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 preservation of non-equili
2、brium structures and their attendant properties-2-3-Learning Objectives1.(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.2.Given a binary phase diagram,the composition of an alloy,i
3、ts 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).3.Given the composition of an ironcarbon alloy containing between 0.022 wt%C and 2.14 wt%C,be able to(a)specify whether
4、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 the eutectoid.Phase:Mass(or part)of system that is uniform chemically and phys
5、ically 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 of the system is a minimum for a given set of external conditions(pressure,temperature,composition,etc)Example:H2O-0-10
6、0 oC and 1 atm pressure,liquidiiXXXTSPVG.2211G:Gibbs free energyP:pressure of the system;V:volume of the systemi:chemical potential of ith componentsXi:mole fraction of ith components Component:The smallest number of independently variable chemical constituents necessary and sufficient to express th
7、e composition of each phase present.Examples:in Cu-Zn brass,the components are Cu an Zn.-5-ONE-COMPONENT PHASE DIAGRAMSThere are three externally controllable parameters that will affect phase structureviz.temperature,pressure,and composition and phase diagrams are constructed when various combinati
8、ons of these parameters are plotted against one another.aO:solid vapor boundarybO:solid liquid boundarycO:liquid vapor boundaryO:trip pointAt any 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 diagramBINARY PHASE DIAGR
9、AMS-Thermodynamic considerationComponents:A and BFor simple mechanical mixtureBBAAMGXGXG1BAXXMixing heat and configurational entropy need to be considered mMsSTHGGmMsidSTGG,mMsrSTHGG,For idea solution,0HFor regular solution,0H-6-BINARY ISOMORPHOUS SYSTEMSPoint A:single phase regionPoint B:+L two pha
10、se regionThis complete solubility is explained by the fact that both Cu and Ni have the same crystal structure(FCC),nearly identical atomic radii and electronegativities,and similarvalences.The coppernickel system is termed isomorphous because of this complete liquid and solid solubility of the two
11、components.-7-8-Determination of Phase Compositions(concerning the two phase region)1)A tie line is constructed across the two-phase region at the temperature of the alloy.2)The intersections of the tie line and the phase boundaries on either side are noted.3)Perpendiculars are dropped from these in
12、tersections to the horizontal composition axis,from which the composition of each of the respective phases is read.Determination 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-Exercises-10-11-DEVEL
13、OPMENT OF MICROSTRUCTURE IN ISOMORPHOUS ALLOYSEquilibriumNon-equilibrium-12-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 diagrams,it is importan
14、t 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-Questions(a)Using this diagram,briefly explain how spreading salt on ice that is at a temperature below 0 Celsius degree can
15、 cause the ice to melt.(b)At what temperature is salt no longer useful in causing ice to melt?-14-For 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 the phase(s)?-15-DEVELOPMENT OF MICROSTRUCTURE IN EUTECTIC ALLOYSLead Tin binary eute
16、ctic phase diagramMicrostructural development of Pb-rich phase-16-Microstructure of a Pb-Sn eutectic alloy,375XFormation of a Pd-Sn eutecitc microstructure-17-Microstructural development of hypoeutecitc Pd-Sn alloyMicrostructure of a hypoeutectic Pb-Sn alloy,400X-18-Lever rule in eutectic systemsFra
17、ction of eutectic phaseFraction of primary phaseFraction of total phaseFraction of total phase-19-EQUILIBRIUM DIAGRAMS HAVING INTERMEDIATE PHASES OR COMPOUNDS-20-This phase diagram can be treated as two simple eutectic phase diagram by consider Mg2Pb phase as a single component.-21-EUTECTOID AND PER
18、ITECTIC REACTIONS-22-CONGRUENT 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 transformations.Examples:pure iron congru
19、ent melting point-23-THE 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 thermodynamicsStatistical mechanicsVector analysisPhysical op
20、ticsA 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 effort to popularize his ideas or to simplify their exposition to
21、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 Europe.When Gibbs submitted his long paper on the equilibrium of hete
22、rogeneous 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 mathematical symbols in the paper.P is the number of phases presentF is termed
23、 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 the number of these variables that can be changed independently without altering the number of phases tha
24、t 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).-25-26-Example:coppersilver systemSince pressure is constant(1 atm),the parameter N is 1temperature is the only noncompositional v
25、ariable.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 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
26、 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 regionTwo phases regionIf we specified T,C and CL will be established.Alternatively,if we specified
27、C,T and CL will be established.Triple pointThis means that the compositions of all three phases as well as the temperature are fixed.-27-28-The IronCarbon SystemBoth steels and cast irons,primary structural materials inevery technologically advanced culture,are essentially ironcarbon alloys.The Fe-C
28、 phase diagram is simple,but has the characters of several types of phase diagram,e.g.pertectic system,eutectic system,isomorphous system,eutectoid system.It is quite helpful in understanding the development of microstructures of different systems.Two types of iron-carbon phase diagramFe-Fe3C phase
29、diagram is most commonly used,but strictly speaking,it should be called as iron-cemmentite phase diagram or partial iron-carbon phase diagram.-29-30-Phase transformations of Fe-C systemL(bcc)(fcc)(bcc)L L+Fe3CL L+Fe3CL L+L+Fe3CL+L+Fe3CL+L+Fe3CL+L+Fe3C+Fe3CL+L+Fe3C+Fe3CL+Fe3C+Fe3CL L+Fe3C+Fe3C+Fe3CLFe3CTCcarbon
