Intermetallic compound melting point

Reference has already been made to the high melting point, boiling point and strength of transition metals, and this has been attributed to high valency electron-atom ratios. Transition metals quite readily form alloys with each other, and with non-transition metals in some of these alloys, definite intermetallic compounds appear (for example CuZn, CoZn3, Cu3,Sng, Ag5Al3) and in these the formulae correspond to certain definite electron-atom ratios. 

A chapter dedicated to the laboratory (small scale) preparation methods of inter-metallics has then been included (Chapter 6). In the preparation of intermetallic phases, indeed (or, more generally, of alloys), in comparison to other chemical compounds a number of interesting and significant peculiarities are pointed out, for instance, those related to their high melting points, insolubility in many common solvents, etc. The presentation of selected examples of preparative methods, therefore,... 

The trend of intermetallic reactivity and alloy stability of V, Nb and Ta with the different elements may be further discussed in terms of the melting points of the compounds as described in the following paragraphs. 

Remarks on the melting point trends in the binary alloys of Be, Mg and of the 12th group metals. The intermetallic reactivity of these metals and the stability of their compounds are also highlighted by the trends of the melting points of their alloys. A selection of these data has been collected in Tables 5.57 and 5.58 where compounds of Be and Mg and of Zn and Hg are listed. For several systems, information only on the existence of intermediate phases with no indication about their melting temperature is reported. 

The intermetallic reactivity of Si and Sn and the stability of their compounds are also indicated by the trends of the melting points of their alloys. A selection of these data has been collected in Table 5.71. 

Betterton-KrollProcess. Metallic calcium and magnesium are added to the lead bullion in a melt and form ternary compounds that melt higher than lead and are lower in density. By cooling the lead bath to a temperature close to the melting point of lead, the intermetallic compounds high in bismuth content solidify and float to the top where they are removed by skimming. 

The melting point of titanium is 1670°C, while that of aluminium is 660°C.142 In kelvins, these are 1943 K and 933 K, respectively. Thus, the temperature 625°C (898 K) amounts to 0.46 7melting of titanium and 0.96 melting of aluminium. Hence, at this temperature the aluminium atoms may be expected to be much more mobile in the crystal lattices of the titanium aluminides than the titanium atoms. This appears to be the case even with the Ti3Al intermetallic compound. The duplex structure of the Ti3Al layer in the Ti-TiAl diffusion couple (see Fig. 5.13 in Ref. 66) provides evidence that aluminium is the main diffusant. Otherwise, its microstructure would be homogeneous. This point will be explained in more detail in the next chapter devoted to the consideration of growth kinetics of the same compound layer in various reaction couples of a multiphase binary system. 

At least with intermetallics, the effect of melting points and atomic radii on the sequence of occurrence of compound layers at the A-B interface seems to be more or less straightforward. On the contrary, the influence of the crystal structure of the compounds is rather obscure. Probably, those with less symmetrical and loosely packed structures may be expected to form first under highly non-equilibrium and stressed conditions usually encountered in reaction-diffusion experiments. 

Table 3.3. Standard enthalphies (heats) of formation of nickel aluminides and their effective heats of formation calculated for the effective concentration at the interface corresponding to the composition (3.5 at.% Ni, 96.5 at.% Al) of the eutectic with the lowest melting point in the Ni-Al binary system.261 For all the intermetallic compounds, the limiting element is nickel...

In pure titanium, the crystal structure is dose-packed hexagonal (a) up to 882°C and body-centered cubic (p) to the melting point. The addition of alloying dements alters the a—p transformation temperature. Elements that raise the transformation temperature are called a-stabilizers those that depress the transformation temperature, p-stabilizers the latter are divided into p-isomorphous and p-eutectoid types. The p-isomorphous elements have limited a-solubility and increasing additions of these dements progressively depresses the transformation temperature. The p-eutectoid elements have restricted p-solubility and form intermetallic compounds by eutectoid decomposition of the p-phase. The binary phase diagram illustrating these three types of alloy... 

---