ENME382 S2010 0201 ch11

ENME382 S2010 0201 ch11 - Chapter 11: Phase Transformations...

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ISSUES TO ADDRESS. .. Transforming one phase into another takes time. Fe γ Eutectoid transformation Fe 3 C (cementite) Chapter 11: Phase Transformations Chapter 11 - ENME 382 Spring 2010 Section 0201 1 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? (Austenite) C FCC α (ferrite) + (BCC)
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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 riving force to nucleate increases as we increase Chapter 11 - ENME 382 Spring 2010 Section 0201 2 Driving force to nucleate increases as we increase Δ T Small supercooling b slow nucleation rate - few nuclei - large crystals Large supercooling b rapid nucleation rate - many nuclei - small crystals
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Rate of Phase Transformation transformation complete raction transformed, y Fixed T 0.5 maximum rate reached – now amount unconverted decreases so rate slows t 0.5 rate increases as surface area increases Chapter 11 - ENME 382 Spring 2010 Section 0201 3 Avrami equation => y = 1- exp (- k t n ) k & n are transformation specific parameters log t Fr fraction transformed time By convention rate = 1 / t 0.5 Adapted from Fig. 11.10, Callister & Rethwisch 3e.
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Temperature Dependence of Transformation Rate Adapted from Fig. 11.11, Callister & Rethwisch 3e. (Fig. 11.11 adapted from B.F. Decker and D. Harker, "Recrystallization in Rolled Copper", Trans AIME , 188 , 1950, p. 888.) 135 ° C 119 ° C 113 ° C 102 ° C 88 ° C 43 ° C 1 10 10 2 10 4 Chapter 11 - ENME 382 Spring 2010 Section 0201 4 • 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!
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For transf. to occur, must cool to below 727°C (i.e., must “undercool”) Eutectoid transf. (Fe-Fe 3 C system): γ α + Fe 3 C 0.76 wt% C 0.022 wt% C 6.7 wt% C 1600 400 L δ T (°C) Adapted from Fig. 10.28, Callister & Rethwisch 3e. (Fig. 10.28 adapted from Binary Alloy Phase Chapter 11 - ENME 382 Spring 2010 Section 0201 5 Fe 3 C (cementite) 1400 1200 1000 800 600 400 0 1 2 3 4 5 6 6.7 γ (austenite) γ + L γ +Fe 3 C α +Fe 3 C L +Fe 3 C (Fe) C, wt%C 1148°C α ferrite 727°C Eutectoid: Equil. Cooling: T transf . = 727
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This note was uploaded on 01/24/2011 for the course ENME enme382 taught by Professor Bruck during the Spring '10 term at Maryland.

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ENME382 S2010 0201 ch11 - Chapter 11: Phase Transformations...

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