Metastable intermetallic compounds

Metastable intermetallic compounds can be obtained from amorphous alloys after partial crystallization. Two exotherms are usually found when the corresponding alloys are heated in a DSC unit. The first exotherm originates from the crystallization of the metastable compound, the second exotherm corresponding to the transformation into the stable equilibrium phases. The metastable compound found are commonly obtained by heating to temperatures close to the first exotherm and subsequently cooling to room temperature. 

In the field of solid-state chemistry an important group of substances is represented by the intermetallic compounds and phases. In binary and multi-component metal systems, in fact, several crystalline phases (terminal and intermediate, stable and metastable) may occur. A few introductory remarks about these substances will be presented in relation to the mentioned figures. 

In the previous chapter we looked at some questions concerning solid intermetallic phases both terminal (that is solubility fields which include one of the components) and intermediate. Particularly we have seen, in several alloy systems, the formation in the solid state of intermetallic compounds or, more generally, intermetallic phases. A few general and introductory remarks about these phases have been presented by means of Figs. 2.2-2.4, in which structural schemes of ordered and disordered phases have been suggested. On the other hand we have seen that in binary (and multi-component) metal systems, several crystalline phases (terminal and intermediate, stable and also metastable) may occur. 

Before closing this section it should be pointed out that the formation of most hydrides of intermetallic compounds is metastable vith respect to disproportionation [35]. Again, taking LaNis as an example, the disproportionation reaction ... 

Now suppose that intermetallic compounds are suppressed in Fig. 2.16. Consider the metastable equilibrium between an amorphous phase and the terminal solutions of a-Zr (bcc Zr) and Ni. Under these conditions, the common tangent construction yields a metastable equilibrium state which consists of a single-phase a-solution for xZr > x4, a two-phase region of liquid (glass)/a-... 

Fig. 2.16. Free-energy diagram for the Ni/Zr system at 300°C showing the free energy of mixing as a function of composition for the hep, bcc, fee, and liquid/amorphous phases. Intermetallic compounds are shown by dots and are assumed to be line compounds. The common tangents for the hep/liquid, and liquid/fee metastable equilibrium are shown. These define compositions XLXj.X3.X4 [2.76]...

Mechanochemical treatment, as already emphasized, produces nanocrystalline/amorphous phases. Such pronounced metastable states readily react at elevated temperatures, thereby related equilibrium phases can be obtained relatively easily. For instance, high-temperature intermetallic compound AlgMos (melting point 2123 K), which, due to significantly different melting points of A1 and Mo is otherwise difficult to prepare from liquid phase, was obtained by continuous heating up to 1400 K of the mechanically alloyed Al-27 at.% Mo powder [46]. 

Aluminium-Copper. Al-Cu forms a simple eutectic system in the range Irom 0 to 53wt% Cu, as shown in Fig. 3.1-11. The a-Al solid solution and the intermetallic compound AI2Q1 (0 phase) are in equilibrium. At intermediate temperatures, metastable transition phases may form and precipitate from the supersaturated solid solution. These metastable phases may be characterised according to their crystal structure, the nature of the phase boundary they form, and their size ... 

Fig. 4. Schematic representation of intermetallic compounds occurring in R-Fe and R-Mn systems. The diagrams were constructed from the data compiled in Buschow s review (1977a). The broken parts in the R-Fe diagram indicate the existence of metastable RFej compounds (Meyer et al., 1981).

Haimovich, J. Kahn, D. Metastable nickel-tin intermetallic compound in tin-based coatings. Proceedings of the 77th AESF Annual Technical Conference, Boston, MA, American Electroplaters and Surface Finishes Society, Orlando, FL, 1990 689-712 pp. 

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