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  • 1.PHASE TRANSFORMATION

2. Why do we study phase transformations? Thetensile strength of an Fe-C alloy of eutectoid composition can bevaried between 700-2000 MPa depending on the heat treatment processadopted. This shows that the desirable mechanical properties of a material can beobtained as a result of phase transformations using the right heat treatmentprocess.In order to design a heat treatment for some alloy with desired RTproperties, time and temperature dependencies of some phasetransformations can be represented on modified phase diagrams. 3. phase transformationsMost phase transformations begin with the formation of numerous smallparticles of the new phase that increase in size until the transformation iscomplete. Nucleation is the process whereby nuclei (seeds) act as templates forcrystal growth.Homogeneous nucleation - nuclei form uniformly throughout the parentphase; requires considerable supercooling (typically 80-300C).Heterogeneous nucleation - form at structural inhomogeneities (containersurfaces, impurities, grain boundaries, dislocations) in liquid phase mucheasier since stable nucleating surface is already present; requires slightsupercooling (0.1-10C ). 4. Thermodynamics andkinectics of PHASETRANSFORMATIONWhat does lie underneath the structure.. 5. phase transformation Phase transformation is predominantly controlled by TEMP. Buttransformation never really start at transformation temp rather it starts ata temp much below the temp predicted for the transformation to occur. Undercooling: It is the gap between the temp predicted for thetransformation to occur and the temp at which the transformationactually occurs. 6. SupercoolingDuring the cooling of a liquid, solidification (nucleation) will begin onlyafter the temperature has been lowered below the equilibrium solidification(or melting) temperature Tm. This phenomenon is termed supercooling (orundercooling. The driving force to nucleate increases as T increasesSmall supercooling slow nucleation rate - few nuclei - large crystalsLarge supercooling rapid nucleation rate - many nuclei - small crystals 7. Nucleation of a spherical solid particle in a liquid The change in free energy G (a function of the internal energy andenthalpy of the system) must be negative for a transformation to occur. The Assume that nuclei of the solid phase form in the interior of the liquidas atoms cluster together-similar to the packing in the solid phase Also, each nucleus is spherical and has a radius r.Free energy changes as a result of a transformation: 1) the differencebetween the solid and liquid phases (volume free energy, GV); and 2) thesolid-liquid phase boundary (surface free energy, GS). 8. phase transformationAllotropic / polymorphic transformation: No change in compositionof the structurePhase transformation: Change in crystal structure+ Change incomposition.Surface creations always hinders the process of transformation. Thenew phase always trys to create the surface, so energy needs to besupplied. So volume free energy will try to decrease the energy butsurface free energy will try to increase the energy. 9. Transforming one phase into another takes time.FeFe C Eutectoid3 transformation (cementite) (Austenite) + C FCC(ferrite) (BCC) G = GS + GV 10. phase transformation In the previous fig it can be observed that as soon as the particles of A phase are formed the free energy of the system should decrease the new phase is developed and has lower energy than the B phase. Fv=Vf V= Vol of the new crystal f=free energies of the new phase formation of the new crystal is linked with the interface between the new and initial phases. Fs = ss = surface area of the new crystal = free energy per unit area 11. phase transformation 12. phase transformation If rate kinetics of phase transformation is increased then the structurewill be finer and this is indicated by the Hall - Petch equation States thatdecrease in grain size and with fineness in the structure the strength inincreased. o = + Ka (-1/2) Hall-Petch EquationWhere, o = Friction stress = in stress a = grain size K= locking parameter 13. Solid state transformation During the solid state transformation still another factor actinginhibiting the nucleation transformation nuclei. A new phase always differs from the initial one in its structure andspecific volume. Since the transformation develops an elastic crystalline medium,change in specific volume should cause an development in elasticstrain energy in one or both the phases. This inhibits the transformationand kinetics the free energy. 14. Solid state transformation Therefore, the certain elastic component Fel makes a +vecontribution to the free energy change in the solid statetransformation 15. Martensite transformation temp is much lower than Pearlitetransformation temp??Tm>>TpReason: Elastic strain energycomponentA M leads to volumetric expansionwhich leads to straining of the lattice andhence a +ve component in the freeenergy. To compensate this +vecomponent an undercooling is there. Sotemp of transformation is so low. 16. Nucleation and Growth Reaction rate is a result of nucleation and growth of crystals. 100 Nucleation rate increases w/T% Pearlite Growth regimeGrowth rate increases w/ T 50Nucleationregime 0t50log (time) Examples: pearlite colony T just below TET moderately belowTE T way below TE Nucleation rate lowNucleation rate medium Nucleation rate high Growth rate is highGrowth rate is mediumGrowth rate is low5 17. FRACTION OF TRANSFORMATION Fraction transformed depends on time. Avrami Eqn.n y = 1 ekt fraction transformedtime Transformation rate depends on T.activation energy Ex: recrystallization of CuC119C103CC y (%) C C 2 51 13118843100 r= = Ae Q /RTt 0.550 0110102 104 log (t) min r often small: equil not possible2 18. Eutectoid Transformation rate ~ T Transformation of austenite to pearlite: Diffusion of CAustenite () cementite (Fe3C) during transformationgrain Ferrite ()boundary pearlite growth direction For this transformation,100Carbondiffusionrate increases with ( T) 600C (T larger) % pearlite[Teutectoid T ]. 650C 50 675C (T smaller)0Coarse pearlite formed at higher temperatures relatively softFine pearlite formed at lower temperatures relatively hard 19. PHASE TRANSFORMATIONSBased onMasstransport Diffusion less militaryDiffusionaltransformation transformation Change inNo change incomposition compositionPHASE TRANSFORMATIONSBased on Order Ist order nucleation 2nd order entireand growthvolume transforms 20. Diffusion-less transformation in solids Majorphase transformations that occur in solid phase are due tothermally activated atomic movementsThe different types of phase transformation that is possible can bedivided into 5 groups: Precipitation Transformation Eutectoid transformation Ordering reactions Massive transformation Polymorphic changes 21. Precipitation Transformations: Generally expressed as + where is a metastable supersaturated solid solution is a stable or metastable precipitate is a more stable solid solution with the same crystal structure as but composition closer to equilibrium 22. Eutectoid Transformations: Generally expressed as + Metastable phase () replaced by a more stable mixture of + Precipitation and eutectoid transformations require compositionalchanges in the formation of the product phase and consequentlyrequire long-range diffusion 23. Ordering Transformations: Generally expressed as (disordered) (ordered) . These do not require long range diffusion 24. Massive Tranformations: Generally expressed as Original phase decomposes into one or more new phases which havethe same composition as the parent phase but different crystalstructures 25. Polymorphic Transformations: Typically exhibited by singlecomponent systems where different crystal structures are stable overdifferent temperature ranges. E.g. bcc-fcc transformation in Fe 26. Possible TransformationsMartensiteT MartensiteStrength Ductilitybainite fine pearlite coarse pearlite spheroidite General Trends 27. Time TemperatureTransformation(TTT) curves 28. Iron-Iron Carbide phase diagramEutectic L + Fe3C PeritecticL L+ 1493C L+0.1 %C2.06 1147C Eutectoid + Fe3C + Fe3C723C 0.025 %C + Fe3C T FeFe3C0.16 0.8 4.3 6.7 %C 29. WHAT ARE TTT CURVES T (Time) T(Temperature) T(Transformation) diagram is aplot of temperature versus the logarithm of time for a steelalloy of definite composition. It is used to determine when transformations begin and endfor an isothermal (constant temperature) heat treatment of apreviously austenitized alloy TTT diagram indicates when a specific transformationstarts and ends and it also shows what percentage oftransformation of austenite at a particular temperature isachieved. 30. Time- Temperature-Transformation (TTT) Curves IsothermalTransformation 800 Eutectoid temperature 723 AusteniteCoarsePearlite 600 Fine Eutectoid steel 500 Pearlite + Bainite T 400 BainiteNot an isothermal 300 MsAustenite 200 transformation Mf 100 Martensite 0.1 1 10102 103104 105 t (s) 31. The dependance of transformation to temperature and time can beanalyzed best using the diagram below: 2 solid curves are plotted: one represents the timerequired at eachtemperature for the start ofthe transformation; the other is fortransformation completion. The dashed curvecorresponds to 50%completion. The austenite to pearlitetransformation will occuronly if the alloy issupercooled to below theeutectoid temperature(727C). Time for process to completedepends on thetemperature. 32. WHY TTT CURVE HAS A C- SHAPE The transformation of austenite doesnot start immediatelyon quenching the austenised sample to a constanttemperature bath Transformation of the austenite to its product occurs after adefinite time interval incubation period Incubation period is that period in which transformationdoesnot proceed because enough diffusion has not takenplacein austenite for the transformation to start 33. Thus the C shape