Al7Cu2Fe and Al12Fe3Si are constituent phases in the Al comm

Al₇Cu₂Fe and Al₁₂Fe₃Si are constituent phases in the Al commercial alloys. Calculate the backscatter coefficient (h) for both phases and the contrasts (C) between Al and each of these two phases. Based on contrasts, can you determine which phase (A or B) is Al₇Cu₂Fe in Fig

Solution

Backscatter coefficient (h)= 0.025+0.016Z-(1.86×10^-4 ×z^2)+(8.3×10^-7 ×z^3)

If more elements are present then the coefficient of mixture becomes h _mix = sum of (C _i × h _i )

Now h for Al= 0.203, for Cu= 0.353, for Fe= 0.329 for Si= 0.215

Now mass fraction in Al7Cu2Fe

Total mass of compound is 372.08g

Mass fraction of Al= (7×27)/372.08=0.508

Of Cu= (2×63.54)/372.08=0.342

Of Fe= 0.15

Now h _mix = 0.273

Now for Al12Fe3Si

Total mass =520g

Mass fraction of Al= 0.623, Fe= 0.323, Si= 0.054

Now h _mix = 0.244

Z-contrast states that phase containing heavier element is lighter while contrast of a lighter element is darker. Of the two the first phase has Cu which heavier compared to Si. Thus contrast of the phase containing Cu will be lighter in comparison to the other.

From the figure Below is the phase containing Al7Cu2Fe.