Intermetallic bonds, strength

In general, there are insufficient data available for quantitative estimates to be made of the hardnesses of intermetallic compounds. However, in some cases trends can be verified. Figure 8.11 illustrates one of these. It indicates that hardnesses and heats of formation tend to be related. In this case for a set of transition metal aluminides. The correlation in this case might have been improved if the heats per molecular volume couls have been plotted, but thr molecular volumes were not available. Nevertheless, the correlation is moderately good indicating that hardness and chemical bond strengths are related as in other compounds. 

Figure 7.7 Experimental volcano curve for H2 evolution on Ni intermetallics. M-H bond strength assumed proportional to the heat of formation of intermetallics. Adapted from [27].

Jaksic has tried to propose a predictive basis for the hydrogen reaction beyond the volcano curve. The idea is based on Brewer-Engel s theory [82] for bonding in metals and intermetallic phases. According to this theory, a maximum in bond strength and stability of the intermetallic phases is expected as a metal lying at the beginning of a transition period in the Periodic Table (e.g., Ti or Zr) is combined with... 

Metallurgical methods are usually needed to prepare intermetallics [535]. Therefore, an extension of the surface area cannot be expected. If there is an increase in activity this can realistically be attributed to electrocatalytic effects. Since in most cases the Tafel slope is not lower but actually higher than for Ni [53], the activity enhancement has to be related to a change in the adsorptive properties of the surface so that both the M - H surface bond strength and the degree of coverage with H are different. For instance, in the case of TiFe, a lower M - H bond strength and a lower... 

Intermetallic compound coatings on carbon steel, such as NiAl with excellent bond strength, were synthesized by solar energy concentrated by a Fresnel lens (Rogriguez et al., 1999). The SHS technique can also be used to immobilize radioactive reactor graphite waste by the self-sustaining reaction ... 

Intermetallic compounds derive their great usefulness by blending metallic and covalent bonds. The former generate toughness, while the latter provide strength and hardness. In many of them dislocations move with great difficulty. 

Intermetallics form because the strength of bonding between the respective unlike atoms is larger than that between like atoms. Accordingly, intermetallics form... 

It may be supposed that the particular crystal structure of an intermetallic reflects the character and strength of bonding in a sensible way, and thus the crystal structure would be a good criterion for phase classification. However, this does not mean that intermetallics with the same crystal structure are similar with respect to bonding and properties since, e.g. the B2 structure is common to the intermetallic Ni Al and to the purely ionic salt CsCl, and in the case of the transition-metal disilicides with a... 

The y-phase is a solid solution with a face-centered crystal lattice and randomly distributed different species of atoms. By contrast, the y -phase has an ordered crystalline lattice of II2 type (Figure 10.2). In pure intermetallic compound NisAl the atoms of aluminum are placed at the vertices of the cubic cell and form the sublattice A. Atoms of nickel are located at the centers of the faces and form the sublattice B. The y -phase has remarkable properties, in particular, an anomalous dependence of strength on temperature. The y -phase first hardens, up to about 1073 K, and then softens. The interatomic bondings Ni-Al are covalent. 

FIGURE 45.4 Cross-sectional micrograph of SAC EGA solder ball bonded to a silver surface finished PWB bond pad. Planar voids are clearly evident in the section and in the magnified inset. Voids in close adjacency above the intermetallic layer and the BGA ball can significantly detract from the solder-joint strength. (Courtesy of Hewlett-Packard). 

---