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For the thin film deposition of some metal A onto another metal B, I understand it is possible for a bit of metal a to diffuse into grain boundaries of metal B. How are these diffused atoms of metal A bonded to metal B and does that constitute an alloy locally? In the case of Al and Nb, Al can diffuse a bit into Nb, but Nb/Al alloys typically don't form at room temperature. What is the difference?

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  • $\begingroup$ I definitely don't know that much about alloying, but my understanding is that it's a chemical process between the metals that creates an alloy, whereas diffusion of atoms without that chemical process occurring is fundamentally different. $\endgroup$
    – Paul Dilley
    Mar 12, 2022 at 20:30
  • $\begingroup$ @PaulDilley Could you convert your comment to an answer and add sources? ASM defines alloying as adding a substance to a base metal; diffusion is not precluded. How can we resolve this discrepancy? $\endgroup$
    – Chemomechanics
    Mar 12, 2022 at 21:05

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Any amount of $Al$ on a $Nb$ matrix is an alloy $NbAl$ by definition.

But if you are asking about a compound like $Nb_3Al$, it requires a minimum amount of $Al$ to form. Below that content, Al is dissolved in the Nb matrix. Once $\Delta G < 0$, the compound can form.

Now there are kinetic considerations. While $\Delta G < 0$, the diffusion rate can be too low for anything happen. One way to boost diffusion is increasing temperature. But normally the diffusion rates are greater at the grain boundaries. So it is expected that the compound forms easily there for low temperatures.

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  • $\begingroup$ Thank you for your answer. Is there any difference in bonding between the two (alloy, compound)? $\endgroup$
    – Frank
    Mar 13, 2022 at 2:54
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    $\begingroup$ Yes, a compound has its own crystalline structure, and its grains have boundaries with the (main) matrix. An alloy can have compounds, but can also consist only of dissolved foreign atoms in the matrix. $\endgroup$
    – Claudio Saspinski
    Mar 13, 2022 at 13:16
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It is indeed true that elements that diffuse along grain boundaries can react with other elements in the bulk to form alloys that are segregated at the grain boundaries. This effect is commonly used to stabilize grain boundaries in metal alloys and thereby make them more resistant to deformation.

For example, carbon that diffuses into red-hot steel will form iron carbide at the grain boundaries, strengthening the steel. However, the next time the steel gets hot the carbon will be once again on the move and the strength will go down. To prevent this, the carbon is stabilized by trace additions of substances like vanadium which form more heat-resistant carbides and thereby allow a chunk of the alloy to remain hard and strong even when red-hot. Tool steel that is used to cut other metals is called high-speed steel ("HSS") if it has these trace elements included in the recipe, and can cut through other steel alloys without getting dull, even when red-hot with frictional heat.

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  • $\begingroup$ Thank you for your answer! $\endgroup$
    – Frank
    Mar 13, 2022 at 2:55

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