To obtain a uniform alloy, cool quickly or slowly?

[DyslexicHobo]

Hello,

My material science teacher posed an interesting question today. During the making of an alloy, would it be better to quench it or let it cool slowly?

At first I thought the answer was obvious: quench it quickly so in the case the two elements have different melting temperatures they don't separate. But then someone brought up the thought of diffusion... if it is cooled slowly then it will have a chance to diffuse, therefore making it more uniform.

Was my intuition correct or is there more behind this?

Thanks.

 

[Andy Resnick]

Quenching allows the material to remain in a highly stressed (unstable or meta-stable) state: martensitic steel and tempered glass, for example.

So ultimately, it depends on the application.

 

[Astronuc]

At first I thought the answer was obvious: quench it quickly so in the case the two elements have different melting temperatures they don't separate. But then someone brought up the thought of diffusion... if it is cooled slowly then it will have a chance to diffuse, therefore making it more uniform.

It depends on the constituents and whether they form a solid solution or one has a tendency to be interstitial or not. Also, for the concentration, one has to consider whether or not there are intermetallic or second phases possible.

Even in melting, some alloys may be incompatible, significantly different densities in which only levetation melting is possible to mix them, and rapid quenching is necessary to prevent segregation.

Finally, some alloys must be made in a vacuum, e.g. alloys of Ti, Zr, Hf, etc and in which case they are at least triple-melted in a water-chilled copper crucible. The triple melting is to get the homogeneity as close to the desired consituent content.

Rapid solidification and powder metallurgy are used for generally incompatible elements.

 

[Mapes]

But then someone brought up the thought of diffusion... if it is cooled slowly then it will have a chance to diffuse, therefore making it more uniform.

Technically, diffusion smooths out not gradients in concentration but gradients in chemical potential; thus, diffusion doesn't necessarily increase uniformity. For example, oil and water separate, rather than mix, by diffusion; similarly, if an alloy's constituents aren't miscible, then diffusion will increase the nonuniformity (as Astronuc indicated) as precipitates nucleate and grow.

 

[DyslexicHobo]

I should have known there was no simple answer to this.

The original problem was posed as a simple bronze alloy (this is not a mixture with interstitial atoms, correct? atomic radii of ~1.5 & 1.7). To simply attain the most well-distributed alloy, is quenching or slow cooling better?Thank you for all the input!

EDIT: Ah, I realize what Astronuc is saying now, thank you Mapes.