Year

1993

Degree Name

Doctor of Philosophy

Department

Department of Materials Engineering

Abstract

The deformation behaviour of 6061 aluminium reinforced with particulate AI2O3 was studied by cold working as well as by high temperature compression. In addition to deformation studies, the ageing behaviour of the composite was also examined by ageing the material at 125 and 170°C isothermally. It was found that peak hardness appeared much earlier in reinforced alloy than in monolithic alloy when the materials were aged at 170°C suggesting that the existence of reinforcement could accelerate ageing. Studies showed that P" phase (or called GP-II zone) was the only hardening agent in 6061Al alloys, and changes in morphologies of P" phase would result in variation in hardness. During ageing, p" phase started appearing in a short needle-shape, then fine needle shape which gave the samples' highest hardness, and as ageing continued, the p" phase became coarse and the hardness of the alloy decreased. Observations using TEM and differential scanning calorimeter (DSC) indicated that formation and coarsening of the P" phase (or called GP-II zone) were accelerated in the reinforced alloys. This acceleration is probably due to high diffusivity resulted from high density of dislocation and elastic strain field generated by thermal mismatch between the reinforcement and the matrix. However, there was little difference in ageing response for reinforced and unreinforced alloys when they were aged at 125°C. This is because at low temperatures, formation and coarsening of the p" phase are very slow, and the effect of the reinforcement is not obvious. The composite and the matrix alloy were also aged in a temperature range of 125 to 350°C for one hour and it was found that accelerated ageing occurred in the composite during isochronal ageing.

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