| The main objective of this study was to examine the possibility of improving the fracture toughness of metal matrix composites via alteration/improvement of the ductility of the matrix through heat treatment. Tensile and fracture toughness testing, based on relevant ASTM standards, have been performed on specimens with different heat treatment conditions. Two solution treatment temperatures of 560 and 530°C were used during the heat treatment. Artificial aging of the specimens was carried out at two temperatures of 150 and 200°C as a function of time over the range of 15 minutes to 450 hours. Both conventional and Field Emission Scanning Electron Microscopy (SEM and FE-SEM) as well as Transmission Electron Microscopy (TEM) have been used to study and correlate fracture mechanism(s) and microstructural aspects. Based on the results from the present project, the effect of heat treatment on fracture toughness values was found similar to that observed in the monolithic alloy. However, the rate of recovery in fracture-related properties of MMCs is lower (or in some cases zero) in the over-aged condition. This “low or no” recovery behaviour for over-aged MMCs can be explained by: (i) clear increase in the size of shear lips with increasing aging time, and (ii) a shift from particle fracture (in under-aged regime) to interface (or near interface) debonding in over-aged regime. The formation/presence of a spinel phase at the interface is recognized as the main cause of this behavior. Although, spinel products mainly form during material processing, their formation is found to continue during the solid state thermal heat treatment as well. Alternatively, the spinel/matrix bonding may go through a drastic change in the long aging process, hence affecting the fracture behaviour. |
