The influence of secondary processing conditions on the mechanical properties and microstructure of a particle reinforced aluminium metal matrix composite

The influence of secondary processing conditions on an aluminium metal matrix composite, comprising of an AA2124 matrix and 3 Jlm particulate SiC reinforcement at 25 volume percent was investigated. The metal matrix composite (MMC) was extruded at three different temperatures, 350°C, 450°C and 550°C, at a ratio of 20:1 and at three different ratios, 5:1, 10:1 and 20:1, at a temperature of 450°C. It was subsequently solution heat treated and naturally aged. A mechanical property assessment was carried out using standard tensile and rotating bend fatigue test methods to determine the properties of the material extruded under each condition. A novel technique using a Focussed Ion Beam (FIB) Microscope was developed to prepare polished specimens and microtextural analysis was performed by FIB imaging. Additionally, techniques were successfully established, through the use of FIB milling and polishing, to provide site-specific electron transparent films, permitting detailed examination ofthe microstructure with a transmission electron microscope. Material extruded at 550°C exhibited a lower yield strength than material extruded at 350°C and 450°C, which was attributed to grain coarsening and recrystallisation. Evidence of recrystallisation was found during texture analysis by X-Ray diffraction, where there was a reduction in the intensity of the fibre texture in the extrusion direction. The phenomenon was also observed during irticrostructural analysis work, where recrystallised grains at grain boundaries were observed. Higher extrusion ratios offered a small improvement in tensile properties, due to an enhanced fibre texture within the microstructure. Microtextural examination gave evidence of the existence of both high angle grain and low angle grain boundaries for the material extruded at 350°C. It is believed that a subgrain structure was partially transformed during extrusion, through subgrain rotation, leading to the formation of high angle grain boundaries. This microstructure was found to offer the optimum mechanical properties.EThOS - Electronic Theses Online ServiceGBUnited Kingdo