Impact of Grain Boundaries on Structural and Mechanical Properties
For some polycrystalline metals with grain sizes in the nano regime, experiments have suggested a deviation away from the Hall-Petch relation relating yield stress to average grain size . The debate continues whether or not such deviations are a result of intrinsically different material properties of nanocrystalline (nc) systems, or due simply to inherent difficulties in the preparation of fully dense nc-samples and in their microstructural characterization. Nevertheless, it suggests that the traditional work hardening mechanism of pile-up of dislocations originating from Frank-Read sources may no longer be valid at the nanometer scale. In-situ deformation testing in the transmission electron microscope (TEM), performed on Cu and Ni3Al nc samples, reveals a limited dislocation activity in grains below 50nm [2,3]. However, due to the presence of large internal stresses which make grain boundaries (GB) in TEM images difficult to observe, and also possible artifacts induced by thin-film geometry such as dislocations emitted from the surface , in-situ tensile tests did not until now, bring convincing evidence for abundant dislocation activity. Mechanical testing also revealed the issue of the “GB state” by means of a property dependence on thermal history and internal strains. It is shown that a substantial strengthening can be obtained by a short heat treatment. The cause of the strengthening is possibly associated with a reduction in internal strains and/or dislocation content produced by the annealing . The effect of strengthening has been measured both on nc materials obtained by grain refinement techniques and those obtained by consolidation of clusters.
KeywordsGrain Boundary Triple Junction Grain Boundary Slide Dislocation Activity Excess Free Volume
Unable to display preview. Download preview PDF.
- 1.Weertman, J.R. (2002) Mechanical behaviour of nanocrystalline metals, Nanostructured Materials: Processing, Properties, and Potential Applications, William Andrew Publishing, Norwich.Google Scholar
- 2.Youngdahl, C.J., Hugo, R.X., Kung, H. and Weertman, J.R. (2002), TEM observation of nanocrystalline copper during deformation, Structure and Mechanical Properties of Nanophase Materials-Theory and Computer Simulation vs. Experiment, MRS Symposium Series Vol. 634, B1.2.Google Scholar
- 3.McFadden, S.X., Sergueeva, A.V., Kruml, T., Martin, J-L. and Mukherjee, A.K. (2000), Superplasticity in nanocrystalline Ni3A1 and Ti Alloys, Structure and Mechanical Properties of Nanophase Materials-Theory and Computer Simulation vs. Experiment, MRS Symposium Series Vol. 634, B1.3.Google Scholar
- 23.Samaras, M., Derlet, P. M., Van Swygenhoven, H., and Victoria, M. (2003) SIA Activity during irradiation of nanocrystalline Ni, J. of Nucl. Mater., Submitted.Google Scholar