiC particles is studied experimentally and numerically. Two types of specimens are prepared for monotonic bending tests. The longitudinal specimen orientation (maximum stress direction) is parallel to or normal to randomly oriented whiskers in plane. The monotonic strength is 18% higher when the hybrid metal matrix composite (MMC) is subjected to an external load parallel to the random whisker orientation in plane than when the load is perpendicular to the whisker orientation. The whisker orientation dependence of composite strength in hybrid composite is weaker than that in whisker-reinforced composite. On the fracture surface of the specimen loaded along the direction parallel to the random whisker orientation in plane, most whiskers are broken while many de-bonded interfaces between the whiskers and matrix are observed on the fracture surface of the specimen loaded along the direction perpendicular to the whisker orientation. To characterize the hybrid effect on the whisker orientation dependence of composite strength, a three-dimensional hybrid composite unit cell model including one whisker and a few particles under a periodic boundary condition is developed using the finite element method. The hybrid composites have higher whisker stress than whisker-reinforced composite when subjected to an external load parallel to the whisker orientation if these composites have the same total volume fraction of reinforcement and the particles are distributed randomly. Under an external load perpendicular to the whisker orientation, the interface stress of hybrid composites is lower than that of whisker-reinforced composite. As a result, the strength difference for parallel and perpendicular loading conditions of the hybrid composites is smaller than that of whisker-reinforced composite. Thus, the weak whisker orientation effect in the hybrid composites is due to a change in microscopic stress distribution induced by interaction between whiskers and particles.

Effect of hybrid reinforcement on crack initiation and early propagation mechanisms in cast metal matrix composites during low cycle fatigue

“…The chemical composition of AC4CH alloy is listed in Table 1. As described in the previous report [18], due to the formation process of the preform the whiskers in the both MMCs are randomly oriented in a plane. 'Longitudinal cross section' is defined as the plane and 'lateral cross section' is defined as the one perpendicular to the plane in this article.…”

“…It may also merely be the result of the load sharing between the high-modulus reinforcement and the low modulus matrix [16]. The fracture mechanism of cast hybrid MMCs under monotonic and cyclic loads was investigated by Rafiquzzaman et al [17,18], who observed fracture surfaces and concluded that the fracture under cyclic load was dominated by interfacial debonding at particle/matrix and whisker/ matrix interfaces. This interfacial debonding occurred due to the cyclic hardening caused by the accumulation of plastic strain at the interface between matrix and reinforcement.…”