Segregation of impurities

In general, greatly reduced rates of attack are observed for impure or dilute nickel alloys compared with pure nickel when exposed to SO2 + O2 atmospheres. Haflan et al. have attributed this to the segregation of impurities at the sulphide/oxide interface causing breakup of the sulphide network. For example in the case of silicon additions, it has been shown that silicates form and it has been proposed that these alter the wetting characteristics of the sulphide and prevent the establishment of an interconnected sulphide network. 

A sample of 99.993% A1 containing 10-fold higher Fe and Cu concentrations, but cooled in such a way as to produce noncellular structure and prevent the segregation of impurities into a separate phase, has maintained the OCP in the same 0.5 M NaCl solution well below the pitting potential and no pitting has been recorded. 

Although there are many features common to synthetic oxides and minerals, fundamental studies of the charge-transfer processes in mixed-valence compounds can only be systematically carried out on synthetic oxides of controlled stoichiometry and impurity concentration. However, with the exception of Seebeck coefficients, transport measurements require single-crystal data if quantitative interpretations are to be made. Nevertheless, conductivity data for polycrystalline samples of cubic phases are useful if the samples are dense and care has been taken to eliminate any segregation of impurities into the grain boundaries. 

Atoms in the free surface of solids (with no neighbors) have a higher free energy than those in the interior and surface energy can be estimated from the number of surface bonds (Cottrell 1971). We have discussed non-stoichiometric ceramic oxides like titania, FeO and UO2 earlier where matter is transported by the vacancy mechanism. Segregation of impurities at surfaces or interfaces is also important, with equilibrium and non-equilibrium conditions deciding the type of defect complexes that can occur. Simple oxides like MgO can have simple anion or cation vacancies when surface and Mg + are removed from the surface,... 

Often, however, it is more realistic to abandon the model of a discontinuous interface. Segregation of impurities and other point defects, as well as elastic and electric fields, broaden the interface region. For this extended boundary, we can formulate j the Gibbs energy of Eqn. (10.1) as (< = A,B) j... 

Several parameters can influence strongly the superplastic behaviour of ceramics, i.e. the strain rate at which the material can be superplastically deformed. Between them can be mentioned the grain size, second phases and segregation of impurities at the grain boundaries, etc. 

Step cooling will not simulate embrittlement of 1 WCM Mb, though it occurs (e.g., a 100°F [38°C] increase in transition temperature was reported after 8 y at 930° F [500° C]). This is because the embrittlement in 11/iCr-1 Mo is caused by precipitation of carbides in the ferrite phase rather than segregation of impurities to the grain boundaries. Temper embrittlement can be reversed by heating at 1,150°F (620°C) for 2 h per inch of thickness. 

Concentration taking into account on segregation of impurity was expressed by (4.1) at the m-c interface ... 

Grain Boundary Segregation of Impurity in Polycrystalline Silicon... 

Fig. 9 A schematic sketch of distribution and segregation of impurities during normal freezing. The dashed lines show the final concentration distribution for the cases of complete and no mixings in the melt, respectively.

Many other nonequilibrium modes of impurity incorporation occur, primarily through segregation of impurities at defect and inclusion sites. As discussed throughout this chapter, these impurities can be systematically reduced from the final product through optimization of crystallization and separation steps. In particular, the thermodynamic approach outlined in Section 3.5.1 can be effectively used commercially to select solvents and optimize processing conditions to greatly improve the purification of crystalline materials. 

This B segregation affects the segregation of impurities such as O, S, and P which embrittle conventional alloys. However, such impurities seem to be of little importance for NijAl since NijAl, and other intermetallics of high purity with clean grain boundaries are still brittle. 

Examples of this type of application abound in the literature. Segregation of impurities in metals and alloys often occurs at grain boundaries. A study of the embrittlement and stress-failure of a tungsten sample showed a nearly uniform distribution of phosphorus across the sample, except for certain grains that appeared to be completely free of it this complete absence of phosphorus can be interpreted as related to cleavage failure at these points. 

Surface contamination has two main sources firstly, reaction of the surface with the residual gases in the UHV chamber, and secondly, the segregation of impurities from the bulk phase. It has to be pointed out that even samples which are of high purity in a chemical sense may have their surfaces severely contaminated due to both uptake and segregation. The high reactivity of rare earth metals makes it quite difficult to achieve the desired cleanliness, and it is therefore worthwhile to comment on the procedures applied in modern surface science to obtain contamination-free surfaces. 

The specimen preparation problem for TEM GBs in ceramics must be considered in terms of both their structure and chemistry (even in so-called pure materials). Just as the wetting behavior of a surface may be altered by the doping of the surface layer, so it is important to identify and characterize the segregation of impurities and additives to GBs in these materials. The problem here is 2-fold the features of the GB (its misorientation and plane) must be identified and the distribution of foreign elements must then be accurately measured. The last part of this process is actually even more difficult than you might expect. The added complexity arises from the methods that are, at present, routinely used to prepare samples of ceramic materials for examination in the TEM. 

Embrittlement by the segregation of impurity elements to grain boundaries leads to failure by fast fracture. A question that has been debated over hundred years is How can minute traces of bismuth cause this ductile metal to fail in a brittle manner Three hypotheses for this embrittlement exist. 

---