Xiao Guang Qiao

A model of grain refinement and strengthening of Al alloys due to cold severe plastic deformation

This paper presents a model which quantitatively predicts grain refinement and strength/hardness of Al alloys after very high levels of cold deformation through processes including cold rolling, equal channel angular pressing (ECAP), multiple forging (MF), accumulative roll bonding (ARB) and embossing. The model deals with materials in which plastic deformation is exclusively due to dislocation movement within grains, which is in good approximation the case for many metallic alloys at low temperature, for instance aluminium alloys. In the early stages of deformation, the generated dislocations are stored in grains and contribute to overall strength. With increase in strain, excess dislocations form and/or move to new cell walls/grain boundaries and grains are refined. We examine this model using both our own data as well as the data in the literature. It is shown that grain size and strength/hardness are predicted to a good accuracy.

Fabrication of MEMS components using ultrafine-grained aluminium alloys

A novel process for the fabrication of a microelectromechanical systems (MEMS) metallic component with features smaller than 10 µm and high thermal conductivity was investigated. This may be applied to new or improved microscale components, such as (micro-) heat exchangers. In the first stage of processing, equal channel angular pressing (ECAP) was employed to refine the grain size of commercial purity aluminium (Al-1050) to the ultrafine-grained (UFG) material. Embossing was conducted using a micro silicon mould fabricated by deep reactive ion etching (DRIE). Both cold embossing and hot embossing were performed on the coarse-grained and UFG Al-1050. Cold embossing on UFG Al-1050 led to a partially transferred pattern from the micro silicon mould and high failure rate of the mould. Hot embossing on UFG Al-1050 provided a smooth embossed surface with a fully transferred pattern and a low failure rate of the mould, while hot embossing on the coarse-grained Al-1050 resulted in a rougher surface with shear bands

Equal Channel Angular Pressing of Magnesium Alloy Containing Quasicrystal Phase

ZWK510 (Mg-5.0wt%Zn-0.9wt%Y-0.2wt%Zr) magnesium alloy containing Mg3YZn6 quasicrystal phase was prepared by conventional permanent mold casting. Part of the cast ingot was subjected to equal channel angular pressing (ECAP) directly; another part of the cast ingot was extruded initially, then ECAP was applied to the extruded alloy. After 4-pass ECAP, the fraction of coarse grains of the as-cast alloy was decreased to about 30%, and the grain size of fine grain was decreased to about 2 μm. Both strength and ductility of the as-cast ZWK510 alloy were significantly improved with increasing ECAP passes, which was resulted from broken and dispersed I-phase, and fine grains formed due to recrystallization. The as-extruded ZWK510 had an initial grain size of about 2 μm and bands of quasicrystal phase parallel to the extrusion direction. After the extruded alloy was subjected to ECAP, the grain size of the extruded alloy was further refined, the grain size was refined to below 0.5 um after 8-pass ECAP; and the quasicrystal phase was further broken and dispersed in the matrix. After ECAP, the elongation to failure of the extruded alloy was improved. However, both yield strength and ultimate tensile strength were decreased, which is considered to be resulted from the texture modification during ECAP.

The effect of high-pressure torsion on the behaviour of intermetallic particles present in Al–1Mg and Al–3Mg

The effect of severe plastic deformation on intermetallic particles was investigated using high-pressure torsion on an Al–1Mg–0.2Si–0.2Fe–0.3Mn alloy and an Al–3Mg–0.2Si–0.2Fe–0.3Mn (wt%) alloy. Extensive optical microscopy and scanning electron microscopy was performed to analyse the intermetallic particles using image analysis software. It was found that all the intermetallic particles decreased in size with increasing strain whilst their spatial distribution was homogenised. A greater decrease in size was found for the intermetallic particles present in Al–1Mg alloy and the possible causes are discussed. Even though the strain near the centre of the sample is close to zero, refinement of intermetallic particles is substantial at this location.

Microstructure and Tensile Properties of Magnesium Alloy Containing Quasicrystallines Processed by Equal-Channel-Angular-Extrusion

Equal channel angular extrusion (ECAE) was performed on as-cast ZW1101 (Mg-11wt%Zn- 0.9wt%Y) Mg alloy containing quasicrystallines. The grain size of α-Mg was effectively refined, and coarse eutectic quasicrystalline phases were broken and dispersed in the alloy by ECAE. The alloy processed by ECAE exhibited a good combination of high strength and high ductility, which is due to the grain refinement and fine dispersed quasicrystallines in the alloy.

Effect of Equal Channel Angular Extrusion on the Microstructure and Mechanical Properties of Magnesium Alloy Containing Quasicrystallines

Equal channel angular extrusion (ECAE) was applied to an extruded ZW1101 (Mg - 11wt%Zn - 0.9wt%Y) Mg alloy containing quasicrystallines. The as-extruded ZW1101 alloy had an initial grain size of about 12 µm and bands of quasicrystalline phases parallel to the extrusion direction. After the extruded alloy was subjected to ECAE processing, the grain size was refined to about 0.5 µm, and the quasicrystalline phases were further broken and dispersed in the matrix. After the ECAE processing, the micro-hardness and yield strength of the alloy were increased, however, the ultimate tensile strength and the ductility of the alloy were slightly decreased.

Mechanical Properties and Damping Behavior of Magnesium Alloys Processed by Equal Channel Angular Pressing

Equal channel angular pressing (ECAP) was applied to commercial pure magnesium alloy, Mg-1wt%Si alloy and Mg-4.2wt%Zn-0.7wt%Y alloy. With increasing ECAP passes, both tensile strength and ductility of the alloys are increased, which are mainly resulted from the grain refinement. At the same time, for the Mg-Zn-Y alloy with inherent low damping capacity, damping capacity is increased after ECAP passes, however, the damping capacity is still low even after 6-pass ECAP. While for the commercial pure magnesium and Mg-Si alloy with inherent high damping capacity, although the damping capacity is decreased obviously after ECAP, Q-1 is still greater than 0.01. The damping capacity after ECAP processing is mainly influenced by grain size and deformation microstructure. ECAP paves a way for the development of magnesium alloys with high strength and high ductility combined with high damping capacity.

Microstructure and Tensile Properties of a Mg-Zn-Y-Zr Alloy Containing Quasicrystal Phase Processed by Equal Channel Angular Pressing

Equal channel angular pressing (ECAP) was performed on extruded Mg-Zn-Y-Zr (Mg-5.0wt%Zn-0.9wt%Y-0.2wt%Zr) alloy at 300 oC. After 8 ECAP passes, average grain size of the alloy was reduced to about 1.4 μm, and the quasicrystalline phases were broken and dispersed in the matrix. In addition, nano- quasicrystallines were precipitated from the matrix during ECAP processing. After ECAP, the elongation to failure of the extruded material was significantly improved. Only after 2 ECAP passes, the elongation to failure was 29%, and after 8 ECAP passes, it reached 35%, which was three times larger than that of the as-extruded alloy. However, both yield strength and ultimate tensile strength were decreased with the increasing ECAP passes, which was considered to be resulted from the {0002} basal plane texture modification during ECAP.

Microembossing of ultrafine grained Al: microstructural analysis and finite element modelling

Ultra-fine-grained (UFG) Al-1050 processed by equal channel angular pressing and UFG Al–Mg–Cu–Mn processed by high-pressure torsion (HPT) were embossed at both room temperature and 300 ?C, with the aim of producing micro-channels. The behaviour of Al alloys during the embossing process was analysed using finite element modelling. The cold embossing of both Al alloys is characterized by a partial pattern transfer, a large embossing force, channels with oblique sidewalls and a large failure rate of the mould. The hot embossing is characterized by straight channel sidewalls, fully transferred patterns and reduced loads which decrease the failure rate of the mould. Hot embossing of UFG Al–Mg–Cu–Mn produced by HPT shows a potential of fabrication of microelectromechanical system components with micro channels.

Shear deformation and texture evolution in Al alloys processed for one pass by ECAP

The evolution of texture and deformation in the grains during one pass of equal-channel angular pressing (ECAP) was examined for fine grained high strength and low strength Al alloys and a coarse grained low strength Al alloy. The materials were analysed using electron back-scatter diffraction (EBSD). The results are consistent with the materials responding to the intense macroscopic shear stress by deformation of individual grains through movement of dislocations on one or more of the slip crystallographic slip planes {hkl} that are favourably oriented, combined with the rotation of grains to directions that bring main crystallographic slip planes parallel to the macroscopic shear direction and crystallographic slip directions parallel to two main shear directions. Contrary to reports claiming up to 4 slip systems are activated, it was observed that only the {111}<110> and {001}<110> shear systems are activated. Macroscopic shear deformation occurs on two shear planes: the main shear plane (MSP), equivalent to the simple shear plane, and a secondary shear plane which is perpendicular to the MSP.