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.
This I can’t do anymore. But…
You are right and wrong.
The answer is that if you want an alloy of 70%A and 30%B then start with a mixture of 70%A and 30%B. Simple, no?
Melt, then cool SLOWLY.
As you cool, a mixture of mostly A with some B will begin to solidify as small grains. As more and more grains solidify, the amount of B in each grain will increase because the liquid phase is becoming richer in B (because the grains are mostly A, remember?)
Eventually, the liquid phase reaches the eutectic composition. If you cooled slowly, this will happen at the eutectic temperature. As soon as you go below this temperature the eutectic liquid will solidify around the grains (which vary in composition).
So you will have a solid of eutectic composition containing these grains which consist mostly of A.
But this is still an alloy because the definition of alloy is pretty darn loose. Look it up.
It must be nice to live in Seychelles.
Moderator’s Note: I don’t think the SDMB is the place for getting people to do your homework for you and getting people to work out the answers to tomorrow’s test for you.
(And even if it was, this still wouldn’t be the right forum for it.)