Phase transformation physical metallurgy

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transformation of phases in physical metallurgy, engineering precipitaion and age hardening

Transcript of Phase transformation physical metallurgy

  • 1.Chapter 11 Phase TransformationsFe3C (cementite)- orthorhombic Martensite - BCT Austenite - FCC Ferrite - BCC

2. Phase Transformations Transformation rate Kinetics of Phase Transformation Nucleation: homogeneous, heterogeneous Free Energy, Growth Isothermal Transformations (TTT diagrams) Pearlite, Martensite, Spheroidite, Bainite Continuous Cooling Mechanical Behavior Precipitation Hardening 3. Phase Transformations Phase transformations change in the number or character of phases. Simple diffusion-dependent No change in # of phases No change in composition Example: solidification of a pure metal, allotropic transformation, recrystallization, grain growth More complicated diffusion-dependent Change in # of phases Change in composition Example: eutectoid reaction Diffusionless Example: metastable phase - martensite 4. Phase Transformations Most 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). 5. Supercooling During 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 increases Small supercooling slow nucleation rate - few nuclei - large crystals Large supercooling rapid nucleation rate - many nuclei - small crystals 6. Nucleation of a spherical solid particle in a liquid Liquid The change in free energy G (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, GV); and 2) the solid-liquid phase boundary (surface free energy, GS). Transforming one phase into another takes time. G = GS + GV Fe (Austenite) Eutectoid transformation C FCC Fe3C (cementite) (ferrite) + (BCC) 7. r* = critical nucleus: for r < r* nuclei shrink; for r >r* nuclei grow (to reduce energy) Homogeneous Nucleation & Energy Effects GT = Total Free Energy = GS + GV Surface Free Energy- destabilizes the nuclei (it takes energy to make an interface) = 2 4 rGS = surface tension Volume (Bulk) Free Energy stabilizes the nuclei (releases energy) = GrGV 3 3 4 volumeunit energyfreevolume = G 8. Solidification TH T r f m = 2 * Note: Hf and are weakly dependent on T r* decreases as T increases For typical T r* ~ 10 nm Hf = latent heat of solidification (fusion) Tm = melting temperature = surface free energy T = Tm - T = supercooling r* = critical radius 9. Transformations & Undercooling For transformation to occur, must cool to below 727C Eutectoid transformation (Fe-Fe3C system): + Fe3C 0.76 wt% C 0.022 wt% C 6.7 wt% C Fe3C(cementite) 1600 1400 1200 1000 800 600 400 0 1 2 3 4 5 6 6.7 L (austenite) +L +Fe3C +Fe3C L+Fe3C (Fe) C, wt% C 1148C T(C) ferrite 727C Eutectoid: Equil. Cooling: Ttransf. = 727C T Undercooling by Ttransf. < 727C 0.76 0.022 10. 2 Fraction transformed depends on time. fraction transformed time y = 1 e ktn Avrami Eqn. Transformation rate depends on T. 1 10 102 1040 50 100 135C 119C113C102C 88C 43C y (%) log (t) min Ex: recrystallization of Cu r = 1 t 0.5 = Ae Q /RT activation energy r often small: equil not possible y log (t) Fixed T 0 0.5 1 t0.5 FRACTION OF TRANSFORMATION 11. Generation of Isothermal Transformation Diagrams The Fe-Fe3C system, for Co = 0.76 wt% C A transformation temperature of 675C. 100 50 0 1 102 104 T = 675C %transformed time (s) 400 500 600 700 1 10 102 103 104 105 0%pearlite 100% 50% Austenite (stable) TE (727C)Austenite (unstable) Pearlite T(C) time (s) isothermal transformation at 675C Consider: 12. Coarse pearlite formed at higher temperatures relatively soft Fine pearlite formed at lower temperatures relatively hard Transformation of austenite to pearlite: pearlite growth direction Austenite () grain boundary cementite (Fe3C) Ferrite () For this transformation, rate increases with ( T) [Teutectoid T ]. 675C (T smaller) 0 50 %pearlite 600C (T larger) 650C 100 Diffusion of C during transformation Carbon diffusion Eutectoid Transformation Rate ~ T 13. 5 Reaction rate is a result of nucleation and growth of crystals. Examples: % Pearlite 0 50 100 Nucleation regime Growth regime log (time)t50 Nucleation rate increases w/ T Growth rate increases w/ T Nucleation rate high T just below TE T moderately below TE T way below TE Nucleation rate low Growth rate high pearlite colony Nucleation rate med . Growth rate med. Growth rate low Nucleation and Growth 14. Isothermal Transformation Diagrams 2 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. 15. Eutectoid iron-carbon alloy; composition, Co = 0.76 wt% C Begin at T > 727C Rapidly cool to 625C and hold isothermally. Isothermal Transformation Diagram Austenite-to-Pearlite 16. Transformations Involving Noneutectoid Compositions Hypereutectoid composition proeutectoid cementite Consider C0 = 1.13 wt% C Fe3C(cementite) 1600 1400 1200 1000 800 600 400 0 1 2 3 4 5 6 6.7 L (austenite) +L +Fe3C +Fe3C L+Fe3C (Fe) C, wt%C T(C) 727C T 0.76 0.022 1.13 17. Strength Ductility Martensite T Martensite bainite fine pearlite coarse pearlite spheroidite General Trends Possible Transformations 18. Coarse pearlite (high diffusion rate) and (b) fine pearlite - Smaller T: colonies are larger - Larger T: colonies are smaller 19. 10 103 105 time (s) 10-1 400 600 800 T(C) Austenite (stable) 200 P B TE 0% 100% 50% A A Bainite: Non-Equil Transformation Products elongated Fe3C particles in -ferrite matrix diffusion controlled lathes (strips) with long rods of Fe3C 100% bainite 100% pearlite Martensite Cementite Ferrite 20. 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). 21. 10 Fe3C particles within an -ferrite matrix diffusion dependent heat bainite or pearlite at temperature just below eutectoid for long times driving force reduction of -ferrite/Fe3C interfacial area Spheroidite: Nonequilibrium Transformation 10 103 105time (s)10-1 400 600 800 T(C) Austenite (stable) 200 P B TE 0% 100% 50% A A Spheroidite 100% spheroidite 100% spheroidite 22. Pearlitic Steel partially transformed to Spheroidite 23. 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 Martensite Formation Isothermal Transformation Diagram 10 10 3 10 5 time (s)10 -1 400 600 800 T(C) Austenite (stable) 200 P B TE 0% 100%50% A A M + A M + A M + A 0% 50% 90% Martensite needles Austenite 24. 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. 25. Isothermal Transformation Diagram Iron-carbon alloy with eutectoid composition. A: Austenite P: Pearlite B: Bainite M: Martensite 26. Other 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 Elements 4340 Steel plain carbon steel nose Plain carbon steel: primary alloying element is carbon. 27. 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 (a) Rapidly cool to 350 C Hold for 104 seconds Quench to room temperature Bainite, 100% 28. Martensite, 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 C Hold for 100 seconds Quench to room temperature Austenite, 100% 29. Bainite, 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: