2材料科学基础英文版课件_(10).ppt

linear defect dislocations features: one dimensional classification and formation edge dislocation and screw dislocation compressive stresses tensile stresses edge dislocation represented by symbol - positive dislocation; - negative screw dislocation representation open circles atom positions above the slip plane; solid circles atom positions below the slip plane ab dislocation line mixed 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 dislocation screw dislocation mixed 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 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 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 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, 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 dislocations tilt boundary twist boundary csl boundary (coincidence site lattice boundary) special boundary use to represent the extent of csl 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 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 segregation on the properties of materials mechanical, corrosion, electrical, and magnetic properties e.g. in 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 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 solute 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 process the lattice near the solute atom is exerted by a compressive stress oversized solute undersized solute the 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) 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 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 boundary thickness, and is the enrichment ratio boundary concentration time equilibrium segregation kinetic curve grain boundary grain boundary grain grain grain excess vacancies complex concentration gradient between the grain center and the boundary non-equilibrium segregation mechanism supersaturated complexes complexes decompose vacancy annihilation solute atoms thermal equilibrium vacancies thermal equilibrium vacancies complex diffusion to the boundary non-equilibrium grain boundary segregation solute atoms solute concentration distance from the boundary matrix level nes point defect-solute complexes es 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.) radiation-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) 226.) 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 coherent low boundary energy stacking faults 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)-type stacking faults if there is an interruption in the stacking sequence, such as abcacabc for fcc or abaabab for hcp stacking faultstacking 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 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 z