I’m currently doing my A-levels in Chemistrt and I’m doing Phase Equilibria as my options topic.
Everything seems to be going well in my studies until I came to this point where no body not even the teacher can give me a really clear answer on this . Hope you guys can help me out.
Question: Suppose you have two Metals A and B. Where A has a higher melting point. You melt these two metals so as to create an alloy. Suppose the eutectic composition of the mixture is 60% A and 40%B. However what I want is an alloy of composition 70%Aand 30% B. How can I get this composition??The reason I’m asking this question is… won’t the A crystallise out thus changing the percentage composition and won’t I be left with the same eutectic somposition again. So can I really get 70%A and 30%B? Remember I’m doing a test on this tomorrow. So it would help if you write down the answer in a way as if you would write this if it was you doing the test.
I’m not going to answer your test question for you, but I’ll try to stear you in the right direction. I think the only way to adequately answer this question is to appeal to non-equilibrium processes. Since you will follow the eutectic in any given phase diagram once you hit it, you really have no other option.
Lenny, you need to study your LEVER Rule. The phase diagram, once you understand it is a very powerful tool for predicting EQUILIBRIUM phase and microstructure. If we assume that A and B are immiscible (solubility of A in B is zero), The answer is quite easy.
First start in the liquid phase: We have a liquid that is 70A and 30B. As the liquid is cooled, the composition crosses the liquidus – we go from a single phase region (L), to a two Phase region (A+L). The point at which this happens can actually be predicted from the melting point of A, and the location of the eutectic.
The crystallization of the A depletes the liquid of A and the liquid composition follows the liquidus line. For any given temperature we can determine how much A and how much L are present. We do this with a isotherm (tie line). The percentage of A will be represented by that inverse (eg. RHS) section of the lever over the total length.
As the A+L mixture cools, you follow the liquidus line until the Eutectic. At the eutectic you go From A+L to A+B.
Just wait until you go to non-equilibrium phase diagrams and ternary phase diagrams.
Just a suggestion. Try to look at an introduction to materials science book for this. IT is covered much better than Chemists can.
And what’s an A-level?
Thanks guys. I figured out the answer on my own. I made a couple of phone calls to some of my friends and they explained it to me. Luckily we had the test today, and there were no questions on that topic.
To Xiao: A-Level is a pre-university certificate in which a two year course is required. It is set by the University of Cambridge in England. It is done by most Commonwealth country. Since I live in Seychelles, so I do it also. It is a certificate which is accepted by most universities around the world.
The Seychelles…A place that I wouldn’t mind visiting. Anyway. I still recommend looking at a materials science text. The chemists (Gibbs) formulated a lot of the phase equilibia, metallurgists and materials scientists are the ones who utilize it as one of the major tools in their arsenal. As a metallurgist, I can tell you a whole lot about a material from the phase diagram, composition, and thermal and mechanical history. Really neat stuff.