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Explore the effects of time and temperature on phase transformations in materials, including mechanical property changes. Learn about different classes of transformations and their impacts on structures. Dive into nucleation, growth, and microstructural development processes. Understand non-equilibrium transformation products like bainite, spheroidite, and martensite. Discover isothermal and continuous cooling transformation diagrams. Gain insights on mechanical properties and processing options.
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Chapter 10: Phase Transformations Chapter 9 assumes equilibrium – takes too long Chapter 10 – effect of time • Transforming one phase into another takes time. • How does the rate of transformation depend on time and T? • How the rate of transformation affect the structures formed? • How are mechanical properties affected?
Chapter 10: Phase Transformations Classes: 1. Simple diffusion-dependent transformation: no change in the number or composition of phases 2. Diffusion-dependent with changes in compositions and/or number of phases (e.g., eutectic reaction) 3. Diffusionless transformation – produces metastable phase • Impediments to PT: • Time-dependent diffusion, • Phase boundary energy • Processes: • Nucleation, • Grain growth
FRACTION OF TRANSFORMATION • Fraction transformed depends on time. • Transformation rate depends on T. • Problems with phase diagrams; - rates not shown - equilibrium cooling is too slow supercooling transformation occurs at a lower temperature
Super Cooling and Eutectoid Transformation Rate • Growth of pearlite from austenite: • Reaction rate increases with DT.
NUCLEATION AND GROWTH • Reaction rate is a result of nucleation and growth of crystals. • Examples:
ISOTHERMAL COOLING • Quickly cool to below TE and keep at constant temp • Fe-C system, Co = 0.77wt%C • Transformation at T = 675C.
Microstructural Development: Example • Eutectoid composition, Co = 0.77wt%C • Begin at T > 727C • Rapidly cool to 625C and hold isothermally.
Isothermal Transformation Diagrams or TTT (time-temp-transformation) plots • Above TE only • For 540<T<726C, • two solid lines: • - left of first line - only • - right of 2nd line – P only • - rate controlled by nucleation • - Coarse vs. fine pearlite: • - high temp: coarse P (faster diffusion thicker layers) • - low temp: fine pearlite • Below 540C • - Bainite formation • - rate is diffusion controlled • Below 215C - Martensite
Coarse vs. Fine Pearlite Two cases: • Ttransf just below TE --Larger T: diffusion is faster --Pearlite is coarser. • Ttransf well below TE --Smaller T: diffusion is slower --Pearlite is finer.
NON-EQUIL TRANSFORMATION PRODUCTS: Fe-C • Bainite: - elongated Fe3C particles in a continuous a matrix - diffusion controlled
Spheroidite • spherelike particles of Fe3C in a continuous ferrite matrix • heating pearlite or bainite to below 726 for a long time (e.g., 700C for 18-24 hours) • Additional diffusion: reduction in surface boundary
Martensite • Martensite: - metastable phase - polymorphic: g(FCC) to Martensite (BCT) • Isothermal Transf. Diagram • g to M transformation - diffusionless - rapid - athermal (independent of time).
Continuous Cooling Transformation (CCT) diagrams • delayed start and end • results depend on cooling rate • - moderately rapid – fine P • - slow – coarse pearlite • no bainite formation
Continuous Cooling • three regions: pure martensite, M+P, pure P • critical cooling rate – minimum rate that produces totally martensitic structure
Mechanical Properties • presence of other allowing elements – (e.g., Cr, Ni, Mo, and W) – delay pearlite formation stronger • other %C compositions – proeutectoid phase
Mechanical Properties • Fine Pearlite vs Martensite: • Hardness: fine pearlite << martensite.
Tempered Martensite • • heat martensitic steel below eutectoid (250-650C) • martensite (BCT - supersaturated) stable and Fe3C • similar to spheroidite but smaller cementite particles. • reduces brittleness of martensite.
