Short-range segregation

In case < 0, bonding of like atoms is energetically preferred, leading in principle to separation of the alloy into a mixture of A and B rich solid-solution phases, each with nearly homoatomic SRO clusters, compared to short range A-B mixed or heteroatomic clusters in the former case of V >0. In other words, the tendency to order (or phase-separate) is manifested to some, local degree also in most solid solutions, where the distribution of atoms in the crystal is not entirely random, and should be incorporated too in any theoretical quantitative evaluation of surface segregation phenomena. Moreover, many alloys of... 

Fig.5. Calculated solute surface concentrations for Ni-8at%Al-4at%Cu(l 11) thick solid lines - FCEM, thin solid lines - the BW-type approximation. Dashed-dotted lines - solute surface concentrations for the binary alloy Ni-8at%Al(lll) and Ni-4at%Cu(l 11) surfaces calculated in the FCEM approximation. Note the enhancement of Cu segregation induced by ternary alloying and short-range order effects.

The subsurface layers of Pt25Co7j have a composition quite different from the 50% Pt of a truncated LIq surface. Due to an oscillating segregation profile, the second layer contains only 5% Pt [54]. This shows us that the surface short-range order cannot be simply understood as truncated LIq phase and should be rather seen as a result of the tendency to form unlike nearest neighbours. This view is supported by the fact that already the second-nearest neighbour correlation A2 n differs in sign from the value expected for an LIq phase. 

The GE is a source of sampling variation at the local level that is, at the small scale where the sample is actually taken. It is not present in classical SR. The GE is due to (r) the distribution heterogeneity of the material, which is random at the small scale area where we take our sample, (//) the selection of groups of particles, rather than individual particles, and ( //) the segregation of material at the short range where we take the sample. 

Thus, in a commonly used procedure [16, 92, 120, 145, 165, 167, 170, 175], each point of the segregation isotherm data z, , ) is analyzed with Eq. (32) to find out the surface concentration ((J) ) assigned by the mean field approach (analogous procedure exists in a self-consistent mean field model [166, 174]). Then, for each such pair (c, c )s) the trajectory 2(icAf)1/2 vs ([) is plotted (for the value (IO and its value is read out at surface concentration s (=c )se or (j)sd in Fig. 15). This value is equal to the surface energy derivative (-dfs/dc ))s at concentration < )s. Such a procedure, repeated for each z ((j)00) data point, yields the concentration dependence of the composition derivative of the short-ranged bare surface energy (-dfs/dc ))s vs < )s (see Fig. 16). 

For smaller values of y S 0.25, despite the deviation from (m the existence of tetrahedra gives evidence of short-range order. Thus, an unexpected tendency of the anionic defects to couple in the neighbourhood of ions such as Fe3+, inducing a change from C. N. 6 to 4 is found. A model taking account of this unusual segregation phenomenon is proposed in Chap. 7. 

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