1Introduction to Materials Science, Chapter 10, Phase Transformations in MetalsUniversity of Tennessee, Dept. of Materials Science and Engineering1Heat Treatment (time and temperature) ⇒⇒ Microstructure ⇒ Mechanical PropertiesChapter Outline: Phase Transformations in Metals¾ Kinetics of phase transformations¾ Multiphase Transformations¾ Phase transformations in Fe-C alloys¾ Isothermal Transformation Diagrams¾ Mechanical Behavior ¾ Tempered Martensite Not tested: 10.6 Continuous Cooling Transformation DiagramsIntroduction to Materials Science, Chapter 10, Phase Transformations in MetalsUniversity of Tennessee, Dept. of Materials Science and Engineering2Phase transformations (change of the microstructure) can be divided into three categories:Phase transformations. Kinetics.¾ Diffusion-dependent with no change in phase composition or number of phases present (e.g. melting, solidification of pure metal, allotropic transformations, recrystallization, etc.)¾ Diffusion-dependent with changes in phase compositions and/or number of phases (e.g. eutectoid transformations)¾ Diffusionless phase transformation - produces a metastable phase by cooperative small displacements of all atoms in structure (e.g. martensitic transformation discussed in later in this chapter)Phase transformations do not occur instantaneously. Diffusion-dependent phase transformations can be rather slow and the final structure often depend on the rate of cooling/heating. We need to consider the time dependence or kinetics of the phase transformations.2Introduction to Materials Science, Chapter 10, Phase Transformations in MetalsUniversity of Tennessee, Dept. of Materials Science and Engineering3Most phase transformations involve change in composition ⇒ redistribution of atoms via diffusion is required.The process of phase transformation involves:Kinetics of phase transformations¾ Nucleation of of the new phase - formation of stable small particles (nuclei) of the new phase. Nuclei are often formed at grain boundaries and other defects.¾ Growth of new phase at the expense of the original phase.S-shape curve: percent of material transformed vs. the logarithm of time. Introduction to Materials Science, Chapter 10, Phase Transformations in MetalsUniversity of Tennessee, Dept. of Materials Science and Engineering4A nucleus is only stable if further growth reduces the energy of the system. For r > rcthe nucleus is stable.Nucleation3Introduction to Materials Science, Chapter 10, Phase Transformations in MetalsUniversity of Tennessee, Dept. of Materials Science and Engineering5Rate of phase transformationsRate of transformation can be defined as reciprocal of time for transformation to proceed halfway to completion:r = 1 / t0.5Rate increases with temperature according to Arrhenius equation, characteristic for thermally activated processes:r = A exp (-QA/kT) = A exp (-Qm/ RT)Per atom Per molePercent recrystallization of pure copper at different TIntroduction to Materials Science, Chapter 10, Phase Transformations in MetalsUniversity of Tennessee, Dept. of Materials Science and Engineering6Superheating / supercooling¾ Upon crossing a phase boundary on the composition-temperature phase diagram phase transformation towards equilibrium state is induced.¾ But the transition to the equilibrium structure takes time and transformation is delayed. ¾ During cooling, transformations occur at temperatures less than predicted by phase diagram: supercooling.¾ During heating, transformations occur at temperatures greater than predicted by phase diagram: superheating.¾ Degree of supercooling/superheating increases with rate of cooling/heating.¾ Metastable states can be formed as a result of fast temperature change. Microstructure is strongly affected by the rate of cooling.¾ Below we will consider the effect of time on phase transformations using iron-carbon alloy as an example.4Introduction to Materials Science, Chapter 10, Phase Transformations in MetalsUniversity of Tennessee, Dept. of Materials Science and Engineering7Let us consider eutectoid reaction as an exampleeutectoid reaction: γ(0.76 wt% C)↓α (0.022 wt% C)+ Fe3CThe S-shaped curves are shifted to longer times at higher T showing that the transformation is dominated by nucleation (nucleation rate increases with supercooling) and not by diffusion (which occurs faster at higher T).Introduction to Materials Science, Chapter 10, Phase Transformations in MetalsUniversity of Tennessee, Dept. of Materials Science and Engineering8Isothermal Transformation (or TTT) Diagrams(Temperature, Time, and % Transformation)5Introduction to Materials Science, Chapter 10, Phase Transformations in MetalsUniversity of Tennessee, Dept. of Materials Science and Engineering9TTT DiagramsThe thickness of the ferrite and cementite layers in pearlite is ~ 8:1. The absolute layer thickness depends on the temperature of the transformation. The higher the temperature, the thicker the layers.Fine pearliteAustenite → pearlitetransformationα ferriteCoarse pearliteFe3CAustenite (stable)Denotes that a transformationis occurringEutectoidtemperatureIntroduction to Materials Science, Chapter 10, Phase Transformations in MetalsUniversity of Tennessee, Dept. of Materials Science and Engineering10TTT Diagrams¾ The family of S-shaped curves at different T are used to construct the TTT diagrams.¾ The TTT diagrams are for the isothermal (constant T) transformations (material is cooled quickly to a given temperature before the transformation occurs, and then keep it at that temperature).¾ At low temperatures, the transformation occurs sooner (it is controlled by the rate of nucleation) and grain growth (that is controlled by diffusion) is reduced.¾ Slow diffusion at low temperatures leads to fine-grained microstructure with thin-layered structure of pearlite (fine pearlite). ¾ At higher temperatures, high diffusion rates allow for larger grain growth and formation of thick layered structure of pearlite (coarse pearlite).¾ At compositions other than eutectoid, a proeutectoid phase (ferrite or cementite) coexist with pearlite. Additional curves for proeutectoid transformation must be included on TTT diagrams.6Introduction to Materials Science, Chapter 10, Phase Transformations in MetalsUniversity of Tennessee, Dept. of Materials Science and Engineering11Formation of Bainite Microstructure (I)If transformation temperature is low enough (≤540°C)bainite rather than fine