Topic10-PhaseTransformations

Topic10-PhaseTransformations - • Transforming one phase...

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Unformatted text preview: • 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 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 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 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 • 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) • Growth of pearlite from austenite: • Reaction rate increases with ∆ T. EUTECTOID TRANSFORMATION RATE ~ ∆ T • Fe-C system, Eutectoid composition (C o = 0.77wt%C) • Transformation at T = 675C. TIME-TEMPERATURE TRANSFORMATION (TTT) DIAGRAMS Also called isothermal transformation diagram • 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- 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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This homework help was uploaded on 02/26/2008 for the course MAT 33 taught by Professor Keith during the Fall '07 term at Lehigh University .

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Topic10-PhaseTransformations - • Transforming one phase...

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