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PROPERTIES 77 flow and on the rotating cold finger is sufficient to almost totally react the powder by compaction alone. In the Cu-Er system, the formation of a partially amorphous phase by a combination of mechanical cold- rolling of elemental foils and their subsequent thermal annealing was reported by Atzmon et al. (1985). Yet, no formation of an amorphous phase was observed in the compacted nanophase pellet, although the composition was in the right range. The most probable reason is that heating of the sample during the compaction process caused by the mechanical energy and the release of the chemical energy of the reacting particles (owing to a large negative heat of mixing), allowed the sample to exceed the fairly low crystallization temperature of about 470 K for this composition. A solution to this problem would be to use a more appropriate system like Ni-Zr with a much higher crystallization temperature or to optimize the compaction process for minimal heating. MECHANICAL PROPERTIES Some of the most significant changes in physical properties of nanometer-scale structures are found in their mechanical properties, elastic as well as plastic. One of the most striking reports in the study of the physical properties of artificial multilayers and in situ composites has been that of the âsupermodulusâ in metallic artificial multilayers. It was found that, for composition modulations around 2 nm, some of the elastic moduli increased by as much as a factor of 4. A similar finding was made in ultrafine in situ composites, as illustrated in Figure 21. The lowering of the modulus with reduction in the unannealed specimens is due to the increase in dislocation density. The increase in the modulus in the annealed specimens is attributed to the âsupermodulusâ effect. Figure 21 Young's modulus of in situ-formed Cu-Nb filamentary composites as a function of wire diameter, thermal history, and composition (Bevk, 1983).
PROPERTIES 78 The experimental situation still needs some clarification, however. A complete and consistent set of moduli for one film needs to be measured. Some investigators, using different measuring techniques or differently prepared samples, have failed to find the effect. The theoretical understanding is in particularly great need of attention. Two categories of theories can be distinguished. Those in the first category consider the interaction between the Fermi surface of the metal and the artificial Brillouin zone boundary created by the composition modulation. The second set of theories is based on the nonlinear elastic effects resulting from the large coherency strains across the phase boundary. The reduction of the modulus of the annealed specimens of Figure 21 as a result of a small amount of plastic formation, for example, points in this direction. Both types of theories, however, are still rather unsatisfactory. Concerning the effects of a nanometer-scale microstructure on the plastic properties, several aspects should be considered. It is well known, for example, that refining the scale of the microstructure leads to an increase in the yield strength of a material. The effect of grain boundaries as obstacles to dislocation motion is reflected in the Hall-Petch relation between grain size and yield strength. It is of great interest to explore to what extent this relation can be extended down to the 1-nm grain-size range. In addition, it would be worthwhile to know if the physical modeling of the hardening, which for âconventionalâ grain sizes consists of the activation of a dislocation source in a new grain by a dislocation pile-up against the boundary in an adjacent grain, should be modified to account for the observation. The effect of second-phase particles on dislocation motion is also well known. A particularly dramatic example of the strengthening effects is found with ultrafine in situ composites. The strength of these composites is considerably above that predicted by the ârule of mixturesâ from the strength of the individual components, which works well for filaments with diameters greater than a micrometer. The role of the filaments in inhibiting the dynamic recovery of the work-hardened matrix seems to be significant, resulting in higher matrix dislocation densities than could be achieved in bulk. The filaments themselves are often found to be almost dislocation-free and whisker-like. Similarly, the plastic properties (especially hardness) of artificial multilayers have also been seen to be very different at small layer thicknesses. For example, the hardness of TiN-VN multilayers is much higher for layer repeat lengths around 2 nm than at higher repeat lengths or in thin films of the individual compounds (Helmersson et al., 1987). The âsupermodulusâ effect may be contributing to this through the line tension of dislocations. At the same time, the interaction of the dislocations with a high density of interfaces, which can include unusual interlayer image stress effects, can lead to strengthening effects.
