2材料科学基础英文版课件_(10)

Linear Defect DislocationsFeatures: one dimensional Classification and formation Edge dislocation and screw dislocationCompressive stressesTensile stressesEdge dislocationRepresented by symbol - positive dislocation; - negative Screw dislocationRepresentationOpen circles atom positions above the slip plane; solid circles atom positions below the slip planeAB dislocation lineMixed dislocation Burgers vector Used to indicate the direction and magnitude of the lattice distortion caused by a dislocation Denoted as b For edge dislocations, b is perpendicular to the dislocation line; for screw dislocations, b is parallel to the dislocation line; for mixed dislocations, b is neither perpendicular nor parallel to the dislocation line One dislocation just has one b For metals, b normally points in a close-packed crystal direction and its magnitude is the interatomic spacing because the slip direction is normally in the close-packed direction Edge dislocationScrew dislocationMixed dislocation Effects of dislocations on the properties of materials Play a crucial role in the plastic deformation of materials Dislocation strengthening is one of the major strengthening mechanisms for metallic materials The properties of LED depend considerably on the dislocation density in light emission materials such as GaN and SiC (the lower, the better) Observation of dislocationsDark lines - dislocationsPlane DefectsFeatures: two dimensional Outline External surfaces Grain boundaries Twin boundaries Stacking faults Phase boundariesExternal Surfaces Surface atoms are not bonded to the nearest neighbors above the surface, leading to a higher energy state, i.e., a surface energy To be stable, materials need to reduce the surface energy. To reduce the surface energy, the materials tend to minimize the total surface area Grain BoundariesIn polycrystalline materials, a grain boundary is the boundary between two adjacent grains which have different orientations Features of grain boundaries 23 atomic layers thick (0.51 nm) Within the boundary, there is some atomic mismatch and the density is lower, so the grain boundary is in a higher energy state, leading to a grain boundary energy To reduce the energy of the system, grains tend to grow to reduce the total grain boundary area. Degree of misorientation low angle boundary (15o). Low angle boundaries are composed of dislocationsTilt boundaryTwist boundaryCSL boundary (coincidence site lattice boundary)Special boundaryUse to represent the extent of CSLe.g., 3, 15, etc.Grain boundary segregation Equilibrium segregation Thermodynamic process Driving force: solute-boundary binding energy (the difference in energy caused by a solute atom between staying in the grain interior and on the grain boundary Non-equilibrium segregation Kinetic process Driving force: supersaturated point defect-solute complex concentration gradient between the grain centre and the boundary Classification of grain boundary segregationEffects of grain boundary segregation on the properties of materialsMechanical, corrosion, electrical, and magnetic propertiese.g. in structural materials, such as steel and Ni alloysSegregation of detrimental elements such as S, P, Sn, and Sb decreasing the grain boundary cohesion grain boundary weakeningSegregation of beneficial elements such as B, C and Be increasing the grain boundary cohesion grain boundary strentheningEquilibrium grain boundary segregation In solid solutions, solute atoms cause lattice distortion, leading to an increase in the energy of the systemGrain boundary area is porous and can accept solute atoms without causing apparent lattice distortionMigration of solute atoms from grain interiors to grain boundaries (segregation) can result in a decrease in the energy of the system (driving force). Therefore, thermodynamically, the segregation is a spontaneous processThe lattice near the solute atom is exerted by a compressive stressOversized soluteUndersized soluteThe lattice near the solute atom is exerted by a tensile stressSegregation thermodynamicsAt a certain temperature, T, there is an equilibrium segregation concentration, C(T)where G is the free energy of segregation (J/mol), R is the gas constant (8.314 J/(mol.K), T is the absolute temperature, and Cg is the bulk concentrationTo reach this equilibrium segregation level, the time is needed. The segregation kinetics is given by where Ds is the solute diffusion coefficient, d is the boundary thickness, and is the enrichment ratio Boundary concentrationTimeEquilibrium segregation kinetic curveGrain boundaryGrainboundaryGrain GrainGrainExcessvacanciesComplex concentration gradient between the grain center and the boundaryNon-equilibrium segregation mechanismSupersaturatedcomplexesComplexesdecomposeVacancyannihilationSoluteatomsThermal equilibriumvacanciesThermal equilibriumvacanciesComplex diffusion to the boundaryNon-equilibrium grain boundar