Papers by Author: Chris H.J. Davies

Abstract: A comparative experimental and simulation study of oxygen-free high conductivity copper produced by equal channel angular pressing (ECAP) one-pass has been carried out by using electron backscatter diffraction (EBSD) and a recently proposed grain refinement model. The grain size and misorientation distributions were extracted from the EBSD measurements. It was found that the microstructure in the ECAP deformed copper was much more refined on the TD plane. The grain size observed experimentally can be fairly well predicted by the grain fragmentation model.

Abstract: The hot deformation behaviour of the alpha / beta Ti-6Al-4V alloy was investigated at various temperatures and strain rates by means of compression and torsion tests. As expected, the peak flow stress increased with increasing strain rate and decreased as the initial sample temperature was increased. The different flow behaviours observed are discussed in terms of the volume fraction of each phase. The dual phase Ti-6Al-4V alloy was assumed to be a composite material containing a soft phase and a hard phase. By taking into consideration the phase-to-phase interaction and volume fraction change with temperature, constitutive models are proposed to simulate the deformation behaviours. By application of the rule of mixtures the modelled yield stresses showed good agreement with the experiment results.

Abstract: Measurements of strain rate sensitivity (SRS) provide a key link between dislocation-based interpretations of plastic deformation and macroscopic measurements made in mechanical tests. It is well known that plastic deformation of hexagonal close-packed (hcp) metals is achieved not only by dislocation glide but also by twinning and that the atomic rearrangement underlying the latter mode is different from that of slip. This leads to an expectation that co-activation of twinning may affect SRS of hcp metals. This assumption was tested in the present work where strain rate jump tests in both tension and compression were conducted on highly textured AZ31 plate. It was found that the SRS of the alloy in tension decreased with strain whereas that in compression increased with strain, exhibiting negative values at low strain and positive values at higher strain. Microstructure analyses revealed that the strain regimes where negative SRS or decreasing trend in SRS with strain was observed correspond to extensive twinning, implying a negative SRS of twinning. It is concluded that dislocation model alone cannot explain the strain rate dependence of flow stress in metals whose deformation is assisted by twinning.

Abstract: Two low carbon steels alloyed with 0.48wt% and 0.78wt% Cr were warm rolled to 65% reduction at 640°C. Annealing was carried out at 710°C to achieve a range of recrystallized volume fractions up to 100%. Texture analysis was performed by X-ray diffraction and Electron Back Scattering Diffraction. During the initial stages of recrystallization, more recrystallized grains nucleated at shear bands than at grain boundaries in the 0.48wt%Cr steel, whereas this was not the case in the 0.78wt%Cr steel. This is associated with a decrease in the propensity to form shear bands due to the lower amount of C in solid solution in the high-Cr steel. Additionally, the nuclei showed a preference to develop the same fiber as the deformed region within which they nucleated. In both steels, an increase in the annealing time led to the deterioration of the ND (or γ) –fiber and a strengthening of both the RD (or α) –fiber and Goss component. In the fully recrystallized samples, the ND component was somewhat stronger in the steel with the higher Cr content.

Abstract: Oxygen-free high conductivity copper was subjected to room temperature equal channel angular extrusion of 8 passes using route Bc. The resulting ultra-fine grain copper was then rolled to thickness reductions of up to 96.5% at liquid nitrogen temperatures. Annealed coarse grained copper was rolled to the same strain at room temperature for comparison. Samples from the two routes were isochronally and isothermally annealed, and the microstructure and texture evolution studied by electron back scattered diffraction and x-ray diffraction. Annealing of the ultrafine grained copper led to the development of a strong rotated cube texture from a texture in the rolled material dominated by the Brass component. In contrast the more commonly observed cube texture was found after annealing of the coarse-grained sample. Accompanying the rotated cube texture was the development of a large fraction of boundaries with rotation angle/axis close to 60° <111>.

Abstract: A recent trial investigated the effect of solidification grain refinement of billet on the grain refinement and properties of alloy ZM20. It was found that even at levels of 0.4Mn, significant grain refinement could be obtained when 0.7Zr was added. At 0.2Mn grain sizes as low as 60μm were obtained. Billets of Mg-2Zn-0.2Mn with four different grain sizes, due to different Zr and cooling rates were then cast via vertical direct chill casting and extruded conventionally. Benefits of grain refinement of the billet on extrusion were found to be a slight increase in the size of the operating window, and a reduction of the grain size in the extrudate. However, the effect of the reduction in extrudate grain size due to refinement of the billet was small compared with the amount of grain refinement obtained due to recrystallisation on extrusion.

Abstract: Samples of extruded magnesium alloy AZ31 (nominally 3 wt% Al, 1 wt% Zn, 0.3 wt% Mn, with the balance magnesium) were tested in uniaxial compression at temperatures from room temperature to 200°C. Monotonic and strain rate jump tests were conducted, and strain rate sensitivity, and activation volume were determined. At the lower temperatures tested, strain rate sensitivity was found to increase with strain to a limiting value, whereas at 200°C strain rate sensitivity initially followed a similar trend, but did not reach a limit within the bounds of the test. The effect of temperature on strain rate sensitivity was minor up to 150°C. Activation volume decreased with stress by roughly an order of magnitude over the course of the tests. An analytical model was modified to incorporate strain rate sensitivity, and it is shown that the evolution of material by twinning into an orientation favourable for slip is principally responsible for the trend in strain rate sensitivity with increasing strain.

Abstract: This paper investigates the changes in deformation mechanisms of commercially pure titanium over a range of temperatures for different orientations relative to the initial rolling texture. Samples from grade 1 titanium plate were tested in plane strain compression (PSC). Extremes of orientation relative to the predominant split basal texture were tested at temperatures from 25°C to 700°C. Specimens were subsequently examined using X-ray texture analysis and electron back-scatter diffraction (EBSD). Changing the orientation resulted in substantial yield stress anisotropy. This was found to be largely related to the orientation of the dominant texture relative to the most favorable orientation for the easiest slip mode (prism slip), and significantly but to a lesser extent on differences in twinning behaviour. The most important difference in twinning was the operation of {1012} tensile twinning in c-axis tension and {1122} compression twinning in c-axis extension. Calculations indicated that at low temperature both of these twinning modes accommodate a significant amount of strain. Twinning was also found to be the most significant factor affecting work hardening behaviour, with reorientation hardening occurring for some sample orientations. As temperature was increased above ~350°C {1011} twinning became the dominant twinning mode, but its contribution to the strain was not as large as the low temperature twinning modes, and the total amount of twinning decreased with temperature. The decrease in twinning with increasing temperature led to a reduction in the difference in work hardening behaviour. The quantitative information gathered in the course of this work is discussed in the context of mechanical property prediction.

Abstract: The crystallographic response to deformation – texture evolution, internal elastic strain, and twin evolution – was measured for three load/orientation variants for AZ31 magnesium alloy extrudate tested in-situ in a synchrotron beamline. Specimens were loaded in tension parallel to the extrudate transverse direction, in compression along the same axis, and in compression parallel to the extrudate normal direction. The crystallographic responses are correlated with the mechanical behaviour of the extrudate.

Abstract: Texture development and grain refinement of interstitial-free (IF) steel as a function of ideal simple shear or torsion was undertaken for up to 8 pressings, Route BC. Dominant texture components evolved after a single pass and remained correspondent for all subsequent passes. End-cycle (4 and 8 pass) orientation distribution functions (ODFs) showed an increased spread of contour levels and the re-appearance of texture components similar to the 0-pass condition. Rotation of torsion components away from their ideal pole figure (PF) orientations increased at mid-cycle and decreased to almost equivalent levels after end-cycle passes. This is attributed to the principle slip directions of BCC structures aligning with the shear direction.