Topic10-PhaseTransformations

Topic10-PhaseTransformations - TOPIC 10 PHASE...

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Transforming one phase into another takes time . How does the rate of transformation depend on time and T? How can we control the transformation so that we can engineering non-equilibrium structures? How different are the mechanical properties of non-equilibrium structures? TOPIC 10: PHASE TRANSFORMATIONS
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IMPORTANCE OF COOLING TIME Cu-Ni alloy First α to solidfy: 46wt%Ni Uniform C α : 35wt%Ni Last α to solidfy: < 35wt%Ni Slow cooling Equilibrium phases Fast cooling Non-equilibrium phases
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COOLING AUSTENITE Mainly interested in eutectoid cooling: γ α + Fe 3 C (pearlite), 0.77 wt% C Cooling rate can result in a wide variety of phases and microstructures Equilibrium phases: pearlite, bainite Non-equilibrium phases: martensite Austenite Pearlite
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MECHANICAL PROPERTIES Martensite Tempered martensite Bainite Fine pearlite Coarse pearlite Strength Ductility Can control the formation of specific phases and microstructure so that desired properties result
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Fraction transformed depends on time, at constant temperature (e.g., γ pearlite) Transformation rate , r = 1/t 0.5 FRACTION OF TRANSFORMATION n kt e y - - = 1 Avrami equation ( k, n are constants)
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Growth of pearlite from austenite: Reaction rate increases with T. EUTECTOID TRANSFORMATION RATE ~ T
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Fe-C system, Eutectoid composition (C o = 0.77wt%C) Transformation at T = 675C. TIME-TEMPERATURE TRANSFORMATION (TTT) DIAGRAMS Also called isothermal transformation diagram
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Eutectoid composition, C o = 0.77wt%C Begin at T > 727C Rapidly cool to 625C and hold isothermally. Cooling to lower temperatures results in finer microstructures EX: COOLING HISTORY Fe-C SYSTEM
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- Smaller T: colonies are larger - Larger T: colonies are smaller T transf just below T E --Larger T: diffusion is faster --Pearlite is coarser. Two cases: T transf well below T E --Smaller T: diffusion is slower --Pearlite is finer.
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