Binary Eutec c Phase Diagrams Isomorphous systems dealt with elements that exhibit complete solid solubility most likely due to the same crystal structure and similar atomic size Now we will examine eutec c systems in which the components have limited solid solubility 1 The Cu Ag binary system Both components have a FCC structure r Cu 0 128 nm r Ag 0 144 nm Eutec c Temperature below this temperature any remaining liquid is converted to solid via the eutec c reac on This area is the region where two solid phases coexist The solvus line is a boundary between solid phases showing the solid solubility limit 2 The phase is Cu with Ag in solid solu on in amounts ranging from 0 8 0 wt depending on temperature The phase is Ag with Cu in solid solu on in amounts ranging from 0 8 8 wt 100 91 2 8 8 depending on temperature The region contains a mixture of the two solid phases At a given temperature the composi on of each phase is xed the the phase frac on of each phase varies depending on the alloy composi on For example if we make an alloy that contains much more silver than copper at 400 C we would expect to nd more of the phase which is silver rich than the phase which is copper rich Let s consider an alloy of 20 wt Ag and 80 wt Cu and see what phases are present and 3 determine their composi on and phase frac ons at 1100 900 and 700 C C0 At 1100 C we are in the single phase liquid region Therefore one phase is present and the phase composi on is the same as the alloy composi on 4 C C0 CL At 900 C two phases are present and liquid containing 8 and 42 wt Ag respec vely the balance is Cu I can use the lever rule to nd the phase frac ons of and L but note that I can get a reasonable es mate by no cing that 2 3rds of the e line lies to the right of the composi on line so I expect roughly 2 3rds phase from my calcula on 5 C C0 C At 700 C two phases are present and containing 5 and 93 wt Ag respec vely the balance is Cu Applying the lever rule will show much more phase then as one expects since the alloy contains mostly Cu and is the Cu rich phase 6 We will use the Pb Sn system to examine how the microstructure changes depending on where you are in the phase diagram When one crosses the horizontal line at 183 C during the cooling any liquid present turns into the two solid phases and via the eutec c reac on L Along the horizontal line three phases coexist L and Analogous to the mel ng temperature in a single component system the temperature cannot drop below the eutec c temperature un l all of the liquid is gone F 2 3 1 0 7 Eutec c Reac on in the Pb Sn System L 61 9 wt Sn 18 3 wt 97 8 wt Sn Sn 8 During the eutec c reac on the liquid forms two solid phases of very di erent composi on To minimize the di usion distance that is required for this separa on a characteris c lamellar structure accompanies the eutec c reac on The amount of eutec c structure observed depends on the alloy composi on 9 If the alloy composi on equals the eutec c composi on then all of the liquid phase will undergo the eutec c reac on 10 For C0 40 wt Pb a bit less then half of the liquid apply the lever rule at point l turns into the phase during cooling in the two phase L region Whatever liquid remains at the eutec c temperature of 183 C undergoes the eutec c reac on The phase formed during cooling in the two phase L region is called proeutec c and has a globular structure The phase that forms a lamellar structure with during the eutec c reac on is called eutec c 11 12 For composi ons that do not cross the horizontal line upon cooling the lamellar eutec c structure does not form If you apply Gibbs phase rule to the alloy to the lee you will see that there are no points where F 0 The same sequence of events happens for alloy composi ons to the right of the eutec c composi on except that the phase is the proeutec c phase rather than the phase 13
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