材料科学专业英语课件4

1、Phase diagram-1-Phase diagram-1-WHY STUDY Phase Diagrams? Predicting microstructures Helping designing alloys Understanding the phase transformation path in forming a microstructure Helping the design and control of heat-treating procedures Being useful in understanding the development and preservat

2、ion of non-equilibrium structures and their attendant properties-2-WHY STUDY Phase Diagrams? Pred-3-Learning Objectives(a) Schematically sketch simple isomorphous and eutectic phase diagrams. (b) On these diagrams label the various phase regions. (c) Label liquidus, solidus, and solvus lines.Given a

3、 binary phase diagram, the composition of an alloy, its temperature, and assuming that the alloy is at equilibrium, determine(a) what phase(s) is (are) present, (b) the composition(s) of the phase(s), and (c) the mass fraction(s) of the phase(s).Given the composition of an ironcarbon alloy containin

4、g between 0.022 wt% C and 2.14 wt% C, be able to (a) specify whether the alloy is hypoeutectoid or hypereutectoid, (b) name the proeutectoid phase, (c) compute the mass fractions of proeutectoid phase and pearlite, and (d) make a schematic diagram of the microstructure at a temperature just below th

5、e eutectoid.-3-Learning Objectives(a) SchePhase: Mass (or part) of system that is uniform chemically and physically and bounded by a surface so that it is mechanically separable from the rest of the system.Example: H2O - vapor, liquid (water) and solid (ice) phasesEquilibrium: When the free energy o

6、f the system is a minimum for a given set of external conditions (pressure, temperature, composition, etc)Example: H2O - 0-100 oC and 1 atm pressure, liquidG: Gibbs free energyP: pressure of the system; V: volume of the systemi: chemical potential of ith componentsXi: mole fraction of ith components

7、 Component: The smallest number of independently variable chemical constituents necessary and sufficient to express the composition of each phase present.Examples: in Cu-Zn brass, the components are Cu an Zn.Phase:G: Gibbs free energyComp-5-ONE-COMPONENT PHASE DIAGRAMSThere are three externally cont

8、rollable parameters that will affect phase structureviz. temperature, pressure, and composition and phase diagrams are constructed when various combinations of these parameters are plotted against one another.aO: solid vapor boundarybO: solid liquid boundarycO: liquid vapor boundaryO: trip pointAt a

9、ny point on one of these curves, the two phases on either side of the curve are in equilibrium (or coexist) with one another.P T phase diagram-5-ONE-COMPONENT PHASE DIAGRAMBINARY PHASE DIAGRAMS- Thermodynamic considerationComponents: A and BFor simple mechanical mixtureMixing heat and configurationa

10、l entropy need to be considered For idea solution, For regular solution,-6-BINARY ISOMORPHOUS SYSTEMSBINARY PHASE DIAGRAMS- ThermodPoint A: single phase regionPoint B: +L two phase regionThis complete solubility is explained by the fact that both Cu and Ni have the same crystal structure (FCC), near

11、ly identical atomic radii and electronegativities, and similarvalences. The coppernickel system is termed isomorphous because of this complete liquid and solid solubility of the two components.-7-Point A: single phase region-8-Determination of Phase Compositions(concerning the two phase region)A tie

12、 line is constructed across the two-phase region at the temperature of the alloy.The intersections of the tie line and the phase boundaries on either side are noted.Perpendiculars are dropped from these intersections to the horizontal composition axis, from which the composition of each of the respe

13、ctive phases is read.-8-Determination of Phase CompDetermination of Phase Amounts (Lever rule)based on a mass balance equationWi the mass fraction of i phaseCi the composition of i phaseCombining the two equations-9-Determination of Phase AmountsExercises-10-Exercises-10-11-11-DEVELOPMENT OF MICROST

14、RUCTURE IN ISOMORPHOUS ALLOYSEquilibriumNon-equilibrium-12-DEVELOPMENT OF MICROSTRUCTURE BINARY EUTECTIC SYSTEMSEutectic reactionPoint E: triple point3 phase are in equilibrium with each other. Eutectic isotherm: the horizontal solidus line at TE (line BEG).In the construction of binary phase diagra

15、ms, it is important to understand that one or at most two phases may be in equilibrium within a phase field.Three phases may be in equilibrium, but only at points along the eutectic isotherm.-13-BINARY EUTECTIC SYSTEMSEutectiQuestionsUsing this diagram, briefly explain how spreading salt on ice that

16、 is at a temperature below 0 Celsius degree can cause the ice to melt.At what temperature is salt no longer useful in causing ice to melt?-14-QuestionsUsing this diagram, bFor a 40 wt% Sn60 wt% Pb alloy at 150 Celsius degree (a) What phase(s) is (are) present? (b) What is (are) the composition(s) of

17、 the phase(s)?-15-For a 40 wt% Sn60 wt% Pb alloDEVELOPMENT OF MICROSTRUCTURE IN EUTECTIC ALLOYSLead Tin binary eutectic phase diagramMicrostructural development of Pb-rich phase-16-DEVELOPMENT OF MICROSTRUCTURE Microstructure of a Pb-Sn eutectic alloy, 375XFormation of a Pd-Sn eutecitc microstructur

18、e-17-Microstructure of a Pb-Sn euteMicrostructural development of hypoeutecitc Pd-Sn alloyMicrostructure of a hypoeutectic Pb-Sn alloy, 400X-18-Microstructural development ofLever rule in eutectic systemsFraction of eutectic phaseFraction of primary phaseFraction of total phaseFraction of total phas

19、e-19-Lever rule in eutectic systemsEQUILIBRIUM DIAGRAMS HAVING INTERMEDIATE PHASES OR COMPOUNDS-20-EQUILIBRIUM DIAGRAMS HAVING INThis phase diagram can be treated as two simple eutectic phase diagram by consider Mg2Pb phase as a single component.-21-This phase diagram can be treaEUTECTOID AND PERITE

20、CTIC REACTIONS-22-EUTECTOID AND PERITECTIC REACTCONGRUENT PHASE TRANSFORMATIONSPhase transformations may be classified according to whether or not there is any change in composition for the phases involved. Those for which there are no compositional alterations are said to be congruent transformatio

21、ns.Examples: pure iron congruent melting point-23-CONGRUENT PHASE TRANSFORMATIONTHE GIBBS PHASE RULEThe publication of the paper On the Equilibrium of Heterogeneous Substances (187478) is now regarded as a landmark in the development of physical chemistry.-24-Major scientific contributions:Chemical

22、thermodynamicsStatistical mechanicsVector analysisPhysical opticsA professor of Yale UniversityGibbs worked at a time when there was little tradition of rigorous theoretical science in the United States. His research was not easily understandable to his students or his colleagues and he made no effo

23、rt to popularize his ideas or to simplify their exposition to make them more accessible. His seminal work on thermodynamics was published mostly in the Transactions of the Connecticut Academy, a journal edited by his librarian brother-in-law, which was little read in the USA and even less so in Euro

24、pe. When Gibbs submitted his long paper on the equilibrium of heterogeneous substances to the Academy, both Elias Loomis and H. A. Newton protested that they did not understand Gibbss work at all, but they helped to raise the money needed to pay for the typesetting of the many equations and mathemat

25、ical symbols in the paper. THE GIBBS PHASE RULEThe publicP is the number of phases presentF is termed the number of degrees of freedom or the number of externally controlled variables (e.g., temperature, pressure, composition) which must be specified to completely define the state of the system.F is

26、 the number of these variables that can be changed independently without altering the number of phases that coexist at equilibrium.On the other wayC represents the number of components in the system.N is the number of noncompositional variables (e.g., temperature and pressure).-25-P is the number of

27、 phases pres-26-Example: coppersilver systemSince pressure is constant (1 atm), the parameter N is 1temperature is the only noncompositional variable.Furthermore, the number of components C is 2 (viz. Cu and Ag), then,Consider the case of single-phase fields on the phase diagram (e.g., , and liquid

28、regions). Since only one phase is present, P=1, we have,This means that to completely describe the characteristics of any alloy that exists within one of these phase fields, we must specify two parameters; these are composition and temperature, which locate, respectively, the horizontal and vertical positions of the alloy on the phase diagram.Single phase region-26-Example: coppersilver sysTwo phases regionIf we specified T, C and CL will be established.Alternatively, if we specified C, T and CL will be established.Triple pointThis means that the compositions of all three