What does this example mean? Slow cooling of 40% Sn alloy from 800°C to 600°C: L → L and γ → L, γ, and ε → L and ε

Question

It's in a phase diagram topic. It's about Cu-Sn phase diagram.

And it has a question of: Write the schematic diagram describing the following scenarios and identifies the regions and the phases when Slow cooling of 13.1% Sn alloy from 1000oC to 300oC happens.

I have this schematic diagram that describe the following scenarios and identifies the regions and the phases at 40%Sn alloy from 800°C "L → L and γ → L, γ, and ε → L and ε" as an example but it's confusing to follow. It's just an example and it doesn't follow the Cu-Sn phase diagram anyway.

But does the first one mean L→ L+ γ ?

And L , y is different phases ... That manage to fall both at 40%Sn alloy from 800°C?

Also am i correct to assume that the final one means ε → L + ε?

This is the closest image that I can give based on my assumption:

Here's the Pdf file for more information: https://uh.edu/~hfang2/MECE3345/Lectures/Chapter9.pdf

"L → L and γ → L, γ, and ε → L and ε" is a schematic diagram for the slow cooling which means it is an equilibrium cooling Like this photo:

Another one is:

This image is from pdf file: http://www.engr.mun.ca/~asharan/courses/5911_LECTURES/ch09.ppt

Answer 1

Consider the 40 at% (Sn) composition at 800 °C: it's a liquid.

When you cool it down at around 650 °C you enter a two-phase field: the solid gamma forms (Cu-enriched) and the liquid enriches in Sn. As you further cool down the gamma phase will continue to form, but with progressively higher content of Sn

You cool down at 640 °C and you enter a new different two-phase field: the liquid partially reacts with the gamma phase forming the solid epsilon phase. If you further cool down, the ε phase will continue to form from the liquid, but its composition will progressively enrich in Cu.

Just below 415 °C you enter into another two-phase field: the liquid will solidify into ε and η phase.

That's what occurs by cooling down to 300 °C, but if you further would cool down below 189 °C you will enter another two-phase field; in this case the ε and η' phases will form.

The amounts of the different phases at the different $T$ are obtained by applying the lever rule.

Take care, I described what happened from the thermodynamic point of view (that is what is represented in a phase diagram); the kinetics can remarkably change the things.