材料科学基础-线缺陷位错教学课件

1、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 sli

2、p 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 dislo

3、cations, 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 direc

4、tion 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 mat

5、erials 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 St

6、acking 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 th

7、e total surface area Grain Boundaries In 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

8、, 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 boundaryTw

9、ist 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 betw

10、een 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 segregation Effects of grain boundary se

11、gregation on the properties of materials Mechanical, 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 benef

12、icial elements such as B, C and Be increasing the grain boundary cohesion grain boundary strenthening Equilibrium grain boundary segregation In solid solutions, solute atoms cause lattice distortion, leading to an increase in the energy of the system Grain boundary area is porous and can accept solu

13、te atoms without causing apparent lattice distortion Migration 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 solut

14、e atom is exerted by a compressive stressOversized soluteUndersized soluteThe lattice near the solute atom is exerted by a tensile stress Segregation thermodynamics At a certain temperature, T, there is an equilibrium segregation concentration, C(T)exp()(1)(gRTGCTCTCwhere G is the free energy of seg

15、regation (J/mol), R is the gas constant (8.314 J/(mol.K), T is the absolute temperature, and Cg is the bulk concentration To reach this equilibrium segregation level, the time is needed. The segregation kinetics is given by )2(erfc)4exp(1)()(s22sggbdtDdtDCTCCtCwhere Ds is the solute diffusion coeffi

16、cient, d is the boundary thickness, and is the enrichment ratio Boundary concentrationTimeEquilibrium segregation kinetic curveGrain boundaryGrain boundaryGrainGrainGrainExcessvacanciesComplex concentration gradient between the grain center and the boundaryNon-equilibrium segregation mechanismSupers

17、aturatedcomplexesComplexesdecomposeVacancyannihilationSoluteatomsThermal equilibriumvacanciesThermal equilibriumvacanciesComplex diffusion to the boundaryNon-equilibrium grain boundary segregation solute atomsSolute concentrationDistance from the boundarymatrix levelNESpoint defect-solute complexesE

18、SSegregation mechanismsFor diffusion of complexes, see the paper: S.-H. Song and L.-Q. Weng, Materials Science and Technology 21 (2005) 305. Sub-classification of non-equilibrium segregation Thermal non-equilibrium segregation (T.-D. Xu and S.-H. Song, Acta Metallurgica 37 (1989) 2499.) Radiation-in

19、duced non-equilibrium segregation (R.G. Faulkner, S.-H. Song, P.E.J. Flewitt et al., Journal of Nuclear Materials 255 (1998) 189.) High temperature plastic deformation-induced non-equilibrium segregation (S.-H. Song, Q. Zhang and L.-Q. Weng, Materials Science and Engineering A 473 (2008) 226.)Twin B

20、oundaries A special type of grain boundary across which there is a mirror lattice symmetry: atoms on one side of the boundary are located at mirror image positions of the atoms on the other side. The region of material between the boundaries is termed a twinCoherent low boundary energyStacking Fault

21、s In close-packed crystal structures, fcc and hcp, the stacking sequence of the close-packed plane (111)-type for fcc and (0001)-type for hcp) is ABCABC. for fcc structures and ABABAB. for hcp structures FCC structure (111)-type FCC structure (111)-type HCP structure (0001)-typeStacking Faults If th

22、ere is an interruption in the stacking sequence, such as ABCACABC for fcc or ABAABAB for hcpStacking faultStacking faultPhase Boundaries Boundaries between different phases Phase Boundaries Types of phase boundaries Coherent: lattice sites on the boundary are shared by both phases. Normally, the sec

23、ond phase is unstable Partially coherent (or semi-coherent): lattice sites on the boundary are partially shared by both phases. Normally, the second phase is also unstable Incoherent: lattice sites on the boundary are not shared by both phases. Normally, the second phase is stable (equilibrium phase)GP