Wikipedia article on factors that affect the stacking-fault energy (SFE) of a material says:

Another factor that has a significant effect on the SFE of a material and is very interrelated with alloy content is the e/a ratio, or the ratio of valence electrons to atoms. Thornton showed this in 1962 by plotting the e/a ratio vs SFE for a few Cu based alloys. He found that the valence-electron to atom ratio is a good predictor of stacking fault energy, even when the alloying element is changed. This directly supports the graphs on the right. Zinc is a heavier element and only has two valence electrons, whereas aluminum is lighter and has three valence electrons. Thus each weight percent of aluminum has a much greater impact on the SFE of the Cu-based alloy than does zinc.

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So, my question is why does the e/a ratio of electrons to atoms affect the SFE?

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    $\begingroup$ First, showing the plots in weight % rather than atomic % makes it harder to compare, for sure, and the scaling is not really that great from Al to Zn based on e/a. As to why it changes things, consider the Fermi surface and the available free electrons, and how they would impact screening around dislocations. $\endgroup$ – Jon Custer Jun 27 '17 at 17:50
  • $\begingroup$ I am kind of beginner in material science. So, it would br better if you elaborate. @Jon Custer $\endgroup$ – Mockingbird Jun 28 '17 at 0:21

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