Structural Nanocrystalline Materials: Mechanical properties of structural nanocrystalline materials – experimental observations

In this chapter we will describe and discuss the experimental evidence for the mechanical behavior of nanocrystalline materials. This will include pure metals, alloys, intermetallic compounds, ceramics, and multiphase materials. The range of mechanical properties for which measurements have been made will be covered. While the models and theoretical explanations for the various phenomena believed responsible for mechanical properties of nanocrystalline materials will be emphasized in Chapter 5, some discussion of deformation mechanisms must necessarily accompany the description of experimental results. Elastic properties of nanostructured materials The early measurements of the elastic constants on nanocrystalline materials prepared by the inert-gas-condensation method gave values, for example for Young's modulus, E , which were significantly lower than values for conventional grain size materials (Suryanarayana, 1995). While various reasons were given for the lower values of E , it was suggested by Krstic and co-workers (1993) that the presence of extrinsic defects, e.g. pores and cracks, was responsible for the low values of E in nanocrystalline materials compacted from powders. This conclusion was based upon the observation that nanocrystalline NiP produced by electroplating with negligible porosity levels had an E value comparable to fully dense conventional grain size Ni (Wong et al. , 1993). Krstic et al . (1993) and Boccaccini et al . (1993) developed theories to account for the decrease in E with porosity which agree with E vs.% porosity data on nanocrystalline Fe produced by inert-gas condensation and warm consolidation (Fougere et al. , 1995).