Axial creep and stress-rupture of B-Al composites

Constant load creep and stress-rupture properties of boron and silicon carbide coated boron, BORSIC^®, filament reinforced aluminum alloys were measured at 300°, 400°, and 500°C. The mean stresses for 1000 hr rupture life at the three test temperatures were 96,000, 82,000, and 72,000 psi, respectively. Elongation in creep tests was checked by using scribed lines, and this technique indicated that grip-mounted extensometers were inadequate for precision measurement due to shear of aluminum in the grips. A maximum of 0.2 pct plastic elongation was measured, which corresponds to the creep properties of the filaments. Comparison of Borsic composite results with data for composites fabricated with uncoated boron filament showed that the short-time rupture strengths of the two materials were similar, but that the stress for rupture of the boron filament composites decreased much more rapidly with increasing rupture time than did the stress for rupture in the Borsic filament composites, due to degrading reactions of the uncoated boron with the matrix. Fracture analysis was performed using optical metallography and scanning electron microscopy. The analysis indicated that the major portion of filament failures in the region of the fracture surface at elevated temperatures initiated at the outer surface of the filament, whereas filament failures that initiated in the region of the core were more common in room temperature tensile specimens. The amount of plastic flow in the matrix increased markedly with increasing temperature, but filament pullout lengths were far less than those predicted by a simple shear-lag theory on the basis of the shear strength of the matrix.