Phase diagram of Ag - Cu question

[vigintitres]

Homework Statement

Describe equilibrium cooling from 1200 C to 200 C of an alloy containing 40 wt% Ag. Draw the schematic equilibrium cooling curve

The Attempt at a Solution

Alloy begins as a liquid, begins to solidify as temperature reaches the liquidus line; alpha accumulates and the liquid slides down the liquidus line and the composition of alpha slides down the solidus line until they both reach the eutectic tie line where the last liquid has to solidify, the alpha is already solid but the liquid is coming toward the eutectic point and it has to solidify there. The liquid transforms into some alpha and some beta and there is a thermal arrest until it all solidifies.....

now, I don't know exactly what happens. To be more specific, I am wondering exactly how that cooling curve would look because I'm confused as to how the phases travel around the phase diagram given a specific composition and being cooled... Starting with the liquid alloy, as it cools, I know that the equilibrium alpha + Liquid two phase field can be considered "empty" - meaning nothing is in there (at least that's what my prof. said), so does the liquid actually travel down the liquidus line, hit the eutectic point of equilibrium, the travel back to the left (in the Ag - Cu phase diagram case) along the eutectic tie line, until it regains its original 40-60 composition? Because at the eutectic point, the composition is predetermined...

 

[nascarfan4817]

Hi. I found help for the cooling curve at this web-site:

http://www.doitpoms.ac.uk/tlplib/phase-diagrams/printall.php

About midway down, there is a graph with the cooling curve. For our binary eutectic problem, I believe all of the cooling curves would look like what is on that web-page. Would you happen to be taking ME 311 right now at UT. We have that same question for hw.

 

[Blueshift5]

As the alloy cools from 1200 C to 200 C, the phases and their compositions will change according to the phase diagram of Ag-Cu. Initially, the alloy will be in the liquid phase and it will begin to solidify as it reaches the liquidus line. At this point, the composition of the alloy will be 40 wt% Ag. As the temperature continues to decrease, the composition of the solid phase (alpha) will slide down the solidus line, while the remaining liquid phase will slide down the liquidus line. This will continue until the composition of both phases reaches the eutectic point, where the last liquid phase will solidify and transform into a mixture of alpha and beta phases.

From this point, there will be a thermal arrest as the remaining liquid solidifies and the alpha and beta phases continue to transform. The cooling curve will show a plateau at the eutectic temperature, representing the transformation of the last liquid phase into a mixture of alpha and beta phases.

After the thermal arrest, the cooling curve will continue downward as the remaining alpha and beta phases continue to transform until the temperature reaches 200 C. At this point, the alloy will be in a fully solid state with a composition of 40 wt% Ag. The cooling curve will then be a straight line representing the equilibrium state of the alloy at 200 C.

To summarize, the cooling curve for an alloy containing 40 wt% Ag will start at the liquidus line, move down to the eutectic point, and then continue downward until it reaches 200 C. The plateau at the eutectic temperature represents the transformation of the last liquid phase into a mixture of alpha and beta phases. The final equilibrium state of the alloy will be at 200 C with a composition of 40 wt% Ag.