Papers by Author: Cheng Xu

Abstract: Equal-channel angular pressing (ECAP) has been established as an effective method to achieve ultrafine-grained materials and ECAP combined with the Conform process (ECAP-Conform) is considered a promising candidate to realize continuous processing by ECAP. This paper describes experiments conducted on samples of an Al-6061 alloy in order to evaluate the homogeneity development of the alloy processed by ECAP and ECAP-Conform. The samples of the Al-6061 alloy were processed by ECAP and ECAP-Conform at room temperature up to 4 passes via route B_C. The microhardness measurements were recorded both in the cross-sectional planes and along the longitudinal directions of the samples. The results show the alloy obtained significant strengthening using both processes. There was reasonable plastic isotropy after 4 passes of ECAP-Conform in both the cross-sectional plane and the longitudinal direction and the alloy showed a homogeneous microhardness distribution in the cross-sectional plane after 4 passes of conventional ECAP.

Abstract: Solid oxide electrolyser (SOE) has been receiving increasing attention due to its potential applications in large-scale hydrogen production and carbon dioxide recycling for fuels. Improving the performance of SOE cell through oxygen electrode development has been of main interest because the major polarization loss of the SOE cell is at the oxygen electrode during high temperature electrolysis (HTE). In the present study, Ag was doped into (La_0.75Sr_0.25)_0.95MnO_3+δ(LSM) based oxygen electrode of Ni/YSZ cathode-supported SOE cell through a solid state method enhanced by ball milling. Short stacks were manufactured using doped and undoped cells and tested under HTE of steam at 800°C up to 150h for in situ comparative study of doping effect. The cells with doped oxygen electrodes showed less polarization loss, lower resistance and improved performance by comparison with the undoped cell. Post-mortem examination revealed Ag migrated from the current collecting layer to the electrolyte/anode interface, which may promote the cell performance.

Abstract: It is now well established that the application of high-pressure torsion (HPT) leads to exceptionally small grain sizes in bulk solids, usually in the nanometer scale. The HPT method thus attracts considerable attention and has been utilized in processing many materials such as pure metals, alloys and even ceramics. This paper describes experiments conducted on samples of an Al-6061 alloy and an Al-6061 metal matrix composite, reinforced with 20 vol.% Al2O3 particulates, in order to evaluate the mechanical properties of the alloy and its composite processed by HPT. The tensile properties of the materials were obtained both at room temperature and at 773 K and representative microstructures were observed using transmission electron microscopy.

Abstract: This work is focused on the effect of the combination of natural aging and severe plastic deformation (SPD) produced by Equal-Channel Angular Pressing (ECAP) on the microstructure and strength of supersaturated AlZnMg alloys. Following a solution heat-treatment and quenching into water at room temperature, samples were naturally aged for different time periods and then processed by ECAP. The microstructure and mechanical properties of these samples are described and discussed. This investigation leads to proposing an interesting application of ECAP for supersaturated alloys. Using the shear bands created by ECAP in only one pass and applying appropriate subsequent aging treatments, composite-like microstructures can be achieved in conventional age-hardenanble Al alloys.

Abstract: High-pressure torsion (HPT) is an important processing technique in which a disk is subjected to a high pressure with concurrent torsional straining. In principle at least, the imposed strain is zero at the center of the disk and a maximum at the outer edge. This difference suggests, therefore, that materials processed by HPT will exhibit considerable inhomogeneity. This paper describes the results obtained in a series of experiments which were designed to evaluate the evolution of homogeneity during the processing of two materials by HPT. It is demonstrated that it is possible to achieve a reasonable level of homogeneity in both materials but there are important differences which reflect the dependence of the microstructure on the occurrence of dynamic recovery.

Abstract: Equal-channel angular pressing (ECAP) is a convenient processing method for refining the grain size of bulk materials to the submicrometer level. Metallic alloys processed by ECAP often exhibit excellent superplastic characteristics including superplasticity at high strain rates. This paper summarizes recent experiments designed to evaluate the occurrence of superplasticity in representative aluminum and magnesium alloys and in the Zn-22% Al eutectoid alloy.

Abstract: It is now well-established that processing through the application of severe plastic deformation (SPD) leads to a significant reduction in the grain size of a wide range of metallic materials. This paper examines the fabrication of ultrafine-grained materials using high-pressure torsion (HPT) where this process is attractive because it leads to exceptional grain refinement with grain sizes that often lie in the nanometer or submicrometer ranges. Two aspects of HPT are examined. First, processing by HPT is usually confined to samples in the form of very thin disks but recent experiments demonstrate the potential for extending HPT also to bulk samples. Second, since the strains imposed in HPT vary with the distance from the center of the disk, it is important to examine the development of inhomogeneities in disk samples processed by HPT.

Abstract: The mechanisms of creep and superplasticity occurring in conventional large-grained materials are now understood reasonably well. However, very recent advances in the processing of theoretically-dense metals with submicrometer grain sizes have provided the opportunity to extend the understanding of flow behavior to include materials where the grains are exceptionally small. Using processing through the application of severe plastic deformation, as in procedures such as equal-channel angular pressing, it is now feasible to fabricate relatively large samples having ultrafine grain sizes in the submicrometer or nanometer range. This paper examines these recent advances and gives examples of the flow behavior in ultrafine-grained aluminum alloys.

Abstract: Experiments were conducted to evaluate the mechanical properties of a spray-cast Al- 7034 alloy processed by severe plastic deformation. The alloy was received with an average grain size of ~2.1 μm and processed by equal-channel angular pressing (ECAP) at a temperature of 473 K to give a grain size of ~0.3 μm after 6 or 8 passes. Following ECAP, the mechanical properties were evaluated at room temperature (298 K) and at an elevated temperature of 673 K. In tensile testing at ambient temperature, the stress-strain curves show very short regions of strain hardening after ECAP and low values for the ultimate tensile strength by comparison with the unpressed alloy. This lack of strength is due to the high pressures imposed by ECAP and the consequent fragmentation and dissolution of the rod-like MgZn2 precipitates. It is shown that the strength may be restored by performing an appropriate ageing treatment after ECAP. Superplastic ductilities were recorded at a temperature of 673 K with tensile elongations exceeding 1000%. Careful inspection of the polished surfaces of samples pulled to fracture in the superplastic condition revealed the occurrence of extensive internal cavitation. Quantitative measurements showed the development of these internal cavities is consistent with conventional superplastic alloys.

Abstract: The texture and grain boundary evolution during equal channel angular pressing (ECAP) of a spray-cast Al-7034 (Al-Zn-Mg-Cu) alloy containing intermetallic particles with a range of sizes was studied through electron backscatter diffraction (EBSD). Up to 8 passes of ECAP using route Bc were employed. The initial ECAP pass leads to the development of low angle grain boundaries and subsequent passes lead to a relatively rapid increase in the fraction of high angle grain boundaries. Before ECAP, the material possessed a strong <111> and <100> fibre texture. On ECAP, the <111> fibre texture component is mostly retained but the <100> fibre develops to a Cube texture after the first ECAP pass. Goss textures form from about 4 passes of ECAP.