A study of the high strain rate behaviour of particle-reinforced metal matrix composites

Field: Materials Science (Physical Sciences)

This thesis presents the results of an experimental and analytical study of the high strain rate behaviour of ceramic particle-reinforced metal matrix composites (MMC). Two MMC systems, both based on the 6061-T6 aluminum matrix, were selected. The first is an alumina reinforced system, made by a liquid metallurgy (LM) route, with 10, 15 and 20% particle volume fractions. The second is a silicon carbide system, made by powder metallurgy (PM), with 0, 15 and 30% particle volume fractions. Unreinforced 6061-T6 and 7075-T6 were also included for comparison. Quasi-static tensile tests, Taylor impact tests, and high velocity penetration tests were conducted. The tension test results indicated that the reinforcement strongly affects the stiffness, strength and ductility. Some anisotropy was also observed. The Young's modulus values for both system are in good agreement with predictions from simple two-phase theoretical models. An experimental facility was constructed which is capable of accelerating small cylindrical impactors at velocities up to 1000 m/s and allow for accurate measurement of the impact velocity. The facility was designed so that both Taylor and dynamic penetration tests could be performed with only minor modifications. The Taylor test was used to characterize the strength of the MMC selected under conditions comparable to those existing in dynamic penetration. It consists of impacting short cylindrical specimens on a flat rigid anvil at velocities ranging from 150 to 300 m/s. The dynamic yield strength was determined from measurements of the deformed shape of the specimen using one-dimensional analysis models. The results were shown to be quite dependent on the analysis model used for calculation. Results show that the dynamic strength is noticeably increased over the quasi-static values. The strain rate sensitivity of the MMC materials also appeared to be more pronounced. Measurements of the tested specimen profiles revealed some asymmetry which can be attributed to yield strength anisotropy. The MMC specimens also appeared to be more susceptible to radial cracking at the impact face. The effects of adiabatic heating and inertia within the specimen were also investigated. To assess the relative impact performance of the selected materials, dynamic penetration tests were conducted by firing small rigid tungsten rods with spherical noses on to MMC cylindrical targets with a diameter of 50 mm and a length of 150 mm. Tests were performed at three average impact velocities of 475, 750 and 920 m/s. The cavity profiles were determined from X-ray photographs. The dynamic penetration tests indicate that the PM-processed materials are more resistant to penetration than the LM-processed materials, with the difference being more significant at higher volume fractions. At low velocities (475 m/s) large scale radial cracking of the highly reinforced MMC was observed. The penetration depths were predicted using an approximate cavity expansion model developed for monolithic metals and which involves only a few measurable material properties. Sensitivity studies indicate that, for the intermediate velocity regime investigated in this study, the dynamic strength of the target material is the dominating parameter. Sliding friction at the impactor/target interface was also shown to influence the penetration behaviour to a lesser degree. Using the strength values obtained from the Taylor impact tests, the cavity expansion model predicted depths that were in reasonable agreement with the experimental results.