Pete S. Bate

Gradient plasticity and deformation structures around inclusions

Abstract Elastic inclusions in plastically deforming metals provide a useful test of the ability of gradient plasticity models to predict length scale effects in deformation microstructures. Simple finite element analysis shows, however, that further development of such models is required if the effects observed in practice are to be predicted.

Transformation Textures in Steels

The mechanisms by which textures can be inherited in transformed phases are discussed in the light of different transformation mechanisms. Possible origins of variant selection in the different cases are reviewed and classified. Evidence is presented for a hitherto unsuspected source of variant selection that arises from the stresses which are generated during transformation due to the presence of micro-segregation. Some model predictions show the potential effect of this phenomenon on textures in bainite or martensite.

Evolution of Matt Surface Topography in Aluminium Pack Rolling

Roughening of the matt surface of pack rolled aluminium foil has been modelled. The model is based on the finite element method using isotropic plasticity. A distribution in material properties has been used to simulate the distribution of orientations through the material. The predictions of roughness show good quantitative agreement with the experiments.Copyright

Abnormal Grain Growth in Metals

Grain growth may occur in two forms, normal grain growth, characterized by a constant grain size distribution during growth, and abnormal grain growth, where one or more abnormally large grains may form in the microstructure. The presence of abnormally large grains in an otherwise uniform microstructure may be detrimental to the mechanical properties of a polycrystalline structure. Little is understood of the exact cause of abnormal grain growth. The annealing conditions leading to the onset of abnormal grain growth have been investigated via a series of grain growth experiments carried out on an Al-4wt%Cu alloy. The structure of which consisted of equiaxed grains (<8μ) pinned by a fine dispersion of sub-micron second phase particles, which may dissolve upon annealing. Minority texture components may experience accelerated growth due to a higher energy and mobility compared to the surrounding grain structure. The combination of these two events may result in the abnormal growth of some grains. SEM imaging and EBSD data has then made it possible to characterize the influence of particle dissolution and grain boundary misorientation on the onset of abnormal grain growth. The stability of ‘island grains’ found to exist internally in abnormally large grains has also been investigated in relation to the misorientation relationship and localized second phase volume fraction found there. There was only weak evidence of special misorientation relationships between the island grains and the abnormally large grains in which they exist, and although there was evidence of an enhanced fraction of pinning particles at island grain boundaries, this was also true of boundaries in general. The larger size of island grains is their dominant characteristic, and grains which become island grains may have been incipient abnormal grains.

Superplastic Deformation without Relative Grain Translation

It is now almost universally accepted that the mechanism by which superplastic deformation occurs involves relative grain translation, by grain boundary sliding, together with dislocation creep and/or diffusional creep. While there has been much debate on the fine points of the process - principally the so-called accommodation mechanisms, the primary role of the relative translation of grains via grain boundary sliding is virtually undisputed. Although this mechanism appears to be consistent with several aspects of microstructural evolution in superplastic deformation, the direct evidence for a dominant grain translation mechanism is mainly drawn from work using surface markers, not withstanding the fact that the extra degree of freedom at the surface of a superplastically deformed sample may make it somewhat unrepresentative of the bulk material. Research carried out recently on Ti-6Al-4V and AA5083+Cr, two well known superplastic materials, using both internal and surface markers throws into some doubt the importance of relative grain translation in superplastic deformation. The behaviour of aligned grain structures - acting as internal markers - is also difficult to reconcile with relative grain translation mechanisms The fact that such microstructures occur in materials which give a high strain rate sensitivity is inconsistent with mechanisms in which grain boundary sliding is a principal, rather than a subservient or even accommodating, mechanism. Evidence from transmission electron microscopy and plastic anisotropy suggest that intragranular slip may have a fundamental role in superplasticity. Certainly, some consideration must be given to mechanisms other than relative grain translation, though these must include the role of grair boundaries in leading to high strain rate sensitivity.

Study of dynamic grain growth by electron microscopy and EBSD

The effect of hot deformation on fully recrystallized aluminium–copper alloys (Al‐4wt%Cu and Al‐33wt%Cu) with different volume fractions of CuAl2 has been studied. The alloys are Zener pinned systems with different superplastic properties. Strain‐induced grain growth, observed in both alloys, was quantitatively estimated by means of electron microscopy and EBSD and compared with the rate of static grain growth. Surface marker observations and in situ hot‐deformation experiments combined with EBSD were aimed at clarifying the mechanisms responsible for the changes in the deformed microstructures. A sequence of secondary and backscattered electron images and EBSD maps was obtained during in situ SEM deformation with different testing conditions. Overlaying EBSD maps for the Al‐4wt%Cu with channelling contrast images showed that grain boundary motion occurred during deformation, creating a layered structure and leading to an increase in size of some grains and shrinkage of others. Of a particular interest are results related to behaviour of CuAl2 in superplastic Al‐33wt%Cu during deformation, including several problems with the use of EBSD in this alloy.

Microstructural inhomogeneity and biaxial stretching limits in aluminium alloy AA6016

Abstract The ductility of sheet metals in the regime where both principal in-plane strains are positive is strongly dependent on macroscopic inhomogeneity of the sheet. This can be clearly seen when the grain size becomes large compared with the sheet thickness. However, even with quite small grain sizes there is a tendency in aluminium alloys for grains of similar orientation to occur in colonies, and this complicates any simple correlation between grain size and limit strains in stretching. Sheets of the alloy AA6016 Al–Mg–Si were heat treated to give a wide difference in grain size, and the microstructure characterised using electron back scattered diffraction (EBSD) electron microscopy, which can reveal the presence of orientation colonies. Marciniak driving blank tests were used to reveal the development of dimensional inhomogeneity and the limit strains in balanced biaxial tension. An approach using the Marciniak–Kuczynski model with yield functions derived from texture data and the ‘defect’ associated with the measured inhomogeneity of crystallographic texture gives first-order predictions of the limit strains in this material, although issues of texture evolution and adequate quantification of inhomogeneity need further consideration.

In-Situ EBSD Observation of the Recrystallization of an IF Steel at High Temperature

Recrystallization phenomena in an interstitial free (IF) steel have been investigated by in-situ annealing in the SEM, combined with Electron Back Scattered Diffraction (EBSD) mapping. Sequential recrystallization phenomena, such as initiation and growth of new grains, are clearly distinguished by EBSD mapping at temperatures of up to 1070K. More than 70% of the recrystallized grains are of {111}<121>, {111}<123> and {111}<110> orientation. It is found that many recrystallized grains are formed from {111}<123> deformed grains at the beginning of recrystallization. It is observed that some of α-fibre (RD//<110>) orientations have difficulty in recrystallization compared to γ–fibre deformed grains. In particular, many grains of {001}<110> orientation remain un-recrystallized even after holding for 65 minutes at 1050K.

Microstructure and Texture of Dynamically Recrystallized Copper and Copper – Tin Alloys

The microstructure and texture in dynamically recrystallized copper and two copper – tin alloys (2wt% and 4.5wt% tin) has been investigated. Specimens were deformed in channel-die plane strain compression to true strains from 0.1 to 1.22 within the temperature range 200°C to 700°C, and the resulting microstructures were investigated with the use of high resolution electron backscatter diffraction (EBSD). Dynamic recrystallization was initiated by the bulging of preexisting high angle grain boundaries (HAGB), and occurred primarily by strain induced boundary migration (SIBM) and twinning. The addition of tin led to an increase in the temperature at which dynamic recrystallization initiated, and furthermore to a smaller dynamically recrystallized grain size. This was attributed to the effects of solute drag causing lower HAGB mobility. Dynamic recrystallization was observed to weaken the deformation texture components of brass and Goss, as well as introduce a cube texture component which generally tended to strengthen with temperature but weaken with increasing tin additions.

Nucleation and Texture Development during Dynamic Recrystallization of Copper

Dynamic recrystallization and texture development in polycrystalline copper have been investigated. Specimens were deformed in channel-die plane strain compression to true strains from 0.1 to 0.7 within the temperature range 200°C to 600°C, and the resulting microstructures were investigated with the use of high resolution electron backscatter diffraction (EBSD). Dynamic recrystallization in copper was initiated by the bulging of pre-existing high angle grain boundaries (HAGB), and occurred primarily by strain induced boundary migration (SIBM). Increasing misorientations from parent to dynamically recrystallizing grains indicated the occurrence of lattice rotations within the bulges, leading, in some cases to the formation of a HAGB behind the bulge. Discrimination between recrystallized and deformed components in material which had partially undergone dynamic recrystallization was carried out, followed by texture analysis. This revealed most of the recrystallized material to have orientations close to that of the deformed material, however, some remote orientations were observed which could not be related to the deformation texture by twin or 40° <111> relationships.