Thermal degradation of the tensile strength of unidirectional boron/aluminum composites

We have systematically studied the variation of ultimate tensile strength with thermal treatment of B-Al composite materials and of boron fibers chemically removed from these composites in an attempt to determine the mechanism of the resulting strength degradation. This knowledge will be of value in designing to extend the use-temperature of these composites. Our findings indicate that thermally cycling B-Al represents a more severe condition than equivalent time at temperature. Degradation of composite tensile strength from about 1.3 GN/m2 to as low as 0.34 GN/m2 was observed after 3000 cycles to 420°C for 203 μm B-1100 Al composite. In general, the 1100 Al matrix composites degraded somewhat more than the 6061 matrix material studied. Measurement of fiber strengths confirmed a composite strength loss due to the degradation of fiber strength. Microscopy indicated a highly flawed fiber surface. On the basis of the thermal cycling studies in air and in the absence of air and of electron diffraction analysis of the reaction zone, a mechanism is favored in which B reacts with Al, freshly exposed by cold working during cycling, to form AIB2. The nonuniform interface reaction leads to a highly flawed and weakened B fiber.