The values for the intrinsic diffusivities of $Cu$ and $Ni$ in a binary $Cu-Ni$ alloy are

shown below in a$,$ as a function of composition $c_{Cu}.$ Note that $Cu$ and $Ni$ are

completely miscible in the solid state. A diffusion couple is made with the

geometry shown at the top right in $b.$ Markers are placed at the weld interface.

a$.(3$ points$)$ What is the interdiffusivity widetilde$(D)$ for an alloy that is pure $Ni?$

Provide a numerical answer using plot $a.$

b$.(3$ points$)$ Will the markers placed initially at the weld interface move

toward the Cu end of the sample, the Ni end, or stay fixed?

c$.(6$ points$)$ The Cu concentration is sketched in c after diffusion has

occurred for some time. Sketch, qualitatively, the fluxes of

$Cu,Ni,$ and vacancies, defining positive fluxes as those

moving to the right$-$hand side. Assume all three fluxes are

measured with respect to the crystal frame.

Make your sketches using the plots on the next page.

d$.(4$ points$)$

Now sketch the rate at which vacancies are created or

destroyed, ${}^{},$ in order to sustain the vacancy flux from $c.$

e$.(4$ points$)$

Explain in words how vacancies can be created and

destroyed. Consider the interaction of vacancies with

dislocations and$/$or grain boundaries.