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1、Linear Defect Dislocations Features: one dimensional,Classification and formation Edge dislocation and screw dislocation,Edge dislocation,Screw dislocation,Open circles atom positions above the slip plane; solid circles atom positions below the slip plane,Mixed dislocation,Burgers vector Used to ind
2、icate 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 di
3、slocation 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 dislocation,Screw dislocation,Mixed dislocation,Effects of dislocatio
4、ns 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
5、 SiC (the lower, the better),Observation of dislocations,Dark lines - dislocations,Plane Defects Features: two dimensional,Outline,External surfaces Grain boundaries Twin boundaries Stacking faults Phase boundaries,External Surfaces,Surface atoms are not bonded to the nearest neighbors above the sur
6、face, 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 Boundaries,In polycrystalline materials, a grain boundary is the boundary between two ad
7、jacent 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
8、 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 dislocations,Tilt boundary,Twist boundary,CSL boundary (coincidence site lattice boundary) Special boundary,Use to represent the extent of CSL
9、 e.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 Driv
10、ing force: supersaturated point defect-solute complex concentration gradient between the grain centre and the boundary,Classification of grain boundary segregation,Effects of grain boundary segregation on the properties of materials Mechanical, corrosion, electrical, and magnetic properties,e.g. in
11、structural materials, such as steel and Ni alloys Segregation of detrimental elements such as S, P, Sn, and Sb decreasing the grain boundary cohesion grain boundary weakening Segregation of beneficial elements such as B, C and Be increasing the grain boundary cohesion grain boundary strenthening,Equ
12、ilibrium 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 solute atoms without causing apparent lattice distortion Migration of solute atoms from grain interiors to gr
13、ain boundaries (segregation) can result in a decrease in the energy of the system (driving force). Therefore, thermodynamically, the segregation is a spontaneous process,Segregation thermodynamics At a certain temperature, T, there is an equilibrium segregation concentration, C(T),where G is the fre
14、e 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 concentration,To 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 bounda
15、ry thickness, and is the enrichment ratio,Equilibrium segregation kinetic curve,Grain boundary,Grain boundary,Grain,Grain,Grain,Non-equilibrium segregation mechanism,Complexes decompose,Vacancy annihilation,Thermal equilibrium vacancies,Thermal equilibrium vacancies,Non-equilibrium grain boundary se
16、gregation,Segregation mechanisms,For 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.) Ra
17、diation-induced 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) 2
18、26.),Twin Boundaries,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 twin,Stacking Faults,In close-packe
19、d 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)-type,Stacking Faults,If there is an interr
20、uption in the stacking sequence, such as ABCACABC for fcc or ABAABAB for hcp,Stacking fault,Stacking fault,Phase Boundaries,Boundaries between different phases,Phase Boundaries,Types of phase boundaries Coherent: lattice sites on the boundary are shared by both phases. Normally, the second phase is
21、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 zone Cu
22、enriched area, disc shaped, coherent with the matrix (in 1938, Guinier and Preston discovered this by X-ray diffraction, hence called GP zone.,e.g., the precipitation process of -Al2Cu in Al-Cu alloy with increasing ageing time: GP zone (instable, coherent) ”(instable, coherent) (instable, partially
23、 coherent) (stable, incoherent),GP zones,” phase, phase, phase,Course Work - Report,Report titles: Any title about materials science and engineering The report must be written in English and have about 5000 words or more with more than 10 references Information search through websites, academic papers The report must be submitted by the end of this course,Prepare your PPT presentation (no lecture this Friday) and present your work next Friday (3 December 2010),
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