Systems that form intermetallic compounds

Table 5.4. Contact angles for systems that form intermetallic compounds.

Beryllium forms intermetallic compounds with transition metals and phase diagrams are available Some 26 phase diagrams have also been published for Mg-transition metal systems, and intermetallic compound formation is widespread in these systems also. The extent of intermetallic compound formation decreases down group IIA, such that Ca, Sr and Ba show much less tendency for compound formation to the extent that compounds are observed only in the Ba-Pd system. 

Protons are not the sole species that can be incorporated into the lattices of different host materials. At the beginning of the 1960s, Boris N. Kabanov showed that during cathodic polarization of different metals in alkaline solutions, intercalation of atoms of the corresponding alkali metal is possible. As a result of such an electrochemical intercalation, either homogeneous alloys are formed (solid solutions) or heterogeneous polyphase systems, or even intermetallic compounds, are formed. 

The Binary System Silver-Strontium. A somewhat more complicated binary system, that formed by silver and strontium, is represented in Figure 24-7. It is seen that four intermetallic compounds are formed, their formulas being Ag Sr, Ag5Sr3, AgSr, and Ag Srg. These compounds and the pure elements form a series of eutectics for example, the alloy containing 25 weight percent strontium is the eutectic mixture of Ag Sr and AggSr. ... 

Rare earth elements form intermetallic compounds with non-transition metals or transition metals. Such RI compounds exist in a variety of crystalline structures which were described in detail by Taylor (1971) and Wallace (1973). The review articles of Buschow (1977a, 1979) and landelli and Palenzona (1979) contain more recent information about the crystalline structure of new RI compounds. We refer the reader to these articles for details. More recently it has been found that amorphous alloys can be prepared from R-atoms and both non-transition and transition metals at the same concentration of elements found in RI compounds (Cochrane et al., 1979). These alloys therefore serve as useful systems for comparison purposes. 

A somewhat more complicated binary system —that formed by silver and strontium —is represented in Figure 17-8. It is seen that four intermetallic compounds are formed, their formulas being AgsSr, AgjSr.-, AgSr, and... 

For some elements such as calcium (Ca) and a number of rare earth elements (REE) their suitability in binary Mg alloys has already been proven. But further work still needs to be undertaken to assure that the intermetallic compound of Mg-Ca and Mg-REE are also not harmful to the human body. In the case that ternary or even more complex alloys will be developed the same tests have to be applied to make sme that all compounds that may form are safe. The experimental work in this area has to be accompanied by thermodynamic calculations. While binary phase diagrams are available for most elemental combinations, there are also reliable phase diagrams for a number of ternary systems. A major challenge is still the calculation of phase formation in more complex systems. Moreover, it might be the case that Mg alloy systems which contain more than three alloy components will be better solutions for biodegradable implants than those ones which are actually presented in literature. 

A detailed study on the wetting characteristics of eutectic Bi-Sn solder has been reported in terms of impurity effects, fluxes, base metals, and soldering temperature [18]. It was determined that eutectic Bi n solder is far less tolerant of impurities than eutectic Plr n [19]. In particular, the presence of impurity elements which form intermetallic compounds with Bi-Sn solder, such as Cu, Ni, Fe, and Pd, is especially critical, while Sb and Pb appear to be beneficial in terms of promoting wetting characteristics. Table 1 compares the solderabihty of several low-melting-point solders on various surface metallizations [20]. Only the Au/Ni-plated metallization is considered acceptable for Bi n solder, while the Cu, Ni, and Au/Ni metallizations are all acceptable for eutectic Sn Pb solder when a rosin-base flux is used. Another study confirmed that Bi Sn solder does not wet Cu-base metallizations as well as eutectic Sn-Pb solder does when a rosin-base flux is used [21]. However, if the Cu surface is pre-tirmed, then Bi-Sn wetting is acceptable even with a rosin-base flux [22]. Based on the wettabihty studies, eutectic Bi-Sn solder can only be considered a viable candidate if a suitable flux system is developed which allows it to be utilized for metallizations other than a Au/Ni overplate. 

A binary phase system that forms an intermetallic compound normally provides good adhesion because of the mutual attraction between atoms of different species. Examples of this type of system include Ti-Cu, Ni n, and Sn-Cu (Fig. 29) [105]. Intermetallic compound formation occurs during high-temperature process steps, and the consequences are usually undesirable, resulting in increased stress levels, impurity snowplowing, Kirkendall voids, and an increase in electrical resistance. These reactions must be controlled either by hmiting the amount of reactants available or by controlling the time and temperature of the reaction. 

Titanium iron hydrides are among the materials which, at the present time, appear to have potential for practical applications as an energy-storage medium (7). The formation and properties of titanium iron hydride have been studied by Reilly and Wiswall (3), who found that the reaction proceeds in two steps as indicated by Reactions 5 and 6. Both hydrides have dissociation pressures above 1 atm at room temperature in contrast to TiH2 which is very stable. Titanium iron is representative of intermetallic compounds that consist of an element (titanium) capable of forming a stable hydride and another element (iron) that is not a hydride former or at best, forms a hydride with great difficulty. Iron presumably plays a role in destabilizing the hydrides. Titanium also forms a 1 1 compound with copper (there are other intermetallic compounds in the titanium-copper system) and this fact, coupled with the observation that copper... 

Calcium is an excellent reducing agent and is widely used for this purpose. At elevated temperatures it reacts with the oxides or halides of almost all metallic elements to form the corresponding metal. It also combines with many metals forming a wide range of alloys and intermetallic compounds. Among the phase systems that have been better characterized are those with Ag, Al, Au, Bi, Cd, Co, Cu, Hg, Li, Na, Ni, Pb, Sb, Si, Sn, Tl, Zn, and the other Group 2 (HA) metals (13). 

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