Materials Science Forum Vols. 495-497

Abstract: Large shear deformation was successfully introduced in 5182 aluminum alloy sheets by 2-pass differential speed warm rolling under a high friction condition. The roll speed ratio was varied from 1.0 to 2.0. When the roll speed ratio was smaller than 1.4, shear strain increased near the surface, but the strain decreased to zero at the mid-thickness. At a roll speed ratio larger than 1.4, shear strain was introduced even at the mid-thickness, and it increased near the surface. Thus the shear strain increased with the roll speed ratio. After 2-pass differential speed rolling, a large shear strain prevailed throughout the thickness. The rolling direction of the second pass was so selected that the direction of shear deformation introduced in the second pass was similar to (unidirectional shear rolling) or opposite (reverse shear rolling) that in the first pass. A shear texture with main components of {111}<110>, {112}<110> and {001}<110> prevailed throughout the thickness, and conventional rolling textures such as {112}<111> or {123}<634> orientation were not detected in any part of thickness. The rolling direction of the second pass had little effect on the deformation texture. After recrystallization annealing, the shear texture components were retained. The intensity of the shear texture components after recrystallization was almost similar to the deformation texture. The r-value of the annealed sheet was slightly increased and the planar anisotropy of the r-value was decreased by differential speed rolling. Differential speed rolling, by which shear deformation can be introduced throughout the thickness, was thus shown to be a promising process for improving the physical and mechanical properties of rolled and annealed aluminum alloy sheets by texture control.

Abstract: On the basis of Taylor-Bishop-Hill’s theory, many previous theoretical investigations have predicted that, at high rolling reductions, most of orientations should rotate along theβfiber from {110}<112> to {123}<634> and finally into the {112}<111> stable end orientations. Although some exceptions exist, experimental observations have shown, on the other hand, that the maximum on the β fiber is located still at about {123}<634> even after 97 % cold rolling. In the present paper, high purity Al containing 50 ppm Cu was cold rolled up to 99.4 % reduction in thickness and examined whether {112}<111> stable end orientation could be achieved experimentally. It was found that, with increasing rolling reduction above 98 %, {110}<112> decreased, while orientations in the range between {123}<634> and {112}<111> increased, suggesting that crystal rotation along the βfiber from {110}<112> toward {123}<634> and {112}<111> in fact took place. At higher rolling reductions, however, further rotation of this peak toward {112}<111> was extremely sluggish, and even at the highest rolling reduction, it could not arrive at {112}<111>. Such discrepancies between theoretical predictions and experimental observations should be ascribed to the development of dislocation substructures, which were formed by concurrent work hardening and dynamic recovery. Since such development of dislocation substructures are not taken into account in Taylor-Bishop-Hill’s theory, it seems that they can not correctly predict the development of rolling textures at very high rolling reductions, i. e. stable end orientations. On annealing specimens rolled above 98 % reduction in thickness, cube textures were very weak, suggesting that cube bands were almost completely rotated into other orientations during cold rolling. {325}<496>, which lay at an intermediate position between {123}<634> and {112}<111> along theβfiber, developed strongly in the recrystallization textures.

Abstract: The effect of fine particles on the uniformity of grain coarsening in a submicron grained Al-Sc alloy containing significant local variations in texture has been investigated using high resolution EBSD. The alloy was processed by severe plastic deformation and low temperature ageing to generate a fine-grained (0.8 µm diameter) microstructure containing either a dispersion of nanosized Al3Sc particles or a particle-free matrix. The initial processing generated a uniform grain size distribution, but the distribution of grain orientations was inhomogeneous with the microstructure containing colonies of grains consisting predominantly of either HAGBs or LAGBs with the latter possessing orientation gradients of up to 10 o/µm. Despite the marked differences in boundary character between these regions, the alloy undergoes slow and uniform grain coarsening during annealing at temperatures up to 500 oC with no marked change in the grain size distribution, boundary distribution and texture. A model of grain coarsening that takes into account the influence of fine particles on the kinetics of grain growth within an orientation gradient is outlined. The model predicts that a large volume fraction of fine particles (large f/r-value) tends to homogenize the overall rate of grain coarsening despite the presence of orientation gradients in the microstructure.

Abstract: Texture evolution in pure Mg and Mg alloy AZ31 during deformation and annealing was investigated. The poor low temperature ductility can be attributed to both, insufficient shear systems and unfavorable deformation geometry. Static recrystallization was shown to proceed discontinuously despite little texture change. High temperature deformation was accompanied by dynamic recrystallization with similar texture development as during static recrystallization.

Abstract: Magnesium is the lightest structural metal with a density of only 1.74 kg/dm3. Furthermore the yield strength and fracture strain of cast magnesium alloys are as good as those of common grade aluminum alloys. Because of these favorable properties there is a lot of interest from the automotive industry to use magnesium and its alloys in order to produce lighter vehicles. Because of its hexagonal crystal structure magnesium displays a very high mechanical anisotropy and a poor formability. In order to address these problems the influence was studied of dynamic recrystallization (DRX) on the microstructure and texture during compression of AZ31 samples (Mg-3%Al-1%Zn). Cylindrical samples were subjected to uniaxial compression tests at various temperatures and strain rates. The occurrence of DRX is revealed by optical micrographs that display bulging grain boundaries and the formation of newly formed grains. As DRX leads to grain refinement and grain refinement leads to an increased strength, DRX is an appropriate instrument to improve the strength of this alloy. The texture evolution and the volume fraction of recrystallized grains during DRX are monitored by orientation microscopy. The orientation microscopy postprocessing software allows to calculate the textures of both original and recrystallized grains separately. It is shown that the recrystallized grains have a much more random texture than the original ones and that increasing the temperature and decreasing the strain rate have a positive effect on the randomization of the texture. Previous research has shown that a near random initial texture produces a better formability (more than 40% failure strain at room temperature in a compression test) for this alloy [1]. This demonstrates that DRX can give rise to an improved ductility.

Abstract: Orientation mapping based on EBSD technique was applied to reveal the orientations of new grains and their relationships to the surrounding matrices, to analyze Kikuchi band contrast and the influence of strain rates on local orientation evolution. This information is used to understand the dynamic recrystallization mechanism and the relative contribution of plastic slip versus grain boundary glide or grain rotation related with super-plasticity. For this purpose samples with different initial textures were deformed by (quasi-)plain strain compression at two strain rates. It is suggested that the dynamic recrystallization in this alloy proceeds in continuous way by progressive subgrain rotation. No evident non-basal slip of was observed by referring texture evolution in the sample with initial basal texture. A high strain rate promotes more contribution of plastic slip accompanied by fast orientation changes. The fact that groups of grains with very similar orientations in basal oriented samples is discussed in terms of viscous flow.

Abstract: In order to improve corrosion resistance and mechanical properties of magnesium alloys, titanium with high specific strength was warm roll-bonded on the surface of AZ31 magnesium alloy sheets. Although the AZ31 alloy before roll-bonding had a typical basal texture, the AZ31 layer that constitutes a larger part of the clad sheet clearly showed off-basal texture with c-axis inclined by about 10º from the normal direction toward the rolling direction. This texture significantly affected tensile properties of clad sheets, resulting in lower proof stress and higher elongation at the rolling direction than at the transverse direction. In deep drawing tests, the 2-layered clad sheet with an outer titanium layer could be successfully formed at temperatures considerably lower than the limiting forming temperature of an original AZ31 single sheet. This is probably due to an effect of the outer titanium layer bearing tensile stress at a shoulder part of cup and an additional effect of improved deformability by off-basal texture in the AZ31 layer.

Abstract: The evolution of microstructure and texture during cold rolling and recrystallization annealing of commercial-purity Ti (CP-Ti) was established. Cold rolling to 40% reduction activated mechanical twinning- mostly > 3 2 11 < } 2 2 11 { compressive twins and > 1 1 10 < } 2 1 10 { tensile twins. The formation of twins resulted in an inhomogeneous microstructure, in which only the localized regions containing twins were refined and the regions deformed by slip remained coarse. The twinned grains, containing high stored energy and numerous high-angle grain boundaries, became the preferential sites of nucleation during subsequent recrystallization. During recrystallization heat treatment at 500~700°C, the cold-rolling texture (ϕ1=0°, Φ=35°, ϕ2=30°) diminished in intensity, whereas a recrystallization texture component (ϕ1=15°, Φ=35°, ϕ2=35°) appeared. The recrystallization heat treatment temperature affected the rate of recrystallization but not the texture characteristics per se. During the subsequent grain growth stage, the recrystallization texture component increased. This behavior was attributed to the growth of larger-than-average grains of this particular crystal orientation.The evolution of microstructure and texture during cold rolling and recrystallization annealing of commercial-purity Ti (CP-Ti) was established. Cold rolling to 40% reduction activated mechanical twinning- mostly > 3 2 11 < } 2 2 11 { compressive twins and > 1 1 10 < } 2 1 10 { tensile twins. The formation of twins resulted in an inhomogeneous microstructure, in which only the localized regions containing twins were refined and the regions deformed by slip remained coarse. The twinned grains, containing high stored energy and numerous high-angle grain boundaries, became the preferential sites of nucleation during subsequent recrystallization. During recrystallization heat treatment at 500~700°C, the cold-rolling texture (ϕ1=0°, Φ=35°, ϕ2=30°) diminished in intensity, whereas a recrystallization texture component (ϕ1=15°, Φ=35°, ϕ2=35°) appeared. The recrystallization heat treatment temperature affected the rate of recrystallization but not the texture characteristics per se. During the subsequent grain growth stage, the recrystallization texture component increased. This behavior was attributed to the growth of larger-than-average grains of this particular crystal orientation.

Abstract: The texture characteristics and the relationship between texture and tensile properties at room temperature and superplasticity at high temperatures in a hot extruded AZ61 alloy are examined in this paper. After warm extrusion, the alloy exhibits a of refined grains microstructure and a sharp basal {0001} texture. The tensile properties at room and elevated temperatures in extruded with the loading axis oriented at 0°, 45° and 90° to the extrusion direction are tested. It is found that the 45° specimens exhibit has lower yield strength and higher ductility. The possible causes for mechanical anisotropy are also analyzed based on the Schmid factor calculations.