04CHM114_0409

04CHM114_0409 - CHEM 114 Spring 2004, Fri, April 9. Lecture...

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Unformatted text preview: CHEM 114 Spring 2004, Fri, April 9. Lecture No. 30 Two component systems (cont'd) PHASE DIAGRAMS PHASE DIAGRAMS WITH ADDITIONAL COMPLEXITY 1. One crystal dissolves some of the other component. (i.e. some "solid solubility”) {a iv- to .- sMaximum solid solubility at eutectic . 9!. 8 B 2. A compound forms in the system.(two component system with “congruently ' ' compound). (because the components are attracted to each other). A very complicated example of this type of system in which phases of both fixed and variable compositions is the historically important system Cu-Sn. The solid solution of Sn in Cu (maximum solubility 10% Sn) is the alon known as bronze, which is much harder than pure copper, and which gave rise to the early human cultural period known as the “Bronze Age". An example is the " top half" of the iron + carbon binary system system. Fig. 5 in the handout. (Compound Fe3C forms from Fe + C reaction) Here- We have most of the story of cast irons. Addition of carbon to iron lowers the melting point to 1140°C, which is why blast furnaces can work without freezing up. The eutectic occurs at about 4.5 wt % iron.Whal crystallizes on the other side of the eutech is not carbon but a compound of iron and carbon, an iron carbide called cementire. Qu. what is the compasition offhe lowest melting Fe +C liquid solution, in mole ‘56 C? (a) (b) (c) (d) (e) I S‘s-nu ‘9 Cemenrite is a very hard brittle compound, like most metal carbides' and by itself has no value as a structural material, however its occurrence in the Fe-C system is of critical importance to the properties of steels because of the way it is distributed in the steel ( see later) For the moment we need only see how it affects the phase dian —._.I y 2“? may IS BUTEcflg an A F c t n- h‘ G+ ‘5’ "" '3“ch ! Cooling of liquid blast furnace iron to room temperature results in the formation of a mixture of two solids one of them almost pure iron and the other almost pure cementite (though there is some complexity on the way down from the eutectic temperature... see below. GRAPHITIZED CAST IRONS. At ambient temperatures, Fe3C is actually unstable with respect to the combination of elements! ! l! @0— kuhxn‘.‘ is»: +th'. M H165‘C— FB3C “=9 3Fe + C(gr) Fe3C must be stabilized by entropy! I FLSC- G (stability index) #130 T So on long annealing (soaking) at moderate temperatures, cast irons lose their brittleness and become a sort of pure iron with graphite nodules scattered around. (called "graphitized cast irons“ 5. Systems with solid state transitions in one component. (The fcc (Austenite) to bee (ferrite) transition and the structure («1 . __ of steels). Let's take this one bit at a time. Forget about the rest — - of the phase diagram with liquid solutions etc, and 9' concentrate just on the solid solution part. We consider the case, like in the Fe—C system, in which the second component is soluble in the high temperature crystal phase [FCC in the case of iron) but not in the low temperature crystal (BCC in the case of Fe-C system). The addition of the second component must have the same effect on the solid-solid transtion temperature as it does on the melting point in the simple eutectic system considered last time... it must lo er it. t? T 5’5 n+- GI pelvis A %B Sow D h A %B B .t- rot:- .S's 1g“ 151-? New terms: eutectgid {temperature and Hun- eutectoz'd composition. In the case of the Fe + C ‘ system the eutectoid temperglglre is 700°C and the eutectoid composition is 0.8% C. .(Exercise: Convert lwt % carbon into (a) mole % carbon and (b) mole IQC) Pearlite Something special about the structures that form during the eutectoid process. Chemical reaction representation (Fat-Ll -—-7 Put "' F’- S 5 i a ‘ Genoa-1‘ a“ EFF. Salad-u... (Fon- C 6M. ) ,PLO-u‘u The product of reaction is a structure called pearlite. (we will come back to this in some detail as it is at the heart of the toughness of steels). (Refer to hand out) Now let’s look at the phase diagram for systems like Fe-Fe3C, omitting the part with carbon contents above that of the compound Fe3C and compare with the simple eutectic system. (see handout 3, Fig. 1 O 1 2 3 4 5 6 0:! (:0 l G What happens during cooling of differ M compositions in the Fe—C s . Pt ? Q.) OH... 121:“- (t) JI‘M Cddsg ) (I! “*3 Jan“ ...
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This note was uploaded on 11/01/2009 for the course CHM 114 taught by Professor Aksyonov during the Fall '08 term at ASU.

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04CHM114_0409 - CHEM 114 Spring 2004, Fri, April 9. Lecture...

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