Metal matrix composites are known for their excellent specific mechanical properties. Nevertheless, because of the thermal expansion mismatch between the matrix and the reinforcements, thermal stresses arise at the interfaces of such materials. In order to relax these stresses, dislocations are emitted from the interface and propagate in the matrix, which can influence the mechanical behavior of the composite material. Mechanical spectroscopy, which characterizes the ability of a solid to dissipate energy under an external excitation, was used to study the relaxation mechanisms occurring at the interface of MMCs. Indeed, the mobile matrix dislocations near the interface are the main sources of damping, making this technique very sensitive to these mechanisms. Composites studied here were processed by gas-pressure infiltration of the reinforcement preform by the molten metal. The processed composites were based on aluminum, magnesium and Mg-2%Si alloy, which is known for its high damping level. They were reinforced by short misorientated alumina fibers or long and aligned carbon and silicon carbide fibers. In the case of long fibers, two types of orientation were obtained: parallel or perpendicular to the composite axis. In order to modify the interface morphology, a heat treatment was carried out on aluminium matrix composites reinforced by alumina fibers during the infiltration process. By increasing the contact time between the molten matrix and the fibers, alumina crystals were formed on the surface of the fibers, whose size increased with time. The specific elastic moduli of the processed composites were clearly superior to those of matrix alone and their value agreed well with the existing theoretical models. Composites were submitted to thermal cycles from 120 K to 500 K and the internal friction and dynamical modulus were measured as a function of temperature. It was shown that the behavior of these two parameters, the large maximum at low temperatures and the modulus anomaly in the case of magnesium composites was driven by the motion of the dislocations activated in the matrix in order to relax thermal stresses. The internal friction was also characterized by a transient contribution, depending on the heating or cooling rate dT/dt and the excitation frequency ω. By using a model developed by Mayencourt and al., it was possible to determine two parameters C1 and C2 which were sensitive respectively to the mobile dislocation density relaxing the thermal stresses and the interface strength. The difference in the behavior of these two parameters as a function of temperature tended to show the great potential of the magnesium matrix composites. Indeed, the interface strength was decreasing at low temperature, allowing a better toughness whereas it was increasing at high temperature, improving the creep resistance. Finally, in the case of the aluminum matrix composites reinforced by short fibers with alumina crystals at the interface, it was observed that the elastic modulus was decreasing as crystal size increased. However, when the crystals exceeded a certain size, they were acting as pinning points between matrix and fibers, resulting in the increase of the modulus almost to its initial value. By determining the parameter C1 from the transient damping, it was shown that this parameter followed the same trend as the elastic modulus, which made it a good probe for the interface quality.
EPFL2004
