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MME 340 PHASE TRANSFORMATIONS IN MATERIALS ANANDH SUBRAMANIAM Materials Science and Engineering INDIAN INSTITUTE OF TECHNOLOGY KANPUR New Delhi- 110016 Ph: (+91) (512) 259 7215, Fax: (+91) (512) 259 7505 [email protected] http://home.iitk.ac.in/~anandh/ UNIVERSE STRONG WEAK ELECTROMAGNETIC HYPERBOLIC EUCLIDEAN SPHERICAL ENERGY SPACE GRAVITY nD + t FIELDS PARTICLES METAL SEMI-METAL BAND STRUCTURE NON-ATOMIC ATOMIC SEMI-CONDUCTOR INSULATOR STATE / VISCOSITY GAS SOLID LIQUID CRYSTALS LIQUID STRUCTURE AMORPHOUS QUASICRYSTALS RATIONAL APPROXIMANTS CRYSTALS SIZE NANO-QUASICRYSTALS NANOCRYSTALS Thermo-mechanical Treatments Crystal Atom Structure Electromagnetic • Casting • Metal Forming • Welding • Powder Processing • Machining Microstructure Phases Component + Defects • Vacancies • Dislocations • Twins + • Stacking Faults • Grain Boundaries • Voids • Cracks Processing determines shape and microstructure of a component Residual Stress Transformations in Materials Phases Defects Residual stress Phases can transform Defect structures can change Stress state can be altered Phase transformation Defect structure transformation Stress-state transformation Geometrical Physical Structural Property Phases Phases Transformations Microstructure Microstructural Transformations Phase Defined Using GEOMETRICAL Entities E.g. Atoms, Cluster of Atoms Ions, etc. PHYSICAL Properties E.g. Spin, Magnetic Moment ORDER ORIENTATIONAL PROBABILISTIC OCCUPATION POSITIONAL Physical Sources of Residual Stress Property • Thermal • Magnetic • Ferroelectric Geometrical Structural Phase transformations & reactions • Volume, Shape change Defects The product of phase transformation or reaction would have a different morphology as compared to the parent phase • Coherent/semi-coherent precipitates • Epitaxial structures (coherent/semi-coherent) • Vacancies • Dislocations • Twins • Stacking Faults • Grain Boundaries • Voids • Cracks Thermal Sources of Residual Stress Physical property • Thermal • Magnetic • Ferroelectric Structural Geometric structure Phase transformations & reactions Defects • Coherent/semi-coherent precipitates • Epitaxial structures (coherent/semi-coherent) • Vacancies • Dislocations • Twins • Stacking Faults • Grain Boundaries • Voids • Cracks Phase Transition Transitions between two phases of identical chemical composition Phase Transformation Covers a wider spectrum of phenomena Includes PHASE REACTIONS leading to compositional changes E.g.: ’ → + (different crystal structure & chemical composition) → + Note: Identical chemical composition equilibrium point defect concentration may be different in the polymorphs (but considered to be of identical composition) Some Questions? What is a Phase? What kind of phases exist? What constitutes a transformation? How can we cause a phase transformation to occur? The stimuli: P, T, Magnetic field, Electric field What kind of phase transformations are there? Why does a phase transformation occur? Energy considerations of the system? Thermodynamic potentials (G, A…) Length Scales Involved Nucleus Atom Crystal Defects Microstructure Component
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