Alloying elements complex systems

Thermodynamic modelling of solution phases lies at the core of the CALPHAD method. Only rarely do calculations involve purely stoichiometric compounds. The calculation of a complex system which may have literally 100 different stoichiometric substances usually has a phase such as the gas which is a mixture of many components, and in a complex metallic system with 10 or 11 alloying elements it is not unusual for all of the phases to involve solubility of the various elements. Solution phases will be defined here as any phase in which there is solubility of more than one component and within this chapter are broken down to four types (1) random substitutional, (2) sublattice, (3) ionic and (4) aqueous. Others types of solution phase, such as exist in polymers or complex organic systems, can also be modelled, but these four represent the major types which are currently available in CALPHAD software programmes. 

One-dimensional chains of heterometallic polymers with covalent metal-metal bonds are interesting as molecular conductors (see references cited in [33]). A complex system of four different elements surroimded by insulating organic materials is described in [33]. An alloy K6Ag2Sn2Te9 was treated with 1,2-diaminoethane, and a saturated aqueous solution of tetraethylammonium iodide was added to the resulting solution. One-dimensional chains of composition (Et4N)4[Au(Agi c)AuxSn2Te9] (x = 0.32) 10 were obtained. Band struetures are discussed and a band gap of 0.45 eV was found. 

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. 

It is not only the toxicity of alloying elements that is of importance their influence on the property profile and the processability of the alloy is also relevant. Not all biocompatible alloying elements can be used due to restrictions in solubility, interactions with the processes and process parameters, etc. In fact, this is a complex system which, at the moment, has to be investigated in detail with a special regard to any alloying element and their influence on the chosen processing route. 

Determining the bulk properties of various alloys is a starting point, enabling comparison among Pb-free alloys and baseline Sn Pb solders. In applications, the solder attachment process combines the alloy systems with minor elements from the component termination and from the PWB surface finish to form complex systems. The following sections describe the application of several key alloys and the reliability of the resulting complex systems they form. 

Perspectives for fabrication of improved oxygen electrodes at a low cost have been offered by non-noble, transition metal catalysts, although their intrinsic catalytic activity and stability are lower in comparison with those of Pt and Pt-alloys. The vast majority of these materials comprise (1) macrocyclic metal transition complexes of the N4-type having Fe or Co as the central metal ion, i.e., porphyrins, phthalocyanines, and tetraazaannulenes [6-8] (2) transition metal carbides, nitrides, and oxides (e.g., FeCjc, TaOjcNy, MnOx) and (3) transition metal chalcogenide cluster compounds based on Chevrel phases, and Ru-based cluster/amorphous systems that contain chalcogen elements, mostly selenium. 

Notice that the structures presented in this paragraph are unary structures, that is one species only is present in all its atomic positions. In the prototypes listed (and in the chemically unary isostructural substances) this species is represented by a pure element. In a number of cases, however, more than one atomic species may be equally distributed in the various atomic positions. If each atomic site has the same probability of being occupied in a certain percentage by atoms X and Y and all the sites are compositionally equivalent, the unary prototype is still a valid structural reference. In this case, from a chemical point of view, the structure will correspond to a two-component phase. Notice that there can be many binary (or more complex) solid solution phases having for instance the Cu-type or the W-type structures. Such phases are formed in several metallic alloy systems either as terminal or intermediate phases. 

Solution databases now exist for a niunber of the major metallic alloy systems such as steels, Ni- based superalloys and other alloy systems, and highly accurate calculation have been made which even a few years ago would have been considered impossible. The number of substance databases are increasing and the numbers of substances they include is reaching well into the thousands. Substance and solution databases are increasingly being combined to predict complex reactions such as in gas evolution in cast-irons and for oxidation reactions, and it is already possible to consider calculations of extreme complexity such as the reactions which may occur in the burning of coal in a industrial power generator or the distribution of elements in meteorites. 

---