Complete solid solubility

As you can see from the tables in Chapter 1, few metals are used in their pure state -they nearly always have other elements added to them which turn them into alloys and give them better mechanical properties. The alloying elements will always dissolve in the basic metal to form solid solutions, although the solubility can vary between <0.01% and 100% depending on the combinations of elements we choose. As examples, the iron in a carbon steel can only dissolve 0.007% carbon at room temperature the copper in brass can dissolve more than 30% zinc and the copper-nickel system - the basis of the monels and the cupronickels - has complete solid solubility. 

Describe, using the copper-nickel ("monel") system as an example, the process of zone-refining. (Figure A1.51 shows a system with complete solid solubility.) Flow many phases are present in an alloy of 60 wt% Ni and 40 wt% Cu at ... 

However, just as two liquids may be completely miscible and form a complete range of solutions from one pure liquid to the other, so certain metals, for example copper and nickel, exhibit complete solid solubility over the whole range of compositions from pure copper to pure nickel. Clearly for two metals to be soluble in each other over the whole compositional range, they must have the same crystal structure, i.e. they must be isomorphous. 

In the present case there are no ternary invariant equilibria in the system, partly due to the complete solid solubility of the A-B system. In a ternary system composed from three binary eutectic sub-systems, three univariant lines would meet in a ternary eutectic equilibrium ... 

Two elements or compounds that do not adopt the same crystal structure cannot exhibit complete solid solubility except when one of the space groups is a subgroup of the other. The energetics of solid solutions of compounds with different structures are obviously difficult to treat systematically and trends may be impossible to obtain, since the energetics is largely related to structural short-range order. We will thus confine our discussion of solid solutions to systems where the two end-members take the same crystal structure. 

Person 1 Do Cu and Ni satisfy the first and second Hnme-Rothery rnles for complete solid solubility ... 

The transient behavior at the interface of two ternary alloy compositions in a system with complete solid solubility will lead to a path in composition space as shown in Fig. 6.1. Evolution is initially parallel to the fast eigendirection / and, after its gradients become small, finally proceeds parallel to the slow direction s. 

Complete solid solubility requires that components have the same crystal structure, similar atomic size, electronegativity and valency. If any of these conditions are not met, a miscibility gap will occur in the solid state. 

Solid solutions with complete solid solubility, i.e., solid solubility over the entire range of the composition, are possible to form, but always of the substitutional kind. For a metallic binary solution to exhibit a complete solid solubility, for instance, both metals must have the same type of crystal structure, because it must be possible to replace, progressively, all the atoms of the initial solvent with solute atoms without causing a change in crystal structure. 

Fig. 12-1 Phase diagram of two metals, showing complete solid solubility.

The consequence of the above considerations is that binary systems of alkali metal halides form different types of phase diagrams, starting with the simple eutectic ones through the solid solution eutectic ones, the phase diagrams with the formation of a binary compound up to those with complete solid solubility. Tables 2.4 and 2.5 summarize the main features of individual phase diagrams. 

Complete solid solubility is observed and four phases with related structures are reported for ... 

Mutual solubility and solid solution formation over the entire composition range, also known as complete solid solubility. 

For complete solid solubility to occur between two end members, the following conditions have to satisfied ... 

Structure type. The two end members must have the same structure type. For instance, Si02 and Ti02 would not be expected to form complete solid solubility. 

Valency factor. The two end members must have the same valence. If this condition is not satisfied, compensating defects form in the host crystal in order to maintain charge neutrality. Given that the entropy increase associated with defect formation is not likely to be compensated for by the energy required to form them over the entire composition range, complete solid solubility is unlikely. 

Size factor. As a result of the mismatch in size of the solvent and solute ions, strain energy will develop as one is substituted for the other. For complete solid solubility to occur, that excess strain energy has to be low. Hence, in general, the size difference between the ions has to be less than 15 percent. 

A typical phase diagram for two compounds that form a complete solid solubility over their entire composition range is shown in Fig. 8.4. Both NiO and MgO crystallize in the rock salt structure, and their cationic radii are very similar. 

Sometimes systems that exhibit complete solid solubility will also exhibit either a maximum (rare in ceramic systems) or a minimum, as shown in Fig. 8.5. 

Figure 8.5 System with complete solid solubility with a minimum in temperature.

The objective of this section is much less ambitious and can be formulated as follows If the free-energy function for all phases in a given system were known as a function of temperature and composition, how could one construct the corresponding phase diagram In other words, what is the relationship between free energies and phase diagrams Two examples are considered below polymorphic transformation in unary systems and complete solid solubility. 

The free-energy versus composition diagram for a system that exhibits complete solid solubility is shown in Fig. 8.11. The components of the diagram are the two vertical axes that represent pure AO (left) and pure... 

Figure 8.11 Free-energy versus composition diagram for an AO-BO mixture exhibiting complete solid solubility at a temperature that is lower than the solidus line.

Crystal structures have been reported for the and y modifications of VsAsa and CrsAsa and for NbaAs. Complete solid solubility has been found in the CrP-CrAs, CrAs-CoAs, and FeAs-CoAs systems, with the phosphorus and arsenic atoms randomly arranged in a MnP-type structure. 

Estimation of Liquidus and Solidus for Systems with Complete Solid Solubility... 

HUME-ROTHERY RULES FOR COMPLETE SOLID SOLUBILITY... 

Siek and Szytula (1979) investigated structural and magnetic properties of the section CeMn2(Si ,.Gej )2 at 11 kOe and in the temperature range 100 powder analysis of arc-melted and annealed alloy specimens (800°C, 3 d). Starting materials were Ce-3N, Mn, Ge-4N, Si-5N. 

A complete solid solubility was observed by Skolozdra et al. (1980) for the system HoCo2 ,Fe,jSi2 crystallizing with the ThCrjSi 2-type of structure (I4/mmm X-ray powder data). Magnetic susceptibility data (80-1050 K) were recorded within the temperature range of 80-1050 K and are presented in table 34. 

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