Phase and metal hardening

[richard9678]

Hi. I read that one method of hardening a metal is through strengthening with a second phase.

I'm not entirely sure I understand the phrase "strengthening with a second phase".

Can the phrase "strengthening with a second phase" be shown on a phase diagram? Thanks.

P.S. I seem to have convinced myself that what amounts to a second phase, in this context, is when the result of cooling a metal results in a metal containing a mix of alpha and beta - a heterogeneous material. Not sure if I'm right though.

https://en.wikipedia.org/wiki/File:Eutektikum_new.svg

 

[richard9678]

Okay, I think the key issue is diffusion. If beta cannot be diffused within alpha as the metal cools, it will have to precipitate out as a separate and homogeneous material within the alpha mass. Vice versa if there is more beta than alpha in the composition. This means that the region on the phase diagram labelled a + b or alpha plus beta is indicating a two phase alloy. There are two separate homogeneous parts to the alloy in the region a + b. I believe.

 

[Baluncore]

Can the phrase "strengthening with a second phase" be shown on a phase diagram?

Yes, as richard9678 wrote, it is where it shows α+β on the example diagram.

α and β may have different atomic composition, but the bulk composition is the sum of the two and so decides the vertical line on the phase diagram down which the material cools.

The presence of two distinct homogeneous phases must be expected to change strength or hardness parameters.

There is no time axis on a phase diagram. The lowest energy phase or structure may not appear until after some trigger or time delay.