As engineering devices have reduced to sub-micron scale, materials behavior at small length scale has become increasingly important to understand quantitatively. Over the last few decades, advances in experimental devices for investigating submicron size specimens such as focused ion beam (FIB), nanoindentaion, and submicron-scale uni-axial compression test machine enable novel capabilities to explore extrinsic size effects or sample size effects. Specimen-size-effects-induced mechanical behaviors of FCC and BCC single crystalline metals and their deformation mechanisms are described in the first chapter. In addition, a comparison of the two crystals is summarized. The study on HCP materials is excluded in this review because sufficient experimental data and theories are not available at this point.
Directionally solidified NiAl-Mo composites consisting of well-aligned [100] Mo fibers embedded in a [100] NiAl matrix were pre-strained from 0% to 16% along the [100] direction. Samples were then extracted parallel to the [100] direction using focused ion beam (FIB) techniques and their cross-sections were examined via TEM. Also, the NiAl matrix was selectively etched away and the free standing Mo fibers were obtained to study dislocation networks. Preliminary TEM results showed that as-grown samples (without pre-straining) do not contain dislocations in the Mo fiber. In the NiAl matrix, on the other hand, <100>-type dislocations are observed. At intermediate pre-strain level, the <100>-type dislocations in the NiAl matrix are gathered at the inter-phase boundary and form slip transfer zones where stress builds up, which causes activation of <111>-type dislocations in the Mo fiber. The <111>-type dislocations in the Mo fiber are inhomogeneously distributed and its plasticity is determined by a slip transfer mechanism. With increased pre-strain, <111>-type dislocations in the Mo fiber interact with each other and form binary and ternary junctions so that the dislocation distribution becomes more homogenous. The homogeneity of dislocation distribution can be related to the transition from stochastic to bulk-like mechanical behaviors.
The crystallographically dependent mechanical responses of an α-Ti-7 wt% Al alloy were measured by nanoindentation using spherical indentor. Both elastic moduli and hardness responses of indents in the (0001), {101̅0}, and {112̅0}planes were quantified. The dislocation structures resulting from indentation were characterized by electron microscopy. While scanning electron microscopy techniques were used for the observation of surface structures, site-specific focused ion beam thin foil preparation and scanning transmission electron imaging techniques were employed for the imaging of subsurface dislocation structures. Elastic modulus, hardness, and load at pop-in were found to vary with crystallographic orientation. Indentation induced plasticity was found to occur by multiple mechanisms and to be dependent on crystal orientation; however, slip on (0001) was found to be common to all orientations.