材料科学基础第章合金相图.ppt

* Figure The cooling curve for an isomorphous alloy during solidification. We assume that cooling rates are small so as to allow thermal equilibrium to take place. The changes in slope of the cooling curve indicate the liquidus and solidus temperatures, in this case for a Cu-40% Ni alloy. * Example SOLUTION The vertical line at 40% Ni represents the overall composition of the alloy: - 1300oC: Only liquid is present. The liquid must contain 40% Ni, the overall composition of the alloy. - 1270oC: Two phases are present. The liquid contains 37% Ni and the solid contains 50% Ni. - 1250oC: Again two phases are present. The tie line drawn at this temperature shows that the liquid contains 32% Ni and the solid contains 45% Ni. - 1200oC: Only solid is present, so the solid must contain 40% Ni. * Example Application of Lever Rule Figure A tie line 1250°C in the copper-nickel system that is used in Example to find the amount of each phase. Calculate the amounts of and L at 1250oC in the Cu-40% Ni alloy * Example SOLUTION Let’s say that x = mass fraction of the alloy that is solid . Since we have only two phases, the balance of nickel must be in the liquid phase (L). Thus, the mass fraction of nickel in liquid will be 1 - x. Total mass of nickel in 100 grams of the alloy = mass of nickel in liquid + mass of nickel in So, 100 ? (% Ni in alloy = [(100)(1 – x)](% Ni in L) + (100)[x](% Ni in ) x = (40-32)/(45-32) = 8/13 = 0.62 If we convert from mass fraction to mass percent, the alloy at 1250oC contains 62% a and 38% L. Note that the concentration of Ni in alpha phase (at 1250oC) is 45% and concentration of nickel in liquid phase (at 1250oC) is 32%. * Determine the amount of each phase in the Cu-40% Ni alloy shown in Figure 9.13 at 1300oC, 1270oC, 1250oC, and 1200oC. Example Solidification of a Cu-40% Ni Alloy Figure Tie lines and phase compositions for a Cu-40% Ni alloy at several temperatures * Example SOLUTION * Figure A hypothetical binar