Papers by Author: John F. Humphreys

Abstract: A combination of electron backscatter diffraction (EBSD) and focused ion beam (FIB) techniques were used to obtain 3D EBSD data in an investigation of dynamic recrystallization in a Cu-2%Sn bronze alloy. The results of this investigation show the origin of the nucleation sites for dynamic recrystallization and also elucidates the orientation relationship of the recrystallized grains to the deformed, prior grains and between the dynamically recrystallized grains.

Abstract: A focused ion beam (FIB) coupled with high resolution electron backscatter diffraction (EBSD) has emerged as a useful tool for generating crystallographic information in reasonably large volumes of microstructure. In principle, data generation is reasonably straightforward whereby the FIB is used as a high precision serial sectioning device for generating consecutive milled surfaces suitable for mapping by EBSD. The successive EBSD maps generated by serial sectioning are combined using various post-processing methods to generate crystallographic volumes of the microstructure. This paper provides an overview of the use of 3D-EBSD in the study of various phenomena associated with thermomechanical processing of both crystalline and semi-crystalline alloys and includes investigations on the crystallographic nature of microbands, void formation at particles, phase redistribution during plastic forming, and nucleation of recrystallization within various regions of the deformation microstructure.

Abstract: The effect of second-phase particles on the deformation and annealing behaviour of metals is re-assessed in the light of some new techniques. Using an EBSD method which provides much improved angular resolution, the effect of small non-deformable particles on the homogeneity of the deformation microstructure has been quantified. The presence of micron sized second-phase particles alters the deformation microstructure adjacent to particles, and a 3-d investigation of the deformation structures associated with large (>1μm) second-phase particles in cold rolled aluminium alloys has been carried out using 3-d FIB sectioning combined with EBSD, and the microstructures compared with the predictions of 3-d CPFEM modelling. The effects of grain orientation, strain and particle size have been investigated, and the results compared with earlier TEM investigations of such microstructures.

Abstract: The effect of copper content on dynamic grain growth in Al-Cu-Zr system was investigated by studying the microstructural development and texture evolution during uniaxial tensile deformation of Al-2wt%Cu-0.3wt.%Zr and Al-4wt%Cu-0.4wt.%Zr alloys at 450°C with a strain rate of 10-3s-1, with a similar initial microstructure in both materials. The initial microstructure consisted of layers of different orientations, the layers being separated by high-angle grain boundaries with low-angle boundaries separating grains within the layers. The initial grain spacing was about 5
m and the texture was typical of rolled aluminium alloys. The 4wt.%Cu alloy gave a higher strain rate sensitivity index, m, and a greater ductility compared to the low copper content alloy. An increase in grain size occurred in both materials due to deformation, but this dynamic grain growth (DGG) was much greater in the material with the higher copper content. This was associated with a more rapid conversion of low-angle boundaries to high angle ones in the 4wt%Cu material which is consistent with changes in crystallographic texture occurring during deformation.

Abstract: 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.

Abstract: Copper, Cu-2%Sn and Cu-4.5%Sn alloys have been deformed in plane strain compression at temperatures up to 700oC and the evolution of the microstructures and textures determined by high resolution EBSD. The effect of the solute is to raise the temperature at which dynamic recrystallization occurs and to significantly reduce the size of the dynamically recrystallized grains. In all the materials, there is a small increase in the cube texture component on dynamic recrystallization. The boundary bulges which precede recrystallization are different in the copper and Cu-Sn alloys, although in both materials there is evidence that local deformation in the boundary regions plays a significant role in dynamic recrystallization.

Abstract: A typical dual-beam platform combines a focused ion beam (FIB) microscope with a field emission gun scanning electron microscope (FEGSEM). Using FIB-FEGSEM, it is possible to sequentially mill away > ~ 50 nm sections of a material by FIB and characterize, at high resolution, the crystallographic features of each new surface by electron backscatter diffraction (EBSD). The successive images can be combined to generate 3D crystallographic maps of the microstructure. A useful technique is described for FIB milling that allows the reliable reconstruction of 3D microstructures using EBSD. This serial sectioning technique was used to investigate the recrystallization behaviour of a particle-containing nickel alloy, which revealed a number of features of the recrystallizing grains that are not clearly evident in 2D EBSD micrographs such as clear evidence of particle stimulated nucleation (PSN) and twin formation and growth during PSN.

Abstract: 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.

Abstract: 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.

Abstract: A typical dual-beam platform combines a focussed ion beam (FIB) microscope with a field emission gun scanning electron microscope (FEGSEM). Using this platform, it is possible to sequentially mill off > ~ 50 nm slices of a material by FIB and characterise, at high resolution, the crystallographic features of each new surface by electron backscatter diffraction (EBSD). The successive images can be combined to generate 3-D crystallographic maps of the microstructure. This paper describes various aspects of 3-D FIB tomography in the context of understanding the microstructural evolution of metals during deformation and annealing. The first part of the paper describes the influence of both metal type and milling parameters on the quality of EBSD patterns generated from a surface prepared by FIB milling. Single crystals of some face centred cubic metals were examined under varying FIB milling parameters to optimise EBSD pattern quality. It was found that pattern quality improves with increasing atomic number with the FIB milling parameters needed to be adjusted accordingly. The second part of the paper describes a useful technique for FIB milling for the reliable reconstruction of 3-D microstructures using EBSD. There is an initial procedure involving extensive milling to generate a protruding rectangular-shaped volume at the free surface. Serial sectioning is subsequently carried out on this volume. The technique was used to investigate the recrystallization behaviour of a particle-containing nickel sample, which revealed a number of features of the recrystallizing grains that are not clearly evident in 2-D EBSD micrographs.