Crack path morphology in silicon carbide whisker-Reinforced aluminum composite

Abstract

Metal matrix composites (MMC) of aluminum alloys reinforced with whisker or particulate silicon carbide have been studied extensively in recent years. These composites have a high specific modulus and strength and a low coefficient of thermal expansion. In contrast to continuous filament MMC, they can offer isotropic properfies, easier fabricability and formability, and potentially low cost. However, like continuous f'dament MMC, their ductility and fracture toughness are low, they fail in a relatively brittle manner, and their fatigue behavior is not well understood. The objective of this study is to characterize the fatigue behavior of a silicon carbide whisker-reinforced aluminum composite. The specimen material was a silicon carbide whiskerreinforced aluminum composite, SXA24E/20W-T8E510 (19.8 vol pct SiCw/A1-3.7Cu-I.4Mg), fabricated by Advanced Composite Materials Corp., Greer, SC. This material was initially double-extruded from a 305-mm (12-inch) diameter billet to a 102-mm (4-inch) diameter rod, machined to a 76-mm (3-inch) diameter billet, extruded to a 6.4-mm  38.1-mm (0.25-inch  1.5-inch) bar, stretched after solution annealing and cold water quenching, and aged for 10 hours at 160 ~ From this material, rectangular tension-test and center-crackedtension specimens were machined, as shown in Figure 1. The tension and fatigue tests were conducted at room temperature in a laboratory atmosphere using a closedloop electrohydraulic MTS machine. During the tension test, the loading rate was 23 kN/min (5.2 kip/rain). The fatigue test was carried out under constant amplitude tension-tension loading of a haversine waveform with a stress ratio of 0.1 and frequency of 10 Hz. The specimen planes, perpendicular and parallel to the extrusion direction, were polished, etched with Keller's reagent, and examined in an Advanced Metals Research 1000 scanning electron microscope (SEM) operated at an accelerating voltage of 20 kV. The fractographic examination of tension- and fatigue-fracaLred specimens was also done using the same SEM. The scanning electron micrographs of the specimen planes perpendicular and parallel to the extrusion direction are shown in Figures 2(a) and (b), respectively. In the plane perpendicular to the extrusion direction, many white dots (transverse section images of SiC whiskers) can be seen. In the specimen plane parallel to the extrusion direction, many white rods (longitudinal images of SiC whiskers) are observed. Such a micrographic feature