Phase transformation physical metallurgy
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Phase TransformationsFe3C (cementite)- orthorhombicMartensite - BCTAustenite - FCCFerrite - BCC
Phase TransformationsTransformation rateKinetics of Phase TransformationNucleation: homogeneous, heterogeneousFree Energy, GrowthIsothermal Transformations (TTT diagrams)Pearlite, Martensite, Spheroidite, BainiteContinuous CoolingMechanical BehaviorPrecipitation Hardening
Phase TransformationsPhase transformations change in the number or character of phases.Simple diffusion-dependentNo change in # of phasesNo change in compositionExample: solidification of a pure metal, allotropic transformation, recrystallization, grain growthMore complicated diffusion-dependentChange in # of phasesChange in compositionExample: eutectoid reactionDiffusionlessExample: metastable phase - martensite
Phase TransformationsMost phase transformations begin with the formation of numerous small particles of the new phase that increase in size until the transformation is complete.Nucleation is the process whereby nuclei (seeds) act as templates for crystal growth. Homogeneous nucleation - nuclei form uniformly throughout the parent phase; requires considerable supercooling (typically 80-300C).Heterogeneous nucleation - form at structural inhomogeneities (container surfaces, impurities, grain boundaries, dislocations) in liquid phase much easier since stable nucleating surface is already present; requires slight supercooling (0.1-10C).
SupercoolingDuring the cooling of a liquid, solidification (nucleation) will begin only after the temperature has been lowered below the equilibrium solidification (or melting) temperature Tm. This phenomenon is termed supercooling (or undercooling.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
Nucleation of a spherical solid particle in a liquidc11f01LiquidThe change in free energy DG (a function of the internal energy and enthalpy of the system) must be negative for a transformation to occur.Assume that nuclei of the solid phase form in the interior of the liquid as 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 difference between the solid and liquid phases (volume free energy, DGV); and 2) the solid-liquid phase boundary (surface free energy, DGS). Transforming one phase into another takes time.
DG = DGS + DGV
r* = critical nucleus: for r < r* nuclei shrink; for r >r* nuclei grow (to reduce energy) Homogeneous Nucleation & Energy Effects
SolidificationNote: Hf and are weakly dependent on T
Transformations & Undercooling For transformation to occur, must cool to below 727C Eutectoid transformation (Fe-Fe3C system):ga+Fe3C0.76 wt% C0.022 wt% C6.7 wt% C
2 Fraction transformed depends on time. Transformation rate depends on T. r often small: equil not possibleFRACTION OF TRANSFORMATION
Generation of Isothermal Transformation Diagrams The Fe-Fe3C system, for Co = 0.76 wt% C A transformation temperature of 675C.
1005001102104T = 675C
% transformedtime (s)Consider:
Coarse pearlite formed at higher temperatures relatively softFine pearlite formed at lower temperatures relatively hardEutectoid Transformation Rate ~ DT
5 Reaction rate is a result of nucleation and growth of crystals. Examples:Nucleation and Growth
c11f13Isothermal Transformation Diagrams2 solid curves are plotted: one represents the time required at each temperature for the start of the transformation;the other is for transformation completion.The dashed curve corresponds to 50% completion.
The austenite to pearlite transformation will occur only if the alloy is supercooled to below the eutectoid temperature (727C).Time for process to complete depends on the temperature.
c11f14 Eutectoid iron-carbon alloy; composition, Co = 0.76 wt% C Begin at T > 727C Rapidly cool to 625C and hold isothermally.Isothermal Transformation DiagramAustenite-to-Pearlite
Transformations Involving Noneutectoid CompositionsHypereutectoid composition proeutectoid cementite
Consider C0 = 1.13 wt% C
Coarse pearlite (high diffusion rate) and (b) fine pearlitec11f15
Bainite: Non-Equil Transformation Products elongated Fe3C particles in a-ferrite matrix diffusion controlled a lathes (strips) with long rods of Fe3C
Bainite Microstructure Bainite consists of acicular (needle-like) ferrite with very small cementite particles dispersed throughout. The carbon content is typically greater than 0.1%. Bainite transforms to iron and cementite with sufficient time and temperature (considered semi-stable below 150C).
10 Fe3C particles within an a-ferrite matrix diffusion dependent heat bainite or pearlite at temperature just below eutectoid for long times driving force reduction of a-ferrite/Fe3C interfacial area
Spheroidite: Nonequilibrium Transformation
c11f20Pearlitic Steel partially transformed to Spheroidite
single phase body centered tetragonal (BCT) crystal structure BCT if C0 > 0.15 wt% C Diffusionless transformation BCT few slip planes hard, brittle% transformation depends only on T of rapid cooling
An micrograph of austenite that was polished flat and then allowed to transform into martensite. The different colors indicate the displacements caused when martensite forms.
Isothermal Transformation DiagramIron-carbon alloy with eutectoid composition.
A: AusteniteP: PearliteB: BainiteM: Martensite
c11f24Other elements (Cr, Ni, Mo, Si and W) may cause significant changes in the positions and shapes of the TTT curves:Change transition temperature;Shift the nose of the austenite-to-pearlite transformation to longer times;Shift the pearlite and bainite noses to longer times (decrease critical cooling rate);Form a separate bainite nose;
Effect of Adding Other Elements4340 Steel plain carbonsteel nosePlain carbon steel: primary alloying element is carbon.
c11f23Example 11.2: Iron-carbon alloy with eutectoid composition.Specify the nature of the final microstructure (% bainite, martensite, pearlite etc) for the alloy that is subjected to the following timetemperature treatments:Alloy begins at 760C and has been held long enough to achieve a complete and homogeneous austenitic structure.
Treatment (a)Rapidly cool to 350 CHold for 104 secondsQuench to room temperature
c11f23Martensite, 100%Example 11.2: Iron-carbon alloy with eutectoid composition.Specify the nature of the final microstructure (% bainite, martensite, pearlite etc) for the alloy that is subjected to the following timetemperature treatments:Alloy begins at 760C and has been held long enough to achieve a complete and homogeneous austenitic structure.
Treatment (b)Rapidly cool to 250 CHold for 100 secondsQuench to room temperature
c11f23Bainite, 50%Example 11.2: Iron-carbon alloy with eutectoid composition.Specify the nature of the final microstructure (% bainite, martensite, pearlite etc) for the alloy that is subjected to the following timetemperature treatments:Alloy begins at 760C and has been held long enough to achieve a complete and homogeneous austenitic structure.
Treatment (c)Rapidly cool to 650CHold for 20 secondsRapidly cool to 400CHold for 103 secondsQuench to room temperature
Austenite, 100%Almost 50% Pearlite, 50% AusteniteFinal: 50% Bainite, 50% Pearlite
c11f26Continuous Cooling Transformation DiagramsIsothermal heat treatments are not the most practical due to rapidly cooling and constant maintenance at an elevated temperature.Most heat treatments for steels involve the continuous cooling of a specimen to room temperature. TTT diagram (dotted curve) is modified for a CCT diagram (solid curve).For continuous cooling, the time required for a reaction to begin and end is delayed.The isothermal curves are shifted to longer times and lower temperatures.
c11f27Moderately rapid and slow cooling curves are superimposed on a continuous cooling transformation diagram of a eutectoid iron-carbon alloy.The transformation starts after a time period corresponding to the intersection of the cooling curve with the beginning reaction curve and ends upon crossing the completion transformation curve.Normally bainite does not form when an alloy is continuously cooled to room temperature; austenite transforms to pearlite before bainite has become possible.The austenite-pearlite region (A---B) terminates just below the nose. Continued cooling (below Mstart) of austenite will form martensite.
c11f28For continuous cooling of a steel alloy there exists a critical quenching rate that represents the minimum rate of quenching that will produce a totally martensitic structure.This curve will just miss the nose where pearlite transformation begins
c11f29Continuous cooling diagram for a 4340 steel alloy and several cooling curves superimposed. This demonstrates the dependence of the final microstructure on the transformations that occur during cooling.Alloying elements used to modify the critical cooling rate for martensite are chromium, nickel, molybdenum, manganese, silicon an