Papers by Author: Qing Liu

Abstract: The effect of crystal orientation on the recovery and recrystallization of cold-rolled Ni (99.96% purity) has been investigated. Particular attention was paid to the annealing response of regions with either the Copper (C), Brass (B) or S rolling texture orientations. Samples with an initial average grain size of approx. 500μm were deformed to strains of up to εvM = 4.5. As a result of the large initial grain size, even after high rolling reductions it is possible to find sufficiently large regions of material with similar crystal orientation to analyze the recovery and recrystallization behaviour as a function of crystal orientation. Microstructural investigations were carried out in the scanning electron microscope using both electron channeling contrast and electron backscatter diffraction orientation mapping. Both the S and C orientation regions exhibit a heterogeneous microstructure containing bands of localized deformation. The presence of volumes surrounded by high angle boundaries in these regions strongly influences both the recovery and recrystallization behaviour of the material. Twinning was observed also to play an important role in the generation of recrystallized grains, with twin chains of up to 3 generations being observed.

Abstract: A series of Al-Si hypoeutectic alloy ingots were prepared by centrifugal casting (CC) both with and without an external electromagnetic field (EMF). The effects the EMF on grain refinement and on the solidification mechanism during centrifugal casting have been investigated using optical microscopy, scanning electron microscopy (SEM) and electron backscattering diffraction (EBSD) orientation measurements. The electromagnetic stirring effect caused by the EMF promotes the columnar-equiaxed transition (CET) resulting in a finer cast grain size. The silicon and aluminum nucleate and grow independently both with and without EMF

Abstract: An experimental study of microstructure and mechanical properties in the Al2O3 particulate reinforced 6061 Aluminum composites has been used to determine the effect of extrusion and particle volume fraction (20, 26, 30, 40, 50, 60%Vf) in deformed metal matrix composites. The microstructure of Al2O3 /6061Al composite before and after hot extrusion is investigated by TEM and SEM. Results show that dislocation and subgrain generated after hot extrusion as well as the particle distribution of composite become more uniform with extrusion ratio of 10:1. The ultimate strength, yield strength and elongation of the composite also increase after hot extrusion. Dispersion strengthening and subgrain boundary strengthening is discussed and also the effect of precipitate introduced by heat treatment both after casting and after extrusion. The yield stress (0.2% offset) of the composites has been calculated and predicted using a standard dislocation hardening model. Whilst the correlation between this and the measured value of yield stress obtained in previous experimental test is reasonable.

Abstract: The dependence on the grain orientation of the alignment of planar dislocation boundaries in plastically deformed metals has been investigated by examining grains of S orientation ({123}<63-4>) in cold-rolled polycrystalline aluminum. For the ideal S orientation the {111} slip plane associated with the highest resolved shear stress lies either at +40° or -40° to the rolling direction in the longitudinal section, with two S variants corresponding to each case. Boundary traces in S orientation grains in the rolled sample were examined by the combined use of electron channeling contrast imaging and electron backscatter diffraction orientation mapping. In each case the +/- sense of the observed planar boundary traces matched that of the {111} slip plane with the highest resolved shear stress showing that the alignment of the boundaries is predominantly controlled by crystallographic rather than macroscopic considerations.

Abstract: A pure Ni single crystal of Cube orientation (001)/[100] have been deformed by reversible cold rolling to a reduction of 98% following an initial extrusion treatment. The microstructure in the as-deformed samples and in partly recrystallized specimens have been characterized in the scanning electron microscope using electron backscattered diffraction (EBSD). Line scans along the normal direction across the sample thickness show some evidence for a macroscopic pattern of subdivision. The EBSD measurements show that the deformed sample is characterized by large jumps in orientation between alternating S and Br orientations, related by misorientation angles of 50°~60°. In part of the sample a background cumulative change in orientation is seen, though local large change in orientation are also still seen. In annealed samples it is found that the orientations of the new grains

Abstract: Polycrystalline nickel (99.999% purity) cold-rolled to a reduction of 96% has been given a two-step annealing treatment consisting of a pre-annealing at 180 °C or 220 °C for 2 hours, followed by a final annealing at 300 °C for 10 minutes. The changes in microstructure and texture during annealing have been followed using electron backscatter diffraction orientation measurements taken on the longitudinal section of the samples. The results suggest that pre-annealing at low temperature can enhance the formation recrystallized grains of cube orientation. Based on an analysis of the experimental data, possible reasons for the enhancement as a result of pre-annealing in the formation of cube orientation grains are discussed.

Abstract: The influence of an electric field on the annealing of high purity (99.999%) cold rolled nickel has been investigated. Annealing was carried out for 2 hours at temperatures between 300oC and 800oC with and without an electric field of strength 2.0KVcm-1. The microstructure and fraction of cube texture resulting were characterized using electron backscattering pattern (EBSP) technique. Annealing in an electric field leads to somewhat smaller average values of the cube fraction and grain sizes compared to annealing without an electric field. The highest temperature (800oC) annealing in an electric field results in microstructures with a lower fractional twin boundary length.

Abstract: In order to improve the formability of magnesium alloys at low temperatures, a thorough understanding of the twinning behaviour is required. Cylindrical samples were compressed uniaxially from room temperature to 250oC, with the compression axis parallel to the transverse direction (TD) of a hot-rolled Mg-3Al-1Zn sheet. Twinning was investigated by use of orientation maps constructed from electron backscattered diffraction (EBSD) data. Tensile twinning was observed at all temperatures. In some grains two twin variants were observed. For each twin the specific twinning variant among the 6 possible twinning systems was identified by consideration of the twinning rotation. The twinning was then investigated by comparing the specific twin variant with the calculated Schmid factors for all 6 possible twin variants. In cases of grains that are completely reoriented by two twin variants it is possible to back-calculate the initial grain orientation by considering all possible twin rotations for both of the twin variants. It was found that the operative twin variants where those with the highest Schmid factors.