Chap10 Phase Transformations in Metals

Chapter 10: Phase Transformations ISSUES TO ADDRESS... γ Eutectoid transformation Fe C FCC • How does the rate of transformation depend on time and temperature? • How can we slow down the transformation so that we can engineer nonequilibrium structures? • Are the mechanical properties of nonequilibrium structures better? Fe 3 C (cementite) α (ferrite) + 10-1 Anderson 205- (BCC) • Phase transformations take time to occur. Fraction of Transformation • Fraction Transformed depends on time Callister, Fig. 10.1 fraction transformed time y = 1− e −kt n Avrami Eqn. • Transformation rate depends on T r = 1 t 0.5 = Ae −Q/RT • r often small: equil. not possible! activation energy y log (t) Fixed T 0 0.5 1 . t 0 5 Callister, Fig. 10.2 1 10 10 2 10 4 0 50 100 y (%) log (t) min Ex: recrystallization of Cu 10-2 Anderson 205- • Can make it occur at: ...727ºC (cool it slowly) ...below 727ºC (“undercool” it!) • Eutectoid transf. (Fe-C System): Transformati ons & Undercooling α ferrite 1600 1400 1200 1000 800 600 400 0 1 2 3 4 5 6 L γ austenite γ +L γ +Fe 3 C Fe 3 C cementite α+Fe 3 C L+Fe 3 C (Fe) C o , wt% C Eutectoid: 727°C T(°C) ∆T 10-3 Anderson 205- 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 Eutectoid Reaction Rate ~ ∆T • Growth of pear lite from auste nite: • r ⇑ as ∆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 1 10 10 2 10 3 time (s) 0 50 100 0 50 100 600°C (∆T larger) 675°C (∆T smaller) 650°C 10-4 Anderson 205- reaction rate

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Chap10 Phase Transformations in Metals