Chapter 10 notes

Chapter 10 notes - C hapter 10 – Phase T ransformations...

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Unformatted text preview: C hapter 10 – Phase T ransformations How does rate of t ransformation depend on time and temperature? Is i t possible to slow down t ransformations so that non equilibrium structures are formed? Nucleation • Nuclei seeds act as templates on which crystals grow • For nucleus to form rate of addition of atoms to nucleus must be faster t han rate of loss • Once nucleated, growth proceeds until equilibrium is attained Driving force to nucleate increases as we increase temperature • Supercooling – eutectic, eutectoid o Small supercooling->slow nucleation rate – few nuclei – large c rystals o Large supercooling->rapid nucleation rate – many nuclei – small c rystals • Superheating – peri tectic Solidification Nucleation Types Homogeneous nucleation • Nuclei form in the bulk of liquid metal • Requires considerable supercooling Heterogeneous nucleation • Much easier since stable nucleating surface is already present • Only every slight supercooling Kinetics – t he study of reaction rates of phase t ransformations to determine reaction rate – measure M a r tensite • Gamma(FCC) to Martensite (BCT) • ...
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