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1、Linear Defect DislocationsFeatures: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 vectorUsed to indicat
2、e the direction and magnitude of the lattice distortion caused by a dislocationDenoted as bFor 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 l
3、ineOne dislocation just has one bFor 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 prop
4、erties of materialsPlay a crucial role in the plastic deformation of materialsDislocation strengthening is one of the major strengthening mechanisms for metallic materialsThe properties of LED depend considerably on the dislocation density in light emission materials such as GaN and SiC(the lower,th
5、e better),Observation of dislocations,Dark lines-dislocations,Plane DefectsFeatures:two dimensional,Outline,External surfacesGrain boundariesTwin boundariesStacking faultsPhase boundaries,External Surfaces,Surface atoms are not bonded to the nearest neighbors above the surface,leading to a higher en
6、ergy 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
7、 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 energyTo reduce the energy of the system,grains tend to grow to reduce th
8、e 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 CSLe.g.,3,15,etc.,Grain boundary segregation,Equi
9、librium segregationThermodynamic processDriving force:solute-boundary binding energy(the difference in energy caused by a solute atom between staying in the grain interior and on the grain boundaryNon-equilibrium segregationKinetic processDriving force:supersaturated point defect-solute complex conc
10、entration 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 alloysSegregation of
11、 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 strenthening,Equilibrium grain boundary segregation,In solid solutions,solute atoms c
12、ause 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
13、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 free energy of segregation(J/mol),R is the gas constant(8.314 J/(mol.K),T is the abso
14、lute 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,Equilibrium segregation kinetic curve,Grain bound
15、ary,Grain boundary,Grain,Grain,Grain,Non-equilibrium segregation mechanism,Complexesdecompose,Vacancyannihilation,Thermal equilibriumvacancies,Thermal equilibriumvacancies,Non-equilibrium grain boundary segregation,Segregation mechanisms,For diffusion of complexes,see the paper:S.-H.Song and L.-Q.We
16、ng,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 Ma
17、terials 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
18、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-packed crystal structures,fcc and hcp,the stacking sequence of the close-packed plane(111)-type for fcc and(0001)-type for hcp)is ABCA
19、BC.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 fault,Stacking fault,Phase Boundaries,Boundaries betw
20、een different phases,Phase Boundaries,Types of phase boundariesCoherent: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 al
21、so unstable Incoherent:lattice sites on the boundary are not shared by both phases.Normally,the second phase is stable(equilibrium phase),GP zone Cu 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 prec
22、ipitation process of-Al2Cu in Al-Cu alloy with increasing ageing time:GP zone(instable,coherent)”(instable,coherent)(instable,partially 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 referencesInformation search through websites,academic papersThe 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),
