F.J. Humphreys

Dynamic recrystallisation and the development of microstructure during the high temperature deformation of magnesium

Abstract Polycrystalline specimens of Mg-0.8%Al have been deformed in compression at elevated temperatures, and the microstructure and texture investigated. At temperatures above ~425 K. deformation tlinning is folloled by dynamic recrystallisation. Optical and electron microscopy have sholn that lattice rotations occur in the grain boundary regions, and that these are also the sites for recrystallisation. The mechanism of dynamic recrystallisation at temperatures betleen 425 and 600 K is sholn not to involve significant high angle boundary migration, but to be similar to a mechanism reported for several minerals. At temperatures belol ~600 K. the deformation becomes macroscopically inhomogeneous at higher strains and is confined to shear lones, lhich are fine grained regions formed by dynamic recrystallisation and lhich are geometrically softer than the remainder of the specimen.

The nucleation of recrystallization at second phase particles in deformed aluminium

Techniques of X-ray and electron diffraction, and in situ annealing in a High Voltage Electron Microscope have been used to investigate the nucleation of recrystallization at large second phase particles in deformed aluminium alloy single crystals. It is found that particle stimulated nucleation occurs above a critical particle size which increases with decreasing strain. Recrystallization originates within a zone of high dislocation density and large lattice misorientation at the particles, and proceeds by a rapid polygonization process, resulting in grains with orientations closely related to the as deformed structure. The deformation structure, mechanisms of nucleation and the resulting orientations are discussed in terms of a model of the inhomogeneous deformation which occurs in the vicinity of non deformable second phase particles.

A unified theory of recovery, recrystallization and grain growth, based on the stability and growth of cellular microstructures—I. The basic model

An analysis of the growth and stability of cellular microstructures, which takes account of the orientation dependence of boundary energies and mobilities, is presented and used to develop a simple unified theory of continuous and discontinuous recovery, recrystallization and grain growth. The analysis shows that low angle subgrain structures are intrinsically very unstable whilst medium to high angle structures may be very stable. The model is used to analyse discontinuous subgrain growth, primary recrystallization, the effects of texture on grain growth and the stability of highly strained metals.

The application of EBSD to the study of substructural development in a cold rolled single-phase aluminium alloy

Abstract Scanning electron microscopy and high resolution electron backscatter diffraction (EBSD) have been used to study substructural development during cold rolling of a single-phase Al–0.1 Mg alloy, the use of EBSD enabling more detailed quantitative measurements to be made than are possible with the transmission electron microscope (TEM). At low strains, bands of elongated cells, aligned at approximately 35° to rolling direction are formed. As the applied strain was increased, intersecting thinner and more widely spaced bands form within many grains, flow becomes localised within these new bands and they develop into microshear bands, which shear the original elongated cell structures. The changes in the scale of the microstructural features, the development of misorientations of the various types of low angle boundary and the alignment of the features to the rolling plane have been measured as a function of strain. The results are compared with previous TEM investigations of deformed aluminium, and a qualitative model of the microstructural evolution is proposed.

A unified theory of recovery, recrystallization and grain growth, based on the stability and growth of cellular microstructures—II. The effect of second-phase particles

Abstract A simple theory which analyses discontinuous and continuous recovery, recrystallization and grain growth, and which enables the relationships between the various annealing phenomena to be explored, has been extended to include the effects of a dispersion of second-phase particles. Application to grain growth results in predictions which are consistent with those of more complex theories, and the model is also used to analyse the effect of a dispersion of second-phase particles on recrystallization occurring by either strain induced boundary migration or by particle-stimulated nucleation. An analysis of subgrain growth shows that this may occur discontinuously under conditions in which recrystallization is not possible, thereby suggesting the possibility of a thermomechanical processing route in which complete softening may occur without recrystallization.

Local lattice rotations at second phase particles in deformed metals

Abstract An experimental investigation of local lattice rotations at non-deformable second phase particles in the size range 0.02 to 5 μm in aluminium and copper single crystals deformed to tensile strains of up to 0.5 is reported. A TEM method in which the misoriented regions are imaged in dark field as the specimen is accurately tilted has been used, and the results have been confirmed by standard X-ray and electron diffraction techniques. The region of maximum misorientation, close to the particle, is rotated about the roller axis [1 2 1] with respect to the matrix. For particles larger than ~ 2 μm, the misorientation increases linearly with shear strain, and values of up to ~ 45° have been measured. The misorientation for a given strain decreases with decreasing particle size, being very small for particles of diameter less than ~ 0.1 μm. These results, together with previous work, are used to assess the importance of the various deformation mechanisms at particles, over a wide range of strains and particle sizes.

Interaction of recrystallization and precipitation: The effect of Al3Sc on the recrystallization behaviour of deformed aluminium

Abstract The kinetics and mechanisms of recrystallization in high purity binary Al-Sc alloys have been investigated. In an Al-0.25wt.%Sc alloy, precipitation always precedes recrystallization, which is severely inhibited. However, in an Al-0.12wt.%Sc alloy, precipitation is found to either precede, follow or occur concurrently with recrystallization, depending on the processing conditions, and the resultant complex microstructures are interpreted in terms of the interactions between precipitation and recrystallization. Coherent precipitates are found to coarsen and become semi-coherent during the passage of low-angle boundaries during recovery. The passage of high-angle boundaries through semi-coherent precipitates during recrystallization occurs by migration of the boundary through the particle, resulting in the precipitates maintaining semi-coherency with the new grain.

Orientation averaging of electron backscattered diffraction data.

The use of data averaging to improve the angular precision of electron backscattered diffraction (EBSD) maps is discussed. It is shown that orientations may be conveniently and rapidly averaged using the four Euler‐symmetric parameters which are coefficients of a quaternion representation. The processing of EBSD data requires the use of an edge preserving filter and a modified Kuwahara filter has been successfully implemented and tested. Three passes of such a filter have been shown to reduce orientation noise by a factor of ∼10. Application of the method to deformed and recovered aluminium alloys has shown that such data processing enables small subgrain misorientation (< 0.5°) to be detected reliably.

Production of ultra-fine grain microstructures in Al-Mg alloys by coventional rolling

Abstract The conditions under which micron-scale grain structures can be developed in two Al–3%Mg alloys by a process of continuous recrystallization, during rolling and plane strain compression to large strains, have been investigated using high resolution electron backscatter diffraction (EBSD). In an Al–Mg–Cr–Fe alloy, it was found that a lower limit to the processing temperature for forming ultra-fine grain structures is imposed by the low mobility of grain boundaries, whilst an upper limit is imposed by grain growth. In an Al–Mg–Sc–Zr alloy containing stable second-phase dispersoid particles, a fine-grained microstructure is formed only at larger strains and higher temperatures due to the interaction of grain boundaries with the dispersoids. It is concluded that although micron-scale grain structures can be produced in Al–3%Mg alloys by deformation processing in plane strain compression, the processing window, which is controlled by both the solute and particle content, is severely restricted.

Subgrain growth and low angle boundary mobility in aluminium crystals of orientation {110}〈001〉

Single crystals of orientation {110}〈001〉 of a high purity Al–0.05% Si single-phase aluminium alloy have been deformed in channel die plane strain compression at room temperature and 350°C. The specimens were annealed at temperatures between 250 and 400°C and detailed measurements have been made of the extensive subgrain growth which occurs in these crystals. It was found that subgrain growth tended to be discontinuous, confirming earlier experimental and theoretical work, and showing that subgrain growth is quite different from normal grain growth. The mean misorientation between subgrains decreased during annealing and this was shown to have a strong effect on the kinetics of subgrain growth. The mobilities of low angle boundaries (2.6°<θ<5.6°) at temperatures between 250 and 400°C were determined from the subgrain growth kinetics and the activation energies for migration found to be consistent with control by lattice diffusion of solute. The boundary mobilities were found to increase rapidly with increasing misorientation and the results have been compared with the predictions of current theories.