材料科学基础线缺陷位错

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
– To reduce the energy of the system, grains tend to grow to reduce the total grain boundary area.
– Degree of misorientation – low angle boundary (<10o) and high angle boundary (>15o). Low angle boundaries are composed of dislocations
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.
Linear Defect – Dislocations
Features: one dimensional
• Classification and formation
– Edge dislocation and screw dislocation
Edge dislocation
Compressive stresses
– 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
Tensile stresses
Represented by symbol - positive dislocation; - negative
Screw dislocation
Representation
AB – dislocation line
Open circles – atom positions above the slip plane; solid circles – atom positions below the slip plane
Tilt boundary
Twist boundary
Use to represent the extent of CSL e.g., 3, 15, etc.
• 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
Edge dislocation
Screw dislocation
ቤተ መጻሕፍቲ ባይዱixed dislocation
• Effects of dislocations on the properties of materials
– Play a crucial role in the plastic deformation of materials
• Features of grain boundaries
– 2~3 atomic layers thick (0.5~1 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
– 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