Extension of the 'five-parameter' grain boundary stereological method to textured alloys

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Intergranular degradation such as corrosion can be a serious problem in many materials applications. Hence, it is of interest to study which grain boundary crystallographic parameters (i.e. boundary structure) are more resistant than others to this phenomenon. Recently it has become possible to use a stereological technique coupled with electron backscatter diffraction (EBSD) to measure all five grain boundary crystallographic parameters, i.e. both misorientation and boundary plane. This landmark experimental advance is delivering new insights into the link between grain boundary structure and properties. However, at present the 'five-parameter analysis' cannot be applied to materials which have a crystallographic texture, which includes many technologically important alloys.The first aim of this project is to remove that limitation. This will be achieved by the design and implementation of various multiple sectioning sampling routines (i.e. acquisition of EBSD maps from several non-parallel sectioning planes), in parallel with extensive revision of the existing software code to take into account non-random orientation distributions i.e. texture. Then the new scheme will be validated by application to alloys of the Al-Mg-Mn 5xxx series, which is being used to reduce mass in automotive chassis applications, and to commercially pure Ti. Focused ion beam (FIB) microscopy coupled with EBSD will be used to confirm and validate the procedures used. The data will provide information on the distribution of boundary planes, which is hitherto unknown in textured alloys, and will allow insights into the mechanism of texture formation. This information will then be applied to explore the relationship between boundary crystallography and intergranular corrosion in the Al-Mg-Mn alloys, which will in turn input to the larger initiative of reducing susceptibility of Al-Mg alloys to intergranular corrosion by proper control of the final gauge texture. This is a cost-effective project because the experimental work is well suited to be carried out by a research student and there are no equipment costs. The principal project partner is Professor Rohrer's group at Carnegie Mellon University, USA, where the five-parameter analysis was developed, and who will carry out the software revision. Other project partners are Hydro Aluminium, Germany, who have an interest in grain boundary network analysis in aluminium alloys and are providing specimens for the project, and the University of New South Wales, who are supporting the work with complementary FIB analysis. Extension of the five-parameter technique to textured materials would give it much more general applicability as a microstructural tool. Furthermore, given that mapping speeds by EBSD are now very rapid, it is envisaged that in the future five-parameter analysis could become incorporated into mainstream grain boundary analysis.