 Solution

Atomic-number contrast can explain why the gold-rich regions are bright and why the phosphorus-rich NiP${}_y$ layer should be somewhat darker than the other nickel–phosphorus layers. However, the calculated alloy yields differ only modestly: the NiP${}_y$ and NiP${}_z$ yields differ by about $5.5\%$ relative to NiP${}_z$. Composition-only backscattering does not explain all of the strong local edge and defect contrast.

An Everhart–Thornley detector normally collects secondary electrons, not a pure backscattered-electron signal. Secondary-electron escape and collection depend strongly on local slope and edge geometry. Differential polishing, relief at interfaces, a crack or cavity, surface oxidation and local charging can therefore alter brightness without the corresponding change in bulk atomic number. backscattered electrons can also generate secondary electrons elsewhere. Crystallographic channeling and orientation can contribute additional contrast. The observed dark and bright areas should therefore be correlated with topography and composition rather than assigned a phase from intensity alone.