B. Beausir

Disclinations provide the missing mechanism for deforming olivine-rich rocks in the mantle

Mantle flow involves large strains of polymineral aggregates. The strongly anisotropic plastic response of each individual grain in the aggregate results from the interactions between neighbouring grains and the continuity of material displacement across the grain boundaries. Orthorhombic olivine, which is the dominant mineral phase of the Earth’s upper mantle, does not exhibit enough slip systems to accommodate a general deformation state by intracrystalline slip without inducing damage. Here we show that a more general description of the deformation process that includes the motion of rotational defects referred to as disclinations can solve the olivine deformation paradox. We use high-resolution electron backscattering diffraction (EBSD) maps of deformed olivine aggregates to resolve the disclinations. The disclinations are found to decorate grain boundaries in olivine samples deformed experimentally and in nature. We present a disclination-based model of a high-angle tilt boundary in olivine, which demonstrates that an applied shear induces grain-boundary migration through disclination motion. This new approach clarifies grain-boundary-mediated plasticity in polycrystalline aggregates. By providing the missing mechanism for describing plastic flow in olivine, this work will permit multiscale modelling of the rheology of the upper mantle, from the atomic scale to the scale of the flow.

Texture Heterogeneity in ECAP Deformed Copper

Polycrystalline copper of 4N purity has been deformed by equal channel angular pressing at room temperature using route BC. Local textures have been measured by high-energy synchrotron radiation along 3 lines in the cross section from the top to the bottom of the billets. The texture heterogeneity observed in the cross section is presented for 2 passes and discussed with regard to friction-affected material flow.

Simulation of Texture Development of Plane Carbon Steel in Multipass Rolling Using Analytical Flow Function

Using a simple analytical flow function, an analysis of the deformation field in symmetrical rolling has been carried out. The so-obtained varying velocity gradient is incorporated into the Taylor polycrystal plasticity model to simulate the development of the deformation texture. The initial discontinuity in the deformation field of the entering material element on the flow lines is also taken into account. Multiple passes of the material is simulated. A strong texture gradient is obtained in good agreement with experiments carried out for rolling of plane carbon steel. It is shown that the shear component of the texture is strongly related to the nature of multiple passes of the rolling operation.

Back stress in strain hardening of carbon nanotube/aluminum composites

ABSTRACT As demonstrated by the loading–unloading tests and the modeling of the grain size effect and the composite effect, mainly owing to the back stress induced by CNTs, carbon nanotube/aluminum (CNT/Al) composites exhibit higher strain hardening capability than the unreinforced ultrafine-grained Al matrix. The back stress induced by CNTs should arise from the interfacial image force and the long-range interaction between statically stored dislocations and geometrically necessary dislocations around the CNT/Al interface. Therefore, this CNT-induced interfacial back stress strengthening mechanism is supposed to provide a novel route to enhancing the strain hardening capability and ductility in CNT/Al composites. IMPACT STATEMENT The present work investigates the roles and origins of back stress in the strain hardening of the carbon nanotubes/aluminum composites for the first time. GRAPHICAL ABSTRACT

Influence of Additives on Texture Development of Nanocrystalline Nickel

Organic additives such as saccharin have been frequently used in electroplating operations to moderate deposit growth rates and to control film quality. In this study, texture development upon annealing of pulse-electroplated Nickel produced without additives is analyzed by use of electron backscatter diffraction technique. Plating without additives results in a microstructure with slightly elongated grains and a fibre texture in growth direction and this texture is conserved upon annealing up to 600°C. Structural units in form of groups of elongated grains possessing a common zone axis in growth direction and twin relationships between themselves are found in the microstructure. For revealing the influence of additives, the observations are compared with results obtained for Ni and Ni-Fe plated in the presence of additives where during abnormal grain growth the initial fibre texture changes to an energetically more favourable texture by twinning. The lack of additives is assumed to be responsible for the observed differences in texture and microstructure development.

Grain Fragmentation in Equal Channel Angular Pressed Copper

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.

A New Flow Function to Model Texture Evolution in Symmetric and Asymmetric Rolling

Using a new analytic flow function, an analysis of the deformation field in symmetrical and asymmetrical rolling has been carried out. The asymmetry concerns the differences in the angular speeds of the rolling cylinders. The flow function describes the trajectory of the material flow from which the velocity field and the velocity gradient is obtained by partial derivations. The new flow function takes also into account the “discontinuity” at the entry of the material into the die. By introducing a non-homogeneous velocity distribution at the end of the flow line, the shear component in the rolling plane and in the rolling direction that is characteristic to the asymmetric rolling is naturally introduced into the deformation process. The varying velocity gradient along selected flow lines is incorporated into the viscoplastic self-consistent polycrystal plasticity model to simulate the development of the deformation texture. The effect of multiple passes as well as the asymmetries on the evolution of the deformation textures is studied for bcc iron.

Influence of deformation temperature on texture evolution in HPT deformed NiAl

NiAl is an intermetallic compound with a brittle-to-ductile transition temperature at about 300°C and ambient pressure. At standard conditions, it is very difficult to deform, but fracture stress and fracture strain are increased under high hydrostatic pressure. On account of this, deformation at low temperatures is only possible at high hydrostatic pressure, as for instance used in high pressure torsion. In order to study the influence of temperature on texture evolution, small discs of polycrystalline NiAl were deformed by high pressure torsion at temperatures ranging from room temperature to 500°C. At room temperature, a typical shear texture of body centred cubic metals is found, while at 500°C a strong oblique cube component dominates. These textures can be well simulated with the viscoplastic self-consistent polycrystal deformation model using the primary and secondary slip systems activated at low and high temperatures. The oblique cube component is a dynamic recrystallization component.

Effect of back pressure on material flow and texture in ECAP of aluminum

Large billets (5 x 5 x 30) cm3 of technically pure aluminum (AA 1050) taken from thick rolled sheets were deformed at room temperature by single pass equal-channel angular pressing (ECAP). ECAP was done at different back pressures (0 – 60 MPa) using a square die with channels intersecting at 90° in sharp corners. The normal direction of rolling was taken parallel to the transverse direction of ECAP. The flow pattern was visualized by marker lines on split billets. The initial texture of the coarse-grained rolled sheet was measured by neutron diffraction. After ECAP, X-ray diffraction was used to measure the texture gradient from top to bottom of the billets. The results show, that with increasing back pressure the corner gap is closed and the flow line pattern becomes more symmetric. The flow line exponent increases strongly from top to bottom of the billets. Moreover, the inhomogeneous deformed zone at the bottom of the billets becomes smaller. The texture changes from a typical rolling texture to a typical shear texture with the intensity of the different shear texture components changing with back pressure. For the ACcomponent splitting is observed. The texture changes are discussed considering Toths flow line model and grain refinement.

Equal-Channel Angular Pressing of NiAl

Equal-channel angular pressing (ECAP) was applied to polycrystalline NiAl at temperatures around the brittle-to-ductile transition temperature (BDTT). NiAl rods encapsulated in a steel jacket were ECAP-processed in a die with a channel angle of 120°. The microstructure and texture were characterized by electron backscatter diffraction with a scanning electron microscope. The volume fraction of the texture components typical for simple shear in the intersection plane of the channels changes in the range of the BDTT.