Valerie Randle

Mechanism of twinning-induced grain boundary engineering in low stacking-fault energy materials

Abstract The current status of “grain boundary engineering” is overviewed, i.e. the deliberate manipulation of grain boundary crystallography in polycrystals in order to produce a material containing grain boundaries which have superior properties compared to average boundaries. A particular focus of attention is annealing twinning in low stacking-fault energy (SFE) materials as a means to achieve this aim, since the role of such twinning in improving the grain boundary network has not hitherto been satisfactorily explained. The twinning is discussed from the viewpoint of strain retention, imposition of crystallographic constraints at grain junctions, “relative specialness” rather than “absolute specialness” and proposal of a new “Σ3 regeneration model” to explain how twins can enhance the Σ3 boundary fraction in the network.

Overview No. 127The role of the grain boundary plane in cubic polycrystals

This paper presents an appraisal of investigations which feature statistics of grain boundary plane distributions in polycrystals. The approach taken is to set the review against a background which includes the significance of the grain boundary plane parameter and several methodologies for its investigation in both bicrystals and polycrystals, including high resolution electron microscopy and computer simulation. The digest of data in polycrystals is analysed in the light of tilt and twist characteristics, boundary plane reorientation, the occurrence of low-index boundary planes and correlation with boundary properties. It emerges that the key factors which control the crystallography of boundary planes are the combined effects of material type, twinning (where applicable), microtexture, proximity to a coincidence site lattice and boundary plane inclination with respect to the macroscopic specimen geometry. Grain boundary plane engineering, wherein CSL categorisation is secondary to boundary plane crystallography, is recommended as a more advanced approach and its feasibility is demonstrated.

The deformation behaviour of grain boundary regions in polycrystalline aluminium

Abstract The deformation pattern at grain boundaries and at triple junctions in polycrystalline high purity aluminium (99.999%) has been studied by electron back scattering pattern (EBSP) observations. Specimens of two different grain sizes rolled to give 5% and 30% reductions have been examined by these different EBSP scans: (i) scans across grain boundaries, (ii) scans along grain boundaries and (iii) two-dimensional scans near triple junctions. These scans are carried out in small steps (1–5 μm) over long distances (up to 50 μm). The EBSP measurements show that the level of perturbations increases with strain and that enhanced zones of perturbations are observed at grain boundaries and especially near triple junctions. In specimens deformed by 30%, such zones of large perturbation are observed at most of the grain boundaries, and in the specimens deformed by 5%, at some triple junctions. The EBSP measurements are compared to previous microstructural observations by transmission electron microscopy and ...

Fine tuning at Σ3n boundaries in nickel

Abstract The mechanisms for evolution of a grain boundary population are investigated in sequentially strain annealed superpure nickel. Whereas the overall population of coincidence site lattice (CSL) boundaries does not increase with extended annealing times, the proportions of Σ3ns which approach the exact Σ3 configuration increases considerably, which is shown to be due mainly to local lattice rotations. The termfine tuning is coined to describe this type of grain boundary engineering. In general, fine tuning is not observed at other CSLs, and this is considered to be because the potential for energy reduction is far greater at Σ3s than at other CSLs. The fine tuning that occurs at Σ9s and Σ27s is considered to be “geometrically necessary” because of the relationship between Σ3, Σ9 and Σ27 at grain junctions.

The coincidence site lattice and the ‘sigma enigma’

This paper focuses on the most appropriate use of electron back-scatter diffraction (EBSD) post-acquisition analysis for categorisation of grain boundaries in polycrystals. A brief survey of the early literature shows that the most meaningful reference structure for grain boundaries in polycrystals is periodicity in the grain boundary surface, rather than the misorientation-based coincidence site lattice (CSL) and Σ notation. However, use of the CSL is convenient to the experimentalist. It is therefore suggested that wherever possible, misorientation data, obtained by EBSD mapping and classified according to CSL types, should be supplemented with other, more detailed information to aid analysis.

A comparison between three-dimensional and two-dimensional grain boundary plane analysis

An EBSD-based methodology for assessment of grain boundary planes, in particular sigma3 boundaries in face-centred cubic materials, has recently been devised. The method is based on trace analysis in a single, two-dimensional section rather than the more arduous three-dimensional method. The paper reports a data set of grain boundary planes in alpha-brass. mainly sigma3s, which have been analysed using both methods so that they can be compared and a recommendation made about the usefulness of the new method. It is shown that the new, two-dimensional method is a valuable tool in the analysis of grain boundary geometry, especially when used in conjunction with v/v(m), a parameter for assessing the proximity to the misorientation reference structure.

Continuous recrystallization-related phenomena in a commercial Al-Fe-Si alloy

The phenomenon of continuous recrystallization has been investigated in a heavily cold rolled commercial Al–Fe–Si alloy (AA8079) containing a distribution of both large and small FeAl3 second-phase particles. Transmission electron microscopy revealed that at lower temperatures the small particles restricted growth by pinning boundaries and at higher temperatures the particles coarsened in size allowing homogeneous subgrain growth to take place. The large particles did not act as preferential sites for nucleation and critical particle sizes calculated for particle-stimulated nucleation are given. It was concluded that continuous recrystallization in the present investigation is a combination of two processes; subgrain growth controlled by particle coarsening and grain growth commencing from a structure comprising predominantly high-angle boundaries.

Grain boundary engineering and the role of the interfacial plane

Abstract Grain boundary engineering (GBE) involves the use of microstructural design to improve bulk material properties and enhance resistance to intergranular degradation. More specifically, the patented GBE procedure involves the design and control of fcc metallic microstructures using thermomechanical treatments and grain boundary characterisation based on the coincidence site lattice model. The phenomenon of multiple twinning is used to create a ‘twin limited’ microstructure, i.e. a microstructure composed entirely of special grain boundaries and triple junctions that is highly resistant to intergranular degradation. However, the theory behind GBE is not fully developed and therefore further study of the interfacial geometry, including the grain boundary plane and its role in GBE, is required to improve understanding of multiple twinning with the ultimate aim of improving the bulk and intergranular properties of metallic materials. An introduction to GBE is presented, including a number of cases where grain boundary design has improved the properties of fcc alloys for industrial applications. The theoretical characterisation of grain boundaries, including interfacial structure and geometry, is reviewed, highlighting the problems associated with microstructural characterisation based on limited knowledge of the grain boundary geometry. The importance of the grain boundary network is discussed: the grain boundary and triple junction character distributions are known to have a significant influence on bulk properties. Finally, the role of the interfacial plane is considered. It is concluded that although GBE has produced significant results, its theoretical basis and the ultimate creation of twin limited microstructures require further development.

Grain-boundary plane reorientation in copper

Abstract This paper describes an experiment to ‘fine tune’ or reorient grain-boundary planes in the interior of a copper specimen after annealing in air, focusing on the Z-3 boundaries. The initial heat treatment consisted of annealing in air at 900°C for 1 h. This resulted in a population of Z-3 boundaries which was mainly displaced from low-energy configurations and was a ‘snapshot’ of interrupted grain growth. The control specimen was further annealed in air at 540°C for 97 h with the aim of reorienting the grain-boundary plane population. This aim was achieved since, in contrast with the control data, the entire Z-3 population was close to the lowest-energy configurations. The data were interpreted in terms of ‘nucleation’ and ‘impingement’ twins, and also in terms of the atomic-level facetting of vicinal boundaries onto special facets plus steps. The latter strategy infers a useful link between macroscopic boundary plane experiments and results from high-resolution electron microscopy. Finally, the r...

Combined application of electron backscatter diffraction and stereo-photogrammetry in fractography studies.

The main aim of this paper is to report on recent experimental developments that have succeeded in combining electron back‐scatter diffraction (EBSD) with stereo‐photogrammetry, compared with two other methods for study of fracture surfaces, namely visual fractography analysis in the scanning electron microscope (SEM) and EBSD directly from facets. These approaches will be illustrated with data relating to the cleavage plane orientation analysis in a ferritic and C‐Mn steel. It is demonstrated that the combined use of EBSD and stereo‐photogrammetry represents a significant advance in the methodology for facet crystallography analysis. The results of point counting from fractograph characterization determined that the proportions of intergranular fracture in C‐Mn and ferritic steels were 10.4% and 9.4%, respectively. The crystallographic orientation was determined directly from the fracture surface of a ferritic steel sample and produced an orientation distribution with a clear trend towards the {001} plane. A stereo‐photogrammetry technique was validated using the known geometry of a Vickers hardness indent. The technique was then successfully employed to measure the macroscopic orientation of individual cleavage facets in the same reference frame as the EBSD measurements. Correlating the results of these measurements indicated that the actual crystallographic orientation of every cleavage facet identified in the steel specimens is {001}.