Year

2018

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials, Mechatronic and Biomedical Engineering

Abstract

As a technique to produce ultrafine grained (UFG) materials, severe plastic deformation (SPD) has engaged considerable investigation over last two decades. HPT is one of the most important SPD techniques because of its unique advantages, including the extreme grain refinement capability, over other SPD techniques. In this work, the evolution of microstructure, texture, hardness and grain boundary (GB) migration in the one-layer and two-layer stacked polycrystalline Al and Al single crystal processed by HPT have been investigated in detail.

The one-layer polycrystalline aluminium was employed to study material flow behaviour for a wide range of strains. The morphology, texture and hardness were used to characterise the material flow. It was found that the grain refinement is not a linear function of shear strain. The UFG structures were obtained at both middle position and edge position after HPT with the rotation angle of 1800°. The distribution of hardness was inhomogeneous throughout the diameter of the disc, even after a large number of HPT revolutions. The central hardness was significantly lower than the hardness at the periphery of the HPT deformed discs. Initial shaped grains, band-shaped structures and small equiaxed grains dominate at the stages with the low, medium and high strains, respectively. The intensities of textures in HPT were weak, due to the change of the shear direction (SD). The gradients of the microstructure and texture were observed at the low strain stages, but the gradients disappeared at the stages with the high shear strain.

The two-layer polycrystalline aluminium was processed by HPT. It was found that the evolution of microstructure in the two-layer specimen is similar to that of the one-layer specimen. But the gradient of the morphology, grain size and texture along the specimen thickness was reduced compared with the one-layer specimen, indicating that the material flow in the two-layer specimen was more stable. In terms of grain refinement, no advantage could be found in the two-layer specimen at the low and high strain levels. However at the stages with the medium strain, the grain size of the two-layer specimen was obviously smaller, especially after the rotation angle of 720°. Strong evidences of shear-induced GB migration were observed in the HPT processed specimens. The GB migration along the preferred orientation resulted in shrinkage or expansion of the particular grains.

The one-layer and two-layer Al single crystals were deformed by HPT with low strains. At the centre position of the specimen, the deformation does not follow the typical simple shear mode. The material flows around the shear plane normal (SPN) axis. At the middle position, the band-shaped structures prevailed. At the edge position, the band-shaped structures were fragmented into the relatively equiaxed grain. The GB migration was observed in the HPT processed specimens. It is found that the magnitude of the shear strain affected the GB migration. The GB migrations occurred at the HAGBs, whereas the Σ41a and Σ5 GBs were stationary.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.