MAT_101-Winter08_Lecture_15

MAT_101-Winter08_Lec - Chapter 10 Phase Transformations ISSUES TO ADDRESS Transforming one phase into another takes time Fe FCC 3 Eutectoid

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Chapter 10: Phase Transformations ISSUES TO ADDRESS . .. • Transforming one phase into another takes time. • How does the rate of transformation depend on time and T? Fe γ± (Austenite) Eutectoid transformation C FCC Fe 3 C (cementite) α (ferrite) + (BCC) • How can we slow the transformation so that we can engineer non-equilibrium structures? • Are the properties of non-equilibrium structures improved?
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Fraction of Transformation Fraction transformed depends on time. Adapted from Fig. 10.1, Callister 6e. y log (t) Fixed T 0 0.5 1 t 0.5 fraction transformed time y = 1 e k t n Avrami Eqn. Transformation rate depends on T. 11 01 0 2 10 4 0 50 100 1 3 5 ° C 9 0 2 8 4 y (%) log (t) min Ex: recrystallization of Cu r = 1 t 0.5 = Ae Q/RT activation energy r often small: equil not possible! Adapted from Fig. 10.2, (Fig. 10.2 adapted from B.F. Decker and D. Harker, "Recrystallization in Rolled Copper", Trans AIME , 188, 1950, p. 888.)
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Transformations and Undercooling • Can make it occur at: ...727ºC (cool it slowly) ...below 727ºC (“undercool” it!) Eutectoid transf. (Fe-C System): α± ferrite 1600 1400 1200 1000 800 600 400 0 123456 6.7 L γ± austenite γ +L γ +Fe 3 C Fe 3 C cementite α +Fe 3 C α + γ L+Fe 3 C (Fe) C o , wt% C Eutectoid: 0.77 727°C T(°C) T 0.022 γ α + Fe 3 C 0.77wt%C 0.022wt%C 6.7wt%C Undercooling by T: T transf. < 727ºC Equil. cooling: T transf. = 727ºC Adapted from Fig. 9.21, Callister 6e . (Fig. 9.21 adapted from Binary Alloy Phase Diagrams , 2nd ed., Vol. 1, T.B. Massalski (Ed.-in-Chief), ASM International, Materials Park, OH, 1990.)
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Eutectoid Transformation Rate ~ T γ α α α α α α pearlite growth direction Austenite ( γ ) grain boundary cementite (Fe 3 C) ferrite ( α ) γ Diffusive flow of C needed α α γ γ α • Growth of pearlite from austenite: • Reaction rate increases with T. Adapted from Fig. 9.13, Callister 6e . Adapted from Fig. 10.3, . 675°C ( T smaller) 1 10 10 2 10 3 time (s) 0 50 100 y (% pearlite) 0 50 100 600°C ( T larger) 650°C % austenite
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Nucleation and Growth • Reaction rate is a result of nucleation and growth of crystals. • Examples: % Pearlite 0 50 100 Nucleation regime Growth regime log (time) t 50 Nucleation rate increases w/ T Growth rate increases w/ T Nucleation rate high T just below T E T moderately below T E T way below T E Nucleation rate low Growth rate high γ γ γ pearlite colony Nucleation rate med . Growth rate med. Growth rate low Adapted from Fig. 10.1, Callister 6e .
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Isothermal Transformation Diagrams • Fe-C system, C o = 0.77wt%C • Transformation at T = 675C. Adapted from Fig. 10.4, Callister 6e . (Fig. 10.4 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams , American Society for Metals, 1977, p. 369.) 400 500 600 700 11 01 0 2 10 3 10 4 10 5 0 % p e a r l it 1 5 Austenite (stable) T E (727°C) Austenite (unstable) Pearlite T(°C) 100 50 0 0 2 10 4 T=675°C y, % transformed time (s) time (s) isothermal transformation at 675°C
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Ex: Cooling History Fe-C System • Eutectoid composition, C o = 0.77 wt%C • Begin at T > 727 °C • Rapidly cool to 625 °C and hold isothermally.
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MAT_101-Winter08_Lec - Chapter 10 Phase Transformations ISSUES TO ADDRESS Transforming one phase into another takes time Fe FCC 3 Eutectoid

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