Papers by Author: M. Liu

Abstract: Nanostructures and microhardness of a commercial purity Al, three binary Al–Mg alloys and a commercial AA5182 alloy subjected to high pressure torsion (HPT) at room temperature were comparatively investigated using high-resolution transmission electron microscopy, X-ray diffraction (XRD) and high-resolution XRD line profile analysis. The hardness values of HPT samples are twice to three times larger than that of the undeformed counterparts. Grain sizes measured by XRD are in the range 10–200 nm with typical average values ranging from 46 to 120 nm. The hardness values and the dislocation densities increased, whereas, the average grain size decreased significantly with increasing Mg contents. Typical dislocation densities are in the range 1.7 × 1014 m-2 – 2.3 × 1015 m-2. However, local densities in grain boundary and triple junction areas might be as high as 1017 m-2. The strengthening mechanisms contributing to high hardness may primarily be attributed to the cooperative interactions of high dislocation densities, grain boundaries and planar interfaces.

Abstract: An Al–0.5 Mg alloy and a commercial AA5182 alloy were subjected to high pressure torsion (HPT) to five turns under pressure of 6 GPa at room temperature. The grain boundary structure and deformation defects were investigated after HPT using high-resolution transmission electron microscopy (HRTEM). Low-angle, high-angle, equilibrium and non-equilibrium grain/subgrain boundaries, twin boundaries, full dislocations, dipoles, microtwins and stacking faults were identified by HRTEM. Extrinsic 60° dislocations in the form of dipoles were frequently observed in non-equilibrium grain/subgrain boundaries. In addition subgrain size distributions and dislocation densities were quantified by x-ray line profile analysis. It was observed that the average grain size decreased from about 120 nm to 55 nm as the Mg content increased from 0.5 to 4.1 wt%. Concomitantly the average stored dislocation density increased from 1.7 to 12.8  1014 m-2. Based on the HRTEM investigations and the x-ray line profile analyses, the deformation mechanism associated with the typical grain boundaries and deformation defects in the aluminium alloys were discussed.

Abstract: Cyclic extrusion compression (CEC) is an effective severe plastic deformation (SPD) process which can be used for fabricating ultrafine grained light materials such as magnesium alloys. This method introduces three-dimensional compression and shear stresses and the process can be repeated for a certain number of passes until the desired accumulated strain has been introduced. In order to reveal the effect of second phases on the microstructure developed in magnesium alloys during CEC, three different alloys (AZ31, AZ31-1wt.%Si and AZ91) were investigated after CEC 7 passes performed at 225°C. The experimental results show that the CEC process can effectively refine the microstructures of these alloys and the mean grain size achieved is 1.3µm, 1.5µm and 1.4µm, respectively. It is revealed that the grain size, grain shape and grain boundary structures are little affected by coarse phase Mg2Si but strongly affected by the fine phase Mg17Al12. The fine phase Mg17Al12 seems to increase the relative grain misorientations, hence enhancing the formation of high angle grain boundaries (HAGBs). It is expected that such changes are improving mechanical properties, subsequent forming behavior and surface quality.

Abstract: High-resolution transmission electron microscopy investigations revealed different types of deformation structures in a nanostructured commercial Al–Mg alloy processed by high pressure torsion at room temperature. Microtwins and stacking faults were detected within both nanocrystalline grains and ultrafine grains. Full dislocations in the form of dipoles were observed within grains and near the grain boundaries. Two twinning mechanisms previously predicted by molecular-dynamics simulations were directly verified including the heterogeneous twins nucleated by the successive emission of Shockley partials from grain boundaries and homogeneous twins formed in the grain interiors by the dynamic overlapping of stacking faults. Hence, the formation of full dislocations, stacking faults and twins in the present aluminum alloy subjected to severe plastic deformation may be interpreted in terms of molecular-dynamics simulations based on generalized planar fault energy curves for pure metal systems.