Papers by Author: Julian H. Driver

Abstract: Microstructure and texture development in twinned fcc metals is investigated in order to characterize the influence of micro- and macro-scale brass-type shear bands (SB) on structural and textural changes at large deformations. TEM and SEM analyses are focused on bands developed by plane strain compression in twinned C{112}<111> oriented single crystals. The proposed crystallographic model of the shear banding phenomenon refers to the idea of local lattice reorientation within narrow areas. Most of these rotations occur around the TD||<110> axis with significant further rotations about <112> poles. These two rotations explain the influence of SB’s on the formation of Goss{110}<001> and brass{110}<112>-S{123}<634> texture components clearly observed in highly deformed low SFE metals. At high deformations symmetrically equivalent crystal lattice rotations inside narrow areas lead to the formation of positive and negative macroscopic SBs.

Abstract: The development of deformation substructure and texture has been studied up to large plastic strains in some simple Al base alloys by multiple forging. The experiments involve successive forging strains on near-cube samples along 3 orthogonal axes up to cumulative strains of 3 or more (and temperatures from 20 to 400°C). The alloys include the commercial AA 3103 (Al- 1%Mn) and a laboratory Al-3%Mn-Sc-Zr alloy for the high temperature tests. Some complementary experiments have been carried out on oriented single crystals of Al-0.3%Mn. During 3D cross forging of fcc metals a clear texture composed of three symmetrical components is formed; they are the 3 possible variants of the <110> <110> <100> crystal axes along the 3 forging axes. This macroscopic texture is demonstrated by X-ray pole figure analysis, EBSD mapping and confirmed by crystal plasticity (CP) simulations. At room temperature the alloys (particularly Al-Mn) exhibit significant grain refinement by grain fragmentation leading to "grain sizes" of less than 103m. However, at temperatures ≥ 300°C in the stable Al-3%Mn-Sc-Zr alloy the lattice rotations towards just 3 texture components leads to a high frequency of grain "fusions"; each grain becomes surrounded by 3-5 neighbours of the same orientation so that long interpenetrating chains of the texture components are formed; they are also confirmed by FEG-SEM EBSD and spatially resolved texture simulations. The behaviour of stable (Goss) and unstable (cube) single crystal orientations during the same deformation processing is also investigated and shown to agree with the CP simulations.

Abstract: An Al-3%Mg-0.25%Sc-0.12%Zr alloy was deformed by triaxial forging at 20-400°C up to strains of about 3. A study of its textural evolution reveals the tendency towards three symmetrical variants of a <110><1 10 ><001> component. This experimental observation is supported by a 3D spatially resolved crystal plasticity analysis. Samples strained at room temperature undergo grain fragmentation in the form of fine substructures and relatively weak textures. Conversely, at 300°C and above, more homogeneous intergranular deformation and rotations give rise to stronger textures. This eventually encourages grain coalescence and thus the development of interpenetrating “orientation chains”, creating a new type of microstructure. The influence of this texture development on the specific work hardening behaviour is discussed.

Abstract: Two relatively simple schemes are described for the interactions of grain deformations during large plastic deformations with the aim of evaluating their influence on texture development. The stress transfer model basically assumes that there is some degree of stress transfer across the boundaries proportional to the boundary area. The reduced stress incompatibility model minimizes the stress incompatibilities between each grain and their surrounding grains These models assume 3D topological schemes using evolving truncated octahedra for the spatial distributions of the grains. They are applied to the cases of hot rolled and cross forged Al alloys. Both give quite similar predictions for texture development which are moderate improvements on the Taylor models, confirming that the incorporation of grain interaction effects can be useful for texture modeling without major modifications. Moreover, they can yield interesting results for local orientation effects and their influence on orientation stability; an example of cube grains hot rolled in different crystallographic surroundings is also treated.

Abstract: This paper described new characterization methods and data to quantify the influence of solute atoms on grain boundary and sub-grain boundary mobilities in Al-Mn alloys with a view to their integration into recovery and recrystallization modelling. Detailed SEM measurements of grain boundary mobilities during recrystallization have been made by in-situ annealing experiments on cold deformed Al – 0.1 and 0.3wt.% Mn binary alloys. Stored energies are estimated from the sub-grain sizes and misorientations and the boundary velocities directly measured in the temperature range 200-450°C. It is shown that in many cases good agreement with the Cahn, Lücke, Stüwe model for solute drag is obtained, e.g. the activation energies are intermediate between those of boundary and volume solute diffusion. Some particular cases of rapid growth occur in Al-0.1%Mn indicating boundary breakaway from solute clouds. A complementary study of sub-grain boundary mobilities has started on the same alloys; in this case the average mobilities are estimated from FEG-SEM growth data for the average sub-grain size for temperatures in the range 150-300°C. The results are compared with some previous data on Al-Si and show similar rates.

Abstract: Multiple forging (MF) can be used to attain large plastic strains in bulk alloys by successive forging along three orthogonal directions to retain the initial sample shape. An original multiple forging technique enabling 3-D cross forging at constant temperature up to 500°C has been applied to two Al alloys (Al-1%Mn and Al-3%Mg-Sc,Zr). Their rheology, texture and microstructure evolution are compared with those obtained in plane strain compression (PSC). The results are interpreted in terms of slip activity behaviour during both deformation modes. They can also be correlated with the contributions of free dislocations and sub-boundaries.

Abstract: Two recent methods for obtaining flow stress-strain relations up to large strains of order 1.5 by channel-die compression are presented: i) for sheet metal formability tests, composite samples have been made of glued sheet layers and deformed at room temperature in a channel-die with the compression axis directed along one of the sheet metal edge directions, i.e. RD or TD. The sheet plane is parallel to the lateral compression die face. It is shown that, using a suitable lubricant, the sample deformation is homogeneous up to strains of 1.5. Tests carried out on 5xxx and 6xxx alloys to evaluate the stress-strain relations show that a generalized Voce law gives a good quantitative fit for the data. ii) for high temperature plate processing, quantitative flow stress data can be obtained up to 500°C with a rapid quench using a hot channel-die set-up. Some new results are presented here for high strain hot PSC tests on Al-Mn and Al-Mg alloys together with microstructure analyses.

Abstract: The crystallography of recrystallization nucleation has been investigated in channel-die deformed pure aluminium bicrystals with {100}<011>/{110}<001> and {100}<001>/{110}<001> orientations. The new grain orientations and misorientations were followed by systematic local orientation measurements using SEM and semi-automatic measurements in a TEM. In particular, orientation mapping combined with in-situ sample heating was used to investigate the formation and growth of new grains and their crystallographic orientation changes at very early stages of recrystallization. Grain boundary migration and ‘consumption’ of the as-deformed areas was always favoured along directions parallel to the traces of the {111} slip planes that had been most active during deformation. The orientations of the first formed nuclei were misoriented with respect to the orientations identified within the neighbouring deformed areas by α(<111>, <112>, or <100>)relations.

Abstract: A simple and general new approach to predict deformation texture evolution during large plastic strains is presented. The stress in each grain, first calculated by a Taylor model, is then modified by the stresses of adjacent grains thereby making the local slip systems and lattice rotations neighbour dependent. Examples of texture simulations during hot rolling of aluminium alloys are given. The model predictions are compared with the standard Taylor model predictions and with ODF data of the textures measured during hot plane strain compression.

Abstract: An original model, based on a variational formulation for boundary motion by viscous drag, is developed to simulate single grain boundary motion and its interaction with particles. The equations are solved by a 3D finite element method to obtain the instantaneous velocity at each triangular element on the boundary surface, before, during and after contact with one or more particles. After validation by comparison with some simple, analytical and numerical cases, it is adapted to model curvature driven grain growth. For single phase material, the single grain boundary model closely matches the grain coarsening kinetics of a 3D multi boundary vertex model. In the presence of spherical incoherent particles the growth rate slows down to give a growth exponent of 2.5. When the boundary is anchored there is a significantly higher density, by a factor of 4, of particles on the boundary than the density predicted by the classic Zener analysis, and many particles exert less than this Zener drag force. As a result the Zener drag is increased by a factor of about 2.2. The limiting grain radius is compared with some experimental results.