Year

1995

Degree Name

Doctor of Philosophy

Department

Department of Materials Engineering

Abstract

The deformation and fracture behaviour of 7075 aluminium alloy reinforced with 15% vol. of SiC particles and 7075 monohthic alloy were studied. The study involved both compression and tensile modes of deformation at high temperature. The effect of temperature and strain rate on hot workability was examined using uniaxial compression test in the temperature range of 250-450°C and strain rates of 0.001-1 s-1. The results showed that the flow curves exhibited strain hardening at low strain followed by steady state stress at high strain as generally shown in Al-alloys at high temperature. The composite was stronger compared to monohthic alloy at lower temperatures, but both possessed similar strength above 400°C. The higher flow stress at lower temperatures seems to be due to higher dislocation density and slow rate of recovery in the matrix.

The Selected Area Diffraction pattern (SADP) showed that the most subboundaries were low angle boundaries, however, some highly misoriented grains were observed at 450°C. The results indicate that dynamic recovery is the dominant restoration process during deformation in the composite and large SiC particles has no significant effect on deformation behaviour at elevated temperature. The composite showed fine substructure with high dislocation density at lower temperatures up to 300°C, but as temperature increased the dislocation density decreased and the subgrain size increased. The reciprocal subgrain size, d-1, showed a linear relationship with log z. Activation energy was found to be 168 kJ mole-1 which is higher than that for the monohthic alloy (151 kJ mole-1).

The double pass deformation was carried out in compression at 300 and 400°C at a strain rate of 1 s-1. Between two passes the delay time was varied from 6 to 900 sec to study its effect on fractional softening (FS). It was found that fractional softening increased substantially with increasing temperature but marginal increase in FS obtained when delay time between passes increased. The degree of FS observed in 7075Al/SiCp composite and monohthic alloy was found to be lower compared to other aluminium alloys. This can be attributed to the stabilizing effect of dispersions and fine precipitates which are presented in 7075 Al matrix. The insignificant changes in the microstructure during interpass hold times shows that the low FS in these alloys seems to be due to static recovery. It was found that microstructure and substructure generated in the samples either by continuously or interrupted straining in compression were similar at a given temperature.

In the second phase of high temperature deformation of these materials the deformed samples were annealed at high temperature to study the static recrystallization ( SRX ) after hot working. The results showed an absence of SRX in the samples annealed after deformation at the same temperature, however, a rise in annealing temperature of 100-150°C above that the deformation temperature led to full recrystallization. This can be ascribed to the relatively moderate dynamic recovery and the presence of dispersions which stabilize the substructure. Particle stimulated nucleation (PSN) had a significant effect on the grain size in deformed samples at low temperature, but no P S N was observed in samples strained at high temperatures. The possible cause might be that at high temperature the dislocation can be annihilated by climb process around the particles together with absence of deformation zone for nucleating the recrystallization.

In the tensile mode of straining at high temperature, the tensile tests were carried out to study the ductility and fracture behaviour of the composite. The tests were performed in the temperature range from room temperature to 400°C at strain rate of 0.001 s-1. T wo different conditions of heat treatment were used (T6 and annealed). The results showed that the ductility increased as temperature increased up to 350°C and remained nearly constant as temperature increased to 400°C. The composite showed a much lower ductility compared to the monohthic alloy at all temperatures, although both materials exhibited similar strength levels at temperature above 300°C.

The results of fracture behaviour showed that at room temperature the fracture mechanism changed from particle fracture in T6 condition to interface decohesion in annealed condition. In T 6 condition, the fracture mechanism changed from particle fracture at room temperature to interparticle voiding at high temperatures. Particle fracture was found predominantly in cluster of particles regions where large particles were present. This can be attributed to the high local stress in these regions and high probability of flaws in large particles. At high temperature the composite revealed a dimpled fracture surface indicating ductile fracture of the matrix. The internal damage associated with particle cluster was observed in regions below fracture surface and the amount of damage increased with increasing temperature. It was found that the cluster of particles regions and large particles regions were prone locations to permanent damage in these materials at both room and high temperatures.

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