The importance of orientation measurement in polycrystalline materials..
Tutorial 3 The importance of orientation measurement in polycrystalline materials.
I am deviating from my first theme of strain measurement in crystalline materials using EBSD in response to requests on more fundamental aspects of the relevance of crystallography in polycrystalline materials.
Perhaps I may begin with a short introduction how I became interested in this field in the first place. I was at the time (1966) working on a transmission electron microscope project on the contrast of dislocations trapped in grain boundaries. It was the contrast of the dislocations that was of major interest rather than the properties of the boundaries in which the dislocations were trapped. That was to follow two to three years later. I was a post doc. in the materials department at Stanford University California. As part of the normal to and fro of conversation over a cup of coffee I was listening to Prof Oleg Sherby describing what was then the new and curious observation of the phenomena known as superplasticity. (Actually it had been discovered long before, C Jenkins 1928 and C Pearson 1934 but interest in it had fallen by the wayside until work in the Soviet Union encouraged the West to take it up again in 1960s). What was intriguing was the observation that after severe mechanical deformation and annealing to produce a two phase micron sized microstructure then the alloy, (AlZn in his case), could be deformed to 5 or more times its original length. Furthermore it did this under loads far lower than required to activate dislocations from Frank Read sources. The mechanism therefore had to be something to do with the grain boundary sliding and diffusional creep. Grain boundary sliding was well known but could not of itself result in large deformations as it was also associated with sever grain boundary cavitation. A closer look at the grain boundary sliding mechanism was needed and after my move to the Physics Department at Bristol University in 1967, that is what I set out to do.
It was a two pronged assault. One project involved TEM examinations of the behaviour of dislocations trapped in grain boundaries to see if they were ever glissile. We (Robert Pond and I) chose aluminium as the test material. The second thrust was to build a tensile stage for operation in the SEM and find a way of observing directly the grain boundary sliding phenomenon; I chose the superplastic alloy PbSn as my test alloy. The question was did the grains deform by slip followed by recovery and recrystallisation or did rapid diffusional processes, (Coble creep) control the issue? We needed a probe to test for this and it had to be a diffraction based technique. The development of this technique came to dominate my future work and lead eventually to the automated EBSD technique we are familiar with today. (The progress from Prof Venables early experiments in EBSD through my own on line computer assisted indexing to fully automated computer indexing routines can be found in the first chapter of the conference proceedings titled ‘Electron backscatter diffraction in materials science’ and published by Elsevier in 2000). The TEM work showed that dislocations were glissile in some boundaries but we could not find a simple rule to categories them, (Dingley and Pond 1977). The SEM work showed clearly the grain boundary sliding behaviour but the parallel search for a suitable diffraction technique to carry out the crystallographic studies was not successful. It took another decade to achieve this.
By the time I had worked through the different SEM based diffraction methods that were then being researched at Oxford University, (Electron Channelling Patterns, Prof. Joy, Dr Newbury), at Sussex University (EBSD Prof Venables) and at Bristol (Kossel diffraction, by myself) my interest in superplastic materials had waned. However, the outcome was eventually the automated EBSD technique so that in retrospect it was a decade that was well spent. It did provide the tool I sought for grain boundary studies, one that provides the means by which huge numbers of grain boundaries can be examined relatively quickly so that by judicious experimentation it might be established which boundaries have special properties and which do not.
And of course the occurrence of special boundaries as a controlling factor in mechanical properties of metals and alloys now includes rocks and semiconductor materials and encompasses creep, fatigue, intergranular fracture, grain boundary migration and recrystallisation. It also extends to determining if specific boundaries can block dislocation movement resulting in dislocation pile-ups and internal strain, or alternatively become dislocation sinks so relieving internal strain. We must include the role of grain boundaries in controlling electrical resistivity in this list as well as mechanical properties and I recall that very early on in my own research using EBSD, one project was entirely devoted to determining grain boundary types produced during laser annealing of polycrystalline silicon as used in photovoltaic cells and a means of detecting residual strain within them, Dingley and Burns 1983.
The first experiments using EBSD in the SEM were very straight forward and simply aimed at establishing whether the type of neighbouring grain misorientations known as the coincident site relationships were of special importance. It turns out they are not. Apart from twin oriented grains along with their second and higher order relatives these boundaries in fact don’t exist in polycrystalline metals more frequently than expected on purely statistical grounds. The emphasis has thus now changed to a search to determine if there are special grain boundary planes and both statistical methods and 3D EBSD are being employed in the search.
Of course grain boundary type is the result of the orientations of neighbouring grains and in the process of determining neighbouring grain misorientation using normal EBSD procedures the absolute orientations of the grains themselves is determined. The distribution of the absolute grain orientations is known as the ‘crystal texture’. It was the primary objective of many x-ray diffraction studies. EBSD has a great advantage over x-ray diffraction in this respect because the x-ray diffraction methods only determine the orientation distribution of a single crystallographic direction, (actually the normal to the crystal plane, the x-ray diffractometer is set to detect). The resulting distribution is known as a pole figure and a set of such figures, basal plane, pyramidal plane etc., is usually produced for a single specimen. Thus when using the x-ray technique statistical methods are needed to deduce what the likely overall orientations of the grains in the sample must be to give rise to the observed pole figures. The result is presented as an orientation distribution function. (I will discuss this function in more detail in my next tutorial). The result is therefore non spatially specific, i.e. there is no correlation between the function and specific grains in the sample. EBSD is superior to the x-ray method because both the orientation of a grain is obtained on each measurement of a pattern and that orientation is related to a specific grain and to a specific point within the grain. It has taken a long time to convince the x-ray diffraction community that for most purposes EBSD is an alternative and better tool than X-ray diffraction for texture determination and the reader interested in the evolution of EBSD for texture determination is referred to the successive proceedings of the ICOTOM conferences dating from 1987 when the first EBSD paper on texture determination using EBSD was presented.
Crystalline texture is of tremendous importance in metallurgy, geology and in semiconductor device physics. From a mechanical properties point of view it is the controlling factor in formability and a means of maximising strength in a particular direction. This all follows from the intrinsic anisotropic elastic properties of crystals and the restriction of plastic flow to specific crystal planes and directions within these planes. The latter property has given rise to the concept of Taylor factor and Schmidt factor parameters and the modelling of deformation behaviour dependent on the distribution of grain orientations and the values of the Taylor and Schmidt factors associated with each orientation. It leads to a modelling of the likely behaviour at grain boundaries based on whether the boundary separates hard and soft oriented grains or hard and hard oriented grains, hard in this case denoted by a high Schmidt factor and difficulty of activating slip.
Texture also affects thermo mechanical treatment, a texture developed during mechanical deformation, due to restriction of the slip systems, affects the texture produced on subsequent thermal treatment via recovery or recrystallisation processing. Modelling such behaviour is difficult with such a large matrix of parameters to be investigated. But it is quite certain EBSD has a major role to play as it can focus on differential behaviour of individually oriented grains or a particular phase in multiphase material following mechanical deformation and can follow the recrystallisation process of each component in subsequent heat treatments.
Both magnetic and electrical properties can likewise be affected by texture though whereas magnetic properties such as in the FeSi transformer steels are dominated by the strength and character of the texture alone it is mainly the type of grain boundaries resultant from the texture that controls electrical behaviour in metal interconnects and photovoltaic silicon devices. On the other hand, the connection between grain boundary type and crystal texture is not obvious. For example a random crystal texture in the nickel base alloys can actually be associated with a microstructure entirely dominated by the presence of twin boundaries.
Thus it is quite clear that grain orientation is of major importance in crystalline materials and that EBSD is a major tool in its investigation. I must admit however that although I was deeply involved in the development of the tool to solve some particular aspects of grain boundary behaviour right at the beginning, the development of the tool itself has distracted me from pursuing those materials science aspects so that now I must leave it to others to make these advances.
The next tutorial will be on the pictorial presentation of orientations, that is pole figures and orientation distribution plots. In the tutorial I will suggest an additional method of presentation that may make the information contained in these figures more obvious and useful to the material scientist.
Jenkins C.H.M. J.Inst. Metals. 40 (1928) 21
Pearson, C.E.J. J.Inst. Metals. 54 (1934) 111
Dingley D.J. and R. Pond, Acta Met.25 (1979) 677
Dingley D.J. and Burns. Inst. Phys. Conf. Ser. No. 68 Chapter 11 (1983) 433