Sets Of Microbands Research Articles

The influence of deformation texture on nucleation and growth of cube grains in aluminium

The influence of deformation texture on the initial stages of recrystallization of deformed pure Al single crystals and commercial AA1050 alloy were characterized. To precisely quantify the orientation relationship between nuclei and simple deformation structures, single crystalline samples of stable Br(110) [1-1-2] and semi-stable S’(234) [20-28 11] orientations, were plane strain compressed (PSC) to 40% to develop a dominant homogeneous microstructure composed of two sets of microbands. The samples were then lightly annealed to reach the initial stage of primary recrystallization. The results on single crystals were related to the behaviour of the polycrystalline samples in state after hot-rolling then PSC to the same degree and lightly annealed. SEM/EBSD analyses of single crystalline samples demonstrate that the orientations of recrystallized grains are limited to a certain number of orientation groups. However, the orientations of the as-deformed state are not retained in the orientations of new grains. Deformed samples with Br orientation do not show the formation of cube grains during recrystallization, whereas strong nucleation of cube-oriented grains is observed in homogeneously and heterogeneously deformed areas in the semi-stable S’ orientation, despite the cube-oriented nuclei not present in the as-deformed structure. The recrystallization texture components of the AA1050 alloy were related to the standard β-fibre texture formed during the previous cold deformation. Cube grains formed intensively in PSC polycrystalline samples, primarily in association with as-deformed areas with near four variants of S(124) [21-1] and S’ orientations and were characterized by local misorientations around the <111> axes.

Size effects on tensile properties and deformation mechanism of commercial pure tantalum foils

The tensile properties and deformation microstructure of commercial high pure tantalum foils with thickness ranging from 30 μm to 200 μm have been investigated. The origin foils show a large texture intensity of γ fiber. The yield strength and ultimate tensile strength gradually increase with the decrease of thickness due to the change of the scale factor of the surface grains and the increase of volume fraction of passivation film. The constitutive model based on the surface layer theory is well established. Meanwhile, the anomalous yield point phenomenon can be seen. The deformation microstructure is closely related with the orientations. Two sets of microbands are developed in most of the grains. The density of microbands is the largest in the grains with tensile direction // 〈110〉, which belong to α fiber. Microbands are usually concentrated near the grain boundaries, which frequently incline at 25–45° with respect to the tensile direction. The trace of microbands is usually consistent with the slip planes of {110} or {112} with largest or second largest Schmid factor. Therefore, the appearance of wavy slip traces on the fracture is attributed to the dislocation cross-slip on these different kinds of slip planes.

Application of orientation imaging to the study of substructural development in cold deformed Al-0.3%Mn single crystal of {110} orientation

A systematic study of the deformation microstructures over 3 perpendicular surfaces was carried out in the present work in order to correlate better the substructure with slip patterns, initial and final crystal orientation, and the macroscopic coordinate system. The microstructure and texture evolution of high purity Al-0.3 wt% Mn alloy of initial near brass {110}<112> (or Bs) orientation, channel-die compressed to the strain level of 1.5, were studied by TEM (including TEM orientation mapping) and high-resolution FEG-SEM/EBSD techniques to observe crystal subdivision deformation patterns at the microscopic scale. The grain orientation dependent structures were analyzed in terms of active slip systems focussing on the crystallography of extended planar boundaries. It was concluded that the type of dislocation structure (one or two sets of microbands) in {110}<112> oriented crystallites, at moderate strains (< 1.5) depended strongly on the crystallographic grain orientation. In this non-ideal initial crystal orientation the applied deformation mode activates a double slip, of which one system predominates and leads to further rotation away from Bs. A general rotation combined with a wide orientation spread is observed after a strain up to 1.5. The microband boundary alignment corresponds very well to the traces of crystallographic {111} planes, on which most of the slip occurs.

Microbands and crystal orientation metastability in cold rolled interstitial-free steel

At low cold rolling reductions microbands form in grains which have either 〈1 1 0〉 or 〈1 1 1〉 parallel to the transverse direction. They appear to form by crystallographic slip and existing theory is sufficient to explain their morphology. Detailed analysis of the mechanics of crystal rotation shows that the orientations belonging to the sets 〈1 1 0〉//TD and 〈1 1 1〉//TD can rotate between two unstable orientations before a stable orientation is reached. This crystallographic metastability is shown to be responsible for the formation of either one or two sets of microbands, and theory and measurement agree that they should form at 20–40° to the rolling direction when measured in the longitudinal section.

Microstructure evolution in an interstitial-free steel during cold rolling at low strain levels

Microstructure development in an interstitial–free steel during cold rolling at low strain levels (ϵ≤ 9.8%) has been investigated by using transmission electron microscopy. At a strain of 2.2%, {112} slip systems operate in addition to {110} slip planes. Dislocation reactions occur at this stage to produce immobile 100 segments in the scissors configuration and these segments are also found at higher strain in the dislocation walls of microbands. The formation of microbands starts at small reductions (ϵ∼6.7%), and microbands are of lenticular shapes and have habit planes running approximately parallel to {110} planes. Two sets of dislocations comprise the microband walls; one is predominant and has its Burgers vector lying on the microband9s habit plane. The secondary set is much less dense, and its slip plane is not coplanar with the microband habit plane. A considerable misorientation exists between the inner region of a microband and either of its two neighbouring matrices, rather than between the two matrices, which is consistent with Jackson9s double cross–slip model. However, the growing end of a microband indicates the splitting of a dense dislocation sheet. In the specimen that was rolled to ϵ=9.8%, some grains contain one or two sets of microbands, while some are microband–free. The crystallographic measurement and deformation geometry calculation reveal that the habit plane of an observed microband has the largest Schmid factor; and when one 〈111〉 slip direction is intensively activated in this plane, one set of microbands is formed on this plane. Two sets of microbands form if two 〈111〉 slip directions have high and nearly equal shear stresses. In the case of microband–free crystals, up to seven slip systems have similar Schmid factors and thus are activated concurrently. This leads to homogeneous deformation and as a result, no microbands form. Based on these results, a new mechanism is proposed for microband formation involving double cross–slip, dislocation wall splitting and dislocation exchange between the walls.