Presentation is loading. Please wait.

Presentation is loading. Please wait.

Chapter 11 - 1 ISSUES TO ADDRESS... Transforming one phase into another takes time. How does the rate of transformation depend on time and temperature.

Similar presentations


Presentation on theme: "Chapter 11 - 1 ISSUES TO ADDRESS... Transforming one phase into another takes time. How does the rate of transformation depend on time and temperature."— Presentation transcript:

1 Chapter ISSUES TO ADDRESS... Transforming one phase into another takes time. How does the rate of transformation depend on time and temperature ? Is it possible to slow down transformations so that non-equilibrium structures are formed? Are the mechanical properties of non-equilibrium structures more desirable than equilibrium ones? Chapter 11: Phase Transformations

2 Chapter Phase Transformations Nucleation –nuclei (seeds) act as templates on which crystals grow –for nucleus to form rate of addition of atoms to nucleus must be faster than rate of loss –once nucleated, growth proceeds until equilibrium is attained Driving force to nucleate increases as we increase  T –supercooling (eutectic, eutectoid) –superheating (peritectic) Small supercooling  slow nucleation rate - few nuclei - large crystals Large supercooling  rapid nucleation rate - many nuclei - small crystals

3 Chapter Solidification: Nucleation Types Homogeneous nucleation –nuclei form in the bulk of liquid metal –requires considerable supercooling (typically °C) Heterogeneous nucleation –much easier since stable “nucleating surface” is already present — e.g., mold wall, impurities in liquid phase –only very slight supercooling (0.1-10ºC)

4 Chapter r* = critical nucleus: for r r* nuclei grow (to reduce energy) Adapted from Fig.11.2(b), Callister & Rethwisch 3e. Homogeneous Nucleation & Energy Effects  G T = Total Free Energy =  G S +  G V Surface Free Energy- destabilizes the nuclei (it takes energy to make an interface)  = surface tension Volume (Bulk) Free Energy – stabilizes the nuclei (releases energy)

5 Chapter Solidification Note:  H f and  are weakly dependent on  T  r* decreases as  T increases For typical  T r* ~ 10 nm  H f = latent heat of solidification T m = melting temperature  = surface free energy  T = T m - T = supercooling r* = critical radius

6 Chapter Rate of Phase Transformations Kinetics - study of reaction rates of phase transformations To determine reaction rate – measure degree of transformation as function of time (while holding temp constant) measure propagation of sound waves – on single specimen electrical conductivity measurements – on single specimen X-ray diffraction – many specimens required How is degree of transformation measured?

7 Chapter Rate of Phase Transformation Avrami equation => y = 1- exp (-kt n ) –k & n are transformation specific parameters transformation complete log t Fraction transformed, y Fixed T fraction transformed time 0.5 By convention rate = 1 / t 0.5 Adapted from Fig , Callister & Rethwisch 3e. maximum rate reached – now amount unconverted decreases so rate slows t 0.5 rate increases as surface area increases & nuclei grow

8 Chapter Temperature Dependence of Transformation Rate For the recrystallization of Cu, since rate = 1/t 0.5 rate increases with increasing temperature Rate often so slow that attainment of equilibrium state not possible! Adapted from Fig , Callister & Rethwisch 3e. (Fig adapted from B.F. Decker and D. Harker, "Recrystallization in Rolled Copper", Trans AIME, 188, 1950, p. 888.) 135  C119  C113  C102  C88  C43  C

9 Chapter Transformations & Undercooling For transf. to occur, must cool to below 727°C (i.e., must “undercool”) Eutectoid transf. (Fe-Fe 3 C system):  + Fe 3 C 0.76 wt% C wt% C 6.7 wt% C Fe 3 C (cementite) L  (austenite)  +L+L  +Fe 3 C  L+Fe 3 C  (Fe) C, wt%C 1148°C T(°C)  ferrite 727°C Eutectoid: Equil. Cooling: T transf. = 727 º C TT Undercooling by T transf. < 727  C Adapted from Fig ,Callister & Rethwisch 3e. (Fig adapted from Binary Alloy Phase Diagrams, 2nd ed., Vol. 1, T.B. Massalski (Ed.-in- Chief), ASM International, Materials Park, OH, 1990.)

10 Chapter The Fe-Fe 3 C Eutectoid Transformation Coarse pearlite  formed at higher temperatures – relatively soft Fine pearlite  formed at lower temperatures – relatively hard Transformation of austenite to pearlite: Adapted from Fig , Callister & Rethwisch 3e.        pearlite growth direction Austenite (  ) grain boundary cementite (Fe 3 C) Ferrite (  )  For this transformation, rate increases with [T eutectoid – T ] (i.e.,  T). Adapted from Fig , Callister & Rethwisch 3e. 675°C (  T smaller) 0 50 y (% pearlite) 600°C (  T larger) 650°C 100 Diffusion of C during transformation      Carbon diffusion

11 Chapter Adapted from Fig ,Callister & Rethwisch 3e. (Fig adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1977, p. 369.) Generation of Isothermal Transformation Diagrams The Fe-Fe 3 C system, for C o = 0.76 wt% C A transformation temperature of 675°C T = 675°C y,y, % transformed time (s) %pearlite 100% 50% Austenite (stable) T E (727  C) Austenite (unstable) Pearlite T(°C) time (s) isothermal transformation at 675°C Consider:

12 Chapter Eutectoid composition, C 0 = 0.76 wt% C Begin at T > 727°C Rapidly cool to 625°C Hold T (625°C) constant (isothermal treatment) Adapted from Fig ,Callister & Rethwisch 3e. (Fig adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28.) Austenite-to-Pearlite Isothermal Transformation %pearlite 100% 50% Austenite (stable) T E (727  C) Austenite (unstable) Pearlite T(°C) time (s)      

13 Chapter Transformations Involving Noneutectoid Compositions Hypereutectoid composition – proeutectoid cementite Consider C 0 = 1.13 wt% C  T E (727°C) T(°C) time (s) A A A + C P A + P Adapted from Fig , Callister & Rethwisch 3e. Adapted from Fig , Callister & Rethwisch 3e. Fe 3 C (cementite) L  (austenite)  +L+L  +Fe 3 C  L+Fe 3 C  (Fe) C, wt%C T(°C) 727°C TT

14 Chapter time (s) T(°C) Austenite (stable) 200 P B TETE 0% 100% 50% A A Bainite: Another Fe-Fe 3 C Transformation Product Bainite: -- elongated Fe 3 C particles in  -ferrite matrix -- diffusion controlled Isothermal Transf. Diagram, C 0 = 0.76 wt% C Adapted from Fig , Callister & Rethwisch 3e. Adapted from Fig , Callister & Rethwisch 3e. (Fig from Metals Handbook, 8th ed., Vol. 8, Metallography, Structures, and Phase Diagrams, American Society for Metals, Materials Park, OH, 1973.)  Fe 3 C (cementite) 5  m  (ferrite) 100% bainite 100% pearlite

15 Chapter Spheroidite: -- Fe 3 C particles within an  -ferrite matrix -- formation requires diffusion -- heat bainite or pearlite at temperature just below eutectoid for long times -- driving force – reduction of  -ferrite/Fe 3 C interfacial area Spheroidite: Another Microstructure for the Fe-Fe 3 C System Adapted from Fig , Callister & Rethwisch 3e. (Fig copyright United States Steel Corporation, 1971.) 60  m  (ferrite) (cementite) Fe 3 C

16 Chapter Martensite: --  (FCC) to Martensite (BCT) Adapted from Fig , Callister & Rethwisch 3e. (Fig courtesy United States Steel Corporation.) Adapted from Fig , Callister & Rethwisch 3e. Martensite: A Nonequilibrium Transformation Product Martensite needles Austenite 60  m x x x x x x potential C atom sites Fe atom sites Adapted from Fig , Callister & Rethwisch 3e. Isothermal Transf. Diagram  to martensite (M) transformation.. -- is rapid! (diffusionless) -- % transf. depends only on T to which rapidly cooled time (s) T(°C) Austenite (stable) 200 P B TETE 0% 100% 50% A A M + A 0% 50% 90%

17 Chapter  (FCC)  (BCC) + Fe 3 C Martensite Formation slow cooling tempering quench M (BCT) Martensite (M) – single phase – has body centered tetragonal (BCT) crystal structure Diffusionless transformation BCT if C 0 > 0.15 wt% C BCT  few slip planes  hard, brittle

18 Chapter Phase Transformations of Alloys Effect of adding other elements Change transition temp. Cr, Ni, Mo, Si, Mn retard    + Fe 3 C reaction (and formation of pearlite, bainite) Adapted from Fig , Callister & Rethwisch 3e.

19 Chapter Adapted from Fig , Callister & Rethwisch 3e. Continuous Cooling Transformation Diagrams Conversion of isothermal transformation diagram to continuous cooling transformation diagram Cooling curve

20 Chapter Isothermal Heat Treatment Example Problems On the isothermal transformation diagram for a 0.45 wt% C, Fe-C alloy, sketch and label the time-temperature paths to produce the following microstructures: a)42% proeutectoid ferrite and 58% coarse pearlite b)50% fine pearlite and 50% bainite c)100% martensite d)50% martensite and 50% austenite

21 Chapter Solution to Part (a) of Example Problem a)42% proeutectoid ferrite and 58% coarse pearlite Isothermally treat at ~ 680°C -- all austenite transforms to proeutectoid  and coarse pearlite. A + B A + P A +  A B P A 50% time (s) M (start) M (50%) M (90%) Adapted from Fig , Callister 5e. Fe-Fe 3 C phase diagram, for C 0 = 0.45 wt% C T (°C)

22 Chapter b)50% fine pearlite and 50% bainite Solution to Part (b) of Example Problem T (°C) A + B A + P A +  A B P A 50% time (s) M (start) M (50%) M (90%) Adapted from Fig , Callister 5e. Fe-Fe 3 C phase diagram, for C 0 = 0.45 wt% C Then isothermally treat at ~ 470°C – all remaining austenite transforms to bainite. Isothermally treat at ~ 590°C – 50% of austenite transforms to fine pearlite.

23 Chapter Solutions to Parts (c) & (d) of Example Problem c)100% martensite – rapidly quench to room temperature d)50% martensite & 50% austenite -- rapidly quench to ~ 290°C, hold at this T T (°C) A + B A + P A +  A B P A 50% time (s) M (start) M (50%) M (90%) Adapted from Fig , Callister 5e. Fe-Fe 3 C phase diagram, for C 0 = 0.45 wt% C d) c)

24 Chapter Mechanical Props: Influence of C Content Adapted from Fig , Callister & Rethwisch 3e. Increase C content: TS and YS increase, %EL decreases C 0 < 0.76 wt% C Hypoeutectoid Pearlite (med) ferrite (soft) Adapted from Fig , Callister & Rethwisch 3e. C 0 > 0.76 wt% C Hypereutectoid Pearlite (med) Cementite (hard) Adapted from Fig , Callister & Rethwisch 3e. (Fig based on data from Metals Handbook: Heat Treating, Vol. 4, 9th ed., V. Masseria (Managing Ed.), American Society for Metals, 1981, p. 9.) YS(MPa) TS(MPa) wt% C hardness 0.76 HypoHyper wt% C %EL Impact energy (Izod, ft-lb) HypoHyper

25 Chapter Mechanical Props: Fine Pearlite vs. Coarse Pearlite vs. Spheroidite Adapted from Fig , Callister & Rethwisch 3e. (Fig based on data from Metals Handbook: Heat Treating, Vol. 4, 9th ed., V. Masseria (Managing Ed.), American Society for Metals, 1981, pp. 9 and 17.) Hardness: %RA: fine > coarse > spheroidite fine < coarse < spheroidite wt%C Brinell hardness fine pearlite coarse pearlite spheroidite HypoHyper wt%C Ductility (%RA) fine pearlite coarse pearlite spheroidite HypoHyper

26 Chapter Mechanical Props: Fine Pearlite vs. Martensite Hardness: fine pearlite << martensite. Adapted from Fig , Callister & Rethwisch 3e. (Fig adapted from Edgar C. Bain, Functions of the Alloying Elements in Steel, American Society for Metals, 1939, p. 36; and R.A. Grange, C.R. Hribal, and L.F. Porter, Metall. Trans. A, Vol. 8A, p ) wt% C Brinell hardness martensite fine pearlite HypoHyper

27 Chapter Tempered Martensite tempered martensite less brittle than martensite tempering reduces internal stresses caused by quenching Adapted from Fig , Callister & Rethwisch 3e. (Fig copyright by United States Steel Corporation, 1971.) tempering decreases TS, YS but increases %RA tempering produces extremely small Fe 3 C particles surrounded by  Adapted from Fig , Callister & Rethwisch 3e. (Fig adapted from Fig. furnished courtesy of Republic Steel Corporation.) 9  m YS(MPa) TS(MPa) Tempering T (°C) %RA TS YS %RA Heat treat martensite to form tempered martensite

28 Chapter Summary of Possible Transformations Adapted from Fig , Callister & Rethwisch 3e. Austenite (  ) Pearlite (  + Fe 3 C layers + a proeutectoid phase) slow cool Bainite (  + elong. Fe 3 C particles) moderate cool Martensite (BCT phase diffusionless transformation) rapid quench Tempered Martensite (  + very fine Fe 3 C particles) reheat Strength Ductility Martensite T Martensite bainite fine pearlite coarse pearlite spheroidite General Trends

29 Chapter wt% Cu L +L+L    +L+L (Al) T(°C) composition range available for precipitation hardening CuAl 2 A Adapted from Fig , Callister & Rethwisch 3e. (Fig adapted from J.L. Murray, International Metals Review 30, p.5, 1985.) Precipitation Hardening Particles impede dislocation motion. Ex: Al-Cu system Procedure: Adapted from Fig , Callister & Rethwisch 3e. -- Pt B: quench to room temp. (retain  solid solution) -- Pt C: reheat to nucleate small  particles within  phase. Other alloys that precipitation harden: Cu-Be Cu-Sn Mg-Al Temp. Time -- Pt A: solution heat treat (get  solid solution) Pt A (sol’n heat treat) B Pt B C Pt C (precipitate 

30 Chapter Al Alloy: Maxima on TS curves. Increasing T accelerates process. Adapted from Fig (a) and (b), Callister & Rethwisch 3e. (Fig adapted from Metals Handbook: Properties and Selection: Nonferrous Alloys and Pure Metals, Vol. 2, 9th ed., H. Baker (Managing Ed.), American Society for Metals, p. 41.) Influence of Precipitation Heat Treatment on TS, %EL precipitation heat treat time tensile strength (MPa) min1h1day1mo1yr 204°C non-equil. solid solution many small precipitates “aged” fewer large precipitates “overaged” 149°C Minima on %EL curves. %EL (2 in sample) min1h1day1mo1yr 204°C 149°C precipitation heat treat time

31 Chapter Melting & Glass Transition Temps. What factors affect T m and T g ? Both T m and T g increase with increasing chain stiffness Chain stiffness increased by presence of 1.Bulky sidegroups 2.Polar groups or sidegroups 3.Chain double bonds and aromatic chain groups Regularity of repeat unit arrangements – affects T m only Adapted from Fig , Callister & Rethwisch 3e.

32 Chapter Thermoplastics: -- little crosslinking -- ductile -- soften w/heating -- polyethylene polypropylene polycarbonate polystyrene Thermosets: -- significant crosslinking (10 to 50% of repeat units) -- hard and brittle -- do NOT soften w/heating -- vulcanized rubber, epoxies, polyester resin, phenolic resin Adapted from Fig , Callister & Rethwisch 3e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.) Thermoplastics vs. Thermosets Callister, Fig T Molecular weight TgTg TmTm mobile liquid viscous liquid rubber tough plastic partially crystalline solid crystalline solid

33 Chapter Heat treatments of Fe-C alloys produce microstructures including: -- pearlite, bainite, spheroidite, martensite, tempered martensite Precipitation hardening --hardening, strengthening due to formation of precipitate particles. --Al, Mg alloys precipitation hardenable. Polymer melting and glass transition temperatures Summary

34 Chapter Core Problems: Self-help Problems: ANNOUNCEMENTS Reading:


Download ppt "Chapter 11 - 1 ISSUES TO ADDRESS... Transforming one phase into another takes time. How does the rate of transformation depend on time and temperature."

Similar presentations


Ads by Google