Interface degradation in metal-matrix composites under cyclic thermo-mechanical loading

Abstract

Thermal cycles under constant load have been performed on [±45]sCAl and SiCAl composites. Effects of temperature amplitude and tensile stress levels were investigated to improve the design of metal-matrix composites structures and to prevent their failure.Transverse sections of the composites before and after thermal cycling have been examined by scanning electronic microscopy (SEM) and electron-probe micro-analysis (EPMA). For composites with a weak interface the degradation mechanisms resulting from thermal fatigue tests under zero load were associated with debonding and cracking of the fiber/matrix interfaces. However, no plastic deformation of the matrix and no chemical reaction at the interfaces were observed. Conversely, for relatively strong interfaces the degradation mechanisms were associated with the plastic deformation of the matrix and the formation of interfacial reaction products. These degradations are mainly attributed to the thermal expansion mismatch between the fiber and the matrix and lead to a decrease in most mechanical properties.When submitted to thermal cycling under constant loads, both composites exhibit a progressive plastic deformation which increases with the number of cycles, even at stress levels far below the yield stress. This phenomenon, related to thermal ratchetting, is expressed as a function of both the applied stress and the amplitude of thermal cycles by a power creep law. The duration of exposure at the highest temperature of the thermal cycle does not play a significant rôle in determining the ratchetting rate. SEM observations show reaction products at the surfaces of the fibers. However, it appears that the main phenomenon leading to composite failure is ratchetting for high load levels and interface degradation for low load levels.