Complex alloys alloying behaviour

A different, more complex, alloying behaviour is illustrated in a number of figures (Figs. 2.18-2.21) where several examples of alloy diagrams are presented which are typically characterized by the formation, through a range of different mechanisms, of intermediate phases. 

This chapter has shown many examples of the use of CALPHAD methods, ranging from an unusual application in a binary system, through complex equilibrium calculations to calculations for 10-component alloy systems. In all cases the use of CALPHAD methods has enhanced the understanding of processes, clearly defined alloy behaviour and provided vital information for other models, etc. It is also clear that equilibrium calculations can be used in many different areas and under a surprising number of different conditions. For numerous reasons, modelling will never completely replace experimental measurement. However, die quantitative verification of the accuracy of CALPHAD calculations now means that they can be seriously considered as an information source which can be used as an alternative to experimental measurement in a number of areas and can also enhance interpretation of experimental results. 

Theories of the oxidation of tantalum in the presence of suboxide have been developed by Stringer. By means of single-crystal studies he has been able to show that a rate anisotropy stems from the orientation of the suboxide which is precipitated in the form of thin plates. Their influence on the oxidation rate is least when they lie parallel to the metal interface, since the stresses set up by their oxidation to the pentoxide are most easily accommodated. By contrast, when the plates are at 45° to the surface, complex stresses are established which create characteristic chevron markings and cracks in the oxide. The cracks in this case follow lines of pores generated by oxidation of the plates. This behaviour is also found with niobium, but surprisingly, these pores are not formed when Ta-Nb alloys are oxidised, and the rate anisotropy disappears. However, the rate remains linear it seems that this is another case in which molecular oxygen travels by sub-microscopic routes. 

During recent years a considerable amount of information has been published on the anodic behaviour of nickel alloys. The data include studies both of binary alloy systems in which nickel forms the major alloying component and of more complex commercially produced nickel alloys. The data are sufficiently numerous to permit a rational and fairly complete interpretation of many of the corrosion-resistant properties of nickel alloys on the basis of their anodic behaviour. 

The literature on the oxidation of nickel-copper alloys is not extensive and emphasis tends to be placed on the copper-rich materials. The nickel-rich alloys oxidise according to a parabolic law and at a rate similar to that for nickel Corronil (Ni-30Cu) exhibited a parabolic rate behaviour below 850°C but a more complex behaviour involving two parabolic stages above 900°C. Electron diffraction examination of the oxide films formed on a range of nickel-copper alloys showed the structures of the films to be the same as for the bulk oxides of the component metals and on all the alloys examined only copper oxide was formed below 500°C and only nickel oxide above 700°C . 

The difficulty of predicting corrosion behaviour in a complex situation, and of determining what changes in conditions are likely during the required life of an item, makes deliberate overdesign a common approach. An alloy or protective system more resistant than is actually required may be used, or thicker sections than needed for mechanical reasons adopted, giving a... 

There are few systematic guidelines which can be used to predict the properties of AB2 metal hydride electrodes. Alloy formulation is primarily an empirical process where the composition is designed to provide a bulk hydride-forming phase (or phases) which form, in situ, a corrosion— resistance surface of semipassivating oxide (hydroxide) layers. Lattice expansion is usually reduced relative to the ABS hydrides because of a lower VH. Pressure-composition isotherms of complex AB2 electrode materials indicate nonideal behaviour. 

Just as it is effective in the other fields of inorganic descriptive chemistry, the Periodic Table is an essential reference point in intermetallic chemistry too. The general alloying characteristics of the different metals, their reactivity towards the other metals, the variety of their intermetallic derivatives usually are very complex and cannot be easily explained and rationalized on the basis of a few concepts and data. Nevertheless a sound first criterion for a description and classification of the intermetallic behaviour of the various metals lies in their position in the Periodic Table. 

Lanthanides properties and general references. For a systematic treatment and general references of the physical and chemical properties of the rare earths and their compounds and alloys mention can be made to a periodical publication in which several contributions to these subjects are being collected. See for instance Gschneidner and Eyring (1978) and Gschneidner etal. (2005). We would also like to quote a sentence, included in the prefaces of all these books, which hints at the complexity and richness of the rare earth behaviour and the ever-increasing interest in their properties and applications. The mentioned sentence is as follows ... 

It is also evident that there is a complex interplay between the three main TCP phases, a, R and P, with /r occasionally being observed (Darolia et al. 1988, Proctor 1992, Walston et al. 1996). To examine this more closely, calculations were made for CMSX-4 by alternatively suspending two of the three main TCP phases and calculating the behaviour of the alloy with just one of the phases allowed to form at any one time (Fig. 10.50). The solvus temperatures for R, P and temperature range. It is clear, therefore, that the interplay between the various TCP phases will be sensitive to the alloy composition and indeed changes in Re, W and Cr levels will cause either o, R or P to become the dominant TCP phase. 

The first two chapters of this book are devoted to the historical and mineralogical aspects of rare earths followed by seperation techniques. It is obvious that these two chapters cannot be limited only to the discussion of europium. Hence, the whole of the rare earth family has been treated with especial emphasis towards europium. Chapters three to five describe exclusively the chemical behaviour of europium and also the interesting features of its complexes and alloys. However, where necessary, mention of other rare earths has been made. The spectroscopic properties, both absorption and fluorescence, are dealt with in chapter six, which is followed by an account of the possible uses of europium in the last chapter. 

Compared to glass, glass-ceramics are mechanically stronger, more resistant to chemical attack and have a wider range of thermal expansion coefficients. Complex non-linear thermal expansion characteristics can be achieved, resulting in very close thermal expansion matching to a variety of metals and alloys, including those with non-linear behaviour [24],... 

---