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EGN3365-10 - Chapter 10 Phase Transformations ISSUES TO...

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Chapter 10 - 1 ISSUES TO ADDRESS... Transforming one phase into another takes time. How does the rate of transformation depend on time and T ? How can we slow down the transformation so that we can engineering non-equilibrium structures? Are the mechanical properties of non-equilibrium structures better? Fe γ (Austenite) Eutectoid transformation C FCC Fe 3 C (cementite) α (ferrite) + (BCC) Chapter 10: Phase Transformations
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Chapter 10 - 2 Phase transformation Takes time (transformation rates: kinetics ). Involves movement/rearrangement of atoms. Usually involves changes in microstructure 1. “Simple” diffusion-dependent transformation : no change in number of compositions of phases present (e.g. solidification of pure elemental metals, allotropic transformation, recrystallization, grain growth). 2. Diffusion-dependent transformation: transformation with alteration in phase composition and, often, with changes in number of phases present (e.g. eutectoid reaction). 3. Diffusionless transformation: e.g. rapid T quenching to “trap” metastable phases.
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Chapter 10 - 3 T R Adapted from Fig. 7.22, Callister 7e. The influence of annealing T on the tensile strength and ductility of a brass alloy. º º T R = recrystallization temperature
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Chapter 10 - 4 Iron-Carbon Phase Diagram Adapted from Fig. 9.24, Callister 7e . Fe 3 C (cementite) 1600 1400 1200 1000 800 600 400 0 1 2 3 4 5 6 6.7 L γ (austenite) γ + L γ +Fe 3 C α +Fe 3 C α + γ L +Fe 3 C δ (Fe) C o , wt% C 1148°C T (°C) α 727°C = T eutectoid A S R 4.30 γ γ γ γ R S 0.76 C eutectoid B Fe 3 C (cementite-hard) α (ferrite-soft) γ (0.76 wt% C) cool heat α (0.022 wt% C) + Fe 3 C (6.7 wt% C) Eutectoid cooling:
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Chapter 10 - 5 Phase Transformations Nucleation – nuclei (seeds) act as template to grow crystals – for nucleus to form rate of addition of atoms to nucleus must be faster than rate of loss – once nucleated, grow until reach equilibrium Driving force to nucleate increases as we increase Δ T – supercooling (eutectic, eutectoid) – superheating (peritectic) Small supercooling barb2right few nuclei - large crystals Large supercooling barb2right rapid nucleation - many nuclei, small crystals
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Chapter 10 - 6 Solidification: Nucleation Processes Homogeneous nucleation – nuclei form in the bulk of liquid metal – requires supercooling (typically 80-300°C max) Heterogeneous nucleation – much easier since stable “nucleus” is already present Could be wall of mold or impurities in the liquid phase – allows solidification with only 0.1-10ºC supercooling Phase 1 (e.g. liquid) Nucleation of 2 nd phase Growth
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Chapter 10 - 7 Homogeneous Nucleation Thermodynamic parameters: Free energy G (or Gibbs free energy) Enthalpy H: internal energy of the system and the product of its volume multiplied by the pressure Entropy S: randomness or disorder of the atoms or molecules Δ G is important---a phase transformation will occur spontaneously only when Δ G has a negative value.
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Chapter 10 - 8 Homogeneous Nucleation
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Chapter 10 - 9 r * = critical nucleus : nuclei < r * shrink; nuclei> r * grow (to reduce energy) Adapted from Fig.10.2(b), Callister 7e.
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