MSE 230 Solutions for Assignment 9 1 i 25 wt Sn Phases Fall 2013 185 C 181 C L Phase Composition C 18 3 wt Sn CL 61 9 wt Sn Phase Amounts W 61 9 25 0 85 61 9 18 3 W 97 8 25 0 917 97 8 18 3 WL 25 18 3 0 15 61 9 18 3 W 25 18 3 0 084 97 8 18 3 C 18 3 wt Sn C 97 8 wt Sn ii 60 wt Sn Phases 185 C L 181 C Phase Composition C 18 3 wt Sn CL 61 9 wt Sn Phase Amounts W 61 9 60 0 04 61 9 18 3 W 97 8 60 0 475 97 8 18 3 WL 60 18 3 0 96 61 9 18 3 W 60 18 3 0 525 97 8 18 3 C 18 3 wt Sn C 97 8 wt Sn iii 90 wt Sn 185 C L 181 C Phase Composition C 97 8 wt Sn CL 61 9 wt Sn C 18 3 wt Sn C 97 8 wt Sn Phase Amounts W 90 61 9 0 78 97 8 61 9 W 97 8 90 0 10 97 8 18 3 WL 97 8 90 0 22 97 8 61 9 W 90 18 3 0 90 97 8 18 3 Phases 1 Things to Notice 1 For the 25 wt Sn alloy primary forms before the eutectic reaction while for the 90 wt Sn alloy primary forms before the eutectic reaction 2 As we get closer to the eutectic alloy composition less primary or primary forms before the eutectic reaction occurs 3 Notice that as the alloy composition changes the phase compositions stay the same but the amount of each phase changes For example at 181 C much more of the phase is observed for the 20 wt Sn alloy than for the 90 wt Sn alloy b With part a in mind let s sketch the microstructures we would expect to observe at 181 C for each alloy composition 25 wt Sn The microstructure should have mostly primary W 0 85 at 185 C with the remainder being eutectic and 60 wt Sn The microstructure should have mostly eutectic and because there is very little primary W 0 04 at 185 C 90 wt Sn The microstructure should have mostly primary W 0 78 at 185 C with the remainder being eutectic and Pb Sn Microstructures at 181 C Primary Primary Co 25 wt Sn Co 60 wt Sn Co 90 wt Sn 2 Problem 2 Hypoeutectoid steel of composition Fe with 0 45 wt C a Proeutectoid ferrite is formed above the eutectoid temperature in the two phase region Eutectoid ferrite is formed during the eutectoid reaction Fe3C This is analogous to the formation of primary and eutectic in eutectic systems b W total C Fe 3 C Co C Fe3 C C 6 7 0 45 0 936 6 7 0 022 WFe 3 C 1 W 0 064 c W proeutectoid the phase formed just above 727 C in the two phase region W proeutectoid 0 77 0 45 0 42 0 77 0 022 W eutectoid W total W proeutectoid 0 936 0 42 W eutectoid 0 516 d Above 727 C Fe3C Below 727 C 3 Problem 3 Fe3C 0 77 0 022 10 wt eutectoid Fe3C Fe3C 6 7 Fe3C C0 Eutectoid Fe3C Total Fe3C Proeutectoid Fe3C C0 0 022 C0 0 76 6 7 0 022 6 7 0 76 total Fe3C Proeutectoid Fe3C 0 1 0 1 C 0 0 022 C 0 0 76 6 68 5 94 6 68 5 94 0 1 5 94 C0 0 022 6 68 C0 0 76 Solving for Co gives Co 1 34 wt C Checking your answer 1 34 0 022 1 34 0 76 0 1 6 7 0 022 6 7 0 76 4 Problem 4 a Particles Dispersed in an Matrix b The formation of the precipitates requires solid state diffusion If we cooled to a low temperature very quickly a process called quenching we could prevent precipitate formation from occurring because diffusion would be too slow This would trap the copper in solid solution even though not all of it wants to be there In other words there is a large driving force for atomic rearrangement to form the precipitates but the diffusivity is too low for the atoms to respond to the driving force 5
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