6.1.Lattice Defects and Diffusion 113 6.1.9 So far,we have considered only the diffusion of one component Kirkendall (say,material A)into another material (say,B).If both elements have the same diffusivity in each other (that is,if A diffuses with Shift the same velocity into B as B diffuses into A),then a mirror im- age of the diffusion profiles shown in Figure 6.6 can be drawn for the other component.In many cases,however,this condition is not fulfilled.For example,copper diffuses with different velocity into nickel than vice versa (Table 6.1).Let us assume that element A diffuses faster into B than B into A.As a consequence,more A atoms cross the initial interface between the two elements in one direction than B atoms into the opposite direction.The result is that the initial interface shifts in the direction of element A. This can be best observed,as done by Kirkendall,by inserting inert markers (usually fine wires of metals having a high melt- ing point)between the two bars of metals A and B before join- ing them.It is then observed that these markers move into the direction of the A metal upon prolonged heating of this diffusion couple,as schematically depicted in Figure 6.8.The shift is not very large,so that annealings close to the melting point of the el- ements lasting for many days are necessary. After cooling,small sections parallel to the interface may be re- moved in a lathe which are then chemically analyzed for their com- positions.This latter procedure is,incidentally,common for many diffusion experiments.As an alternative,radioactive "tracer ele- ments"are utilized for determining the amount of diffused species rather than the less accurate chemical analysis.An even more ad- vanced technique utilizes the microprobe,which scans an X-ray beam along the specimen whose response signal eventually yields the composition.It might be noted in passing that in cases for which the atom flow of the species under consideration is con- siderably unbalanced,some porosity in the diffusion zone might develop. A B FIGURE 6.8.Schematic representa- tion of the movement of inert markers (inserted at the initial interface)due to faster movement Kirkendall markers Initial location of A atoms into B than vice versa after annealing of interface (Kirkendall shift).So far, we have considered only the diffusion of one component (say, material A) into another material (say, B). If both elements have the same diffusivity in each other (that is, if A diffuses with the same velocity into B as B diffuses into A), then a mirror im￾age of the diffusion profiles shown in Figure 6.6 can be drawn for the other component. In many cases, however, this condition is not fulfilled. For example, copper diffuses with different velocity into nickel than vice versa (Table 6.1). Let us assume that element A diffuses faster into B than B into A. As a consequence, more A atoms cross the initial interface between the two elements in one direction than B atoms into the opposite direction. The result is that the initial interface shifts in the direction of element A. This can be best observed, as done by Kirkendall, by inserting inert markers (usually fine wires of metals having a high melt￾ing point) between the two bars of metals A and B before join￾ing them. It is then observed that these markers move into the direction of the A metal upon prolonged heating of this diffusion couple, as schematically depicted in Figure 6.8. The shift is not very large, so that annealings close to the melting point of the el￾ements lasting for many days are necessary. After cooling, small sections parallel to the interface may be re￾moved in a lathe which are then chemically analyzed for their com￾positions. This latter procedure is, incidentally, common for many diffusion experiments. As an alternative, radioactive “tracer ele￾ments” are utilized for determining the amount of diffused species rather than the less accurate chemical analysis. An even more ad￾vanced technique utilizes the microprobe, which scans an X-ray beam along the specimen whose response signal eventually yields the composition. It might be noted in passing that in cases for which the atom flow of the species under consideration is con￾siderably unbalanced, some porosity in the diffusion zone might develop. 6.1 • Lattice Defects and Diffusion 113 Kirkendall markers after annealing Initial location of interface A B FIGURE 6.8. Schematic representa￾tion of the movement of inert markers (inserted at the initial interface) due to faster movement of A atoms into B than vice versa (Kirkendall shift). 6.1.9 Kirkendall Shift