材料科学基础英文学习课程

1、 Classification and formation Edge dislocation and screw dislocationCompressive stressesTensile stressesEdge dislocationRepresented by symbol - positive dislocation; - negative 第1页/共47页第一页，编辑于星期日：二十一点 三十二分。Screw dislocationRepresentation第2页/共47页第二页，编辑于星期日：二十一点 三十二分。Open circles atom positions above

2、the slip plane; solid circles atom positions below the slip planeAB dislocation line第3页/共47页第三页，编辑于星期日：二十一点 三十二分。Mixed dislocation第4页/共47页第四页，编辑于星期日：二十一点 三十二分。第5页/共47页第五页，编辑于星期日：二十一点 三十二分。 Burgers vector Used to indicate the direction and magnitude of the lattice distortion caused by a dislocation D

3、enoted 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 clos

4、e-packed crystal direction and its magnitude is the interatomic spacing because the slip direction is normally in the close-packed direction 第6页/共47页第六页，编辑于星期日：二十一点 三十二分。Edge dislocation第7页/共47页第七页，编辑于星期日：二十一点 三十二分。Screw dislocation第8页/共47页第八页，编辑于星期日：二十一点 三十二分。Mixed dislocation第9页/共47页第九页，编辑于星期日：二十一

5、点 三十二分。 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 emiss

6、ion materials such as GaN and SiC (the lower, the better)第10页/共47页第十页，编辑于星期日：二十一点 三十二分。 Observation of dislocationsDark lines - dislocations第11页/共47页第十一页，编辑于星期日：二十一点 三十二分。Plane DefectsFeatures: two dimensional 第12页/共47页第十二页，编辑于星期日：二十一点 三十二分。Outline External surfaces Grain boundaries Twin boundaries

7、Stacking faults Phase boundaries第13页/共47页第十三页，编辑于星期日：二十一点 三十二分。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,

8、the materials tend to minimize the total surface area 第14页/共47页第十四页，编辑于星期日：二十一点 三十二分。Grain Boundaries In polycrystalline materials, a grain boundary is the boundary between two adjacent grains which have different orientations第15页/共47页第十五页，编辑于星期日：二十一点 三十二分。 Features of grain boundaries 23 atomic lay

9、ers 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 misorienta

10、tion low angle boundary (15o). Low angle boundaries are composed of dislocations第16页/共47页第十六页，编辑于星期日：二十一点 三十二分。Tilt boundary第17页/共47页第十七页，编辑于星期日：二十一点 三十二分。Twist boundary第18页/共47页第十八页，编辑于星期日：二十一点 三十二分。CSL boundary (coincidence site lattice boundary)Special boundaryUse to represent the extent of CSLe.

11、g., 3, 15, etc.第19页/共47页第十九页，编辑于星期日：二十一点 三十二分。Grain boundary segregation第20页/共47页第二十页，编辑于星期日：二十一点 三十二分。第21页/共47页第二十一页，编辑于星期日：二十一点 三十二分。 Equilibrium segregation Thermodynamic process Driving force: solute-boundary binding energy (the difference in energy caused by a solute atom between staying in the

12、 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第22页/共47页第二十二页，编辑于星期日：二十一点 三十二分。 Effects of gra

13、in boundary segregation 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 weakeningSegreg

14、ation of beneficial elements such as B, C and Be increasing the grain boundary cohesion grain boundary strenthening第23页/共47页第二十三页，编辑于星期日：二十一点 三十二分。 Equilibrium grain boundary segregation In solid solutions, solute atoms cause lattice distortion, leading to an increase in the energy of the system Gra

15、in 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

16、 spontaneous process第24页/共47页第二十四页，编辑于星期日：二十一点 三十二分。The lattice near the solute atom is exerted by a compressive stressOversized soluteUndersized soluteThe lattice near the solute atom is exerted by a tensile stress第25页/共47页第二十五页，编辑于星期日：二十一点 三十二分。 Segregation thermodynamics At a certain temperature,

17、 T, there is an equilibrium segregation concentration, C(T)exp()(1)(gRTGCTCTCwhere 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

18、 segregation kinetics is given by )2(erfc)4exp(1)()(s22sggbdtDdtDCTCCtCwhere Ds is the solute diffusion coefficient, d is the boundary thickness, and is the enrichment ratio 第26页/共47页第二十六页，编辑于星期日：二十一点 三十二分。Boundary concentrationTimeEquilibrium segregation kinetic curve第27页/共47页第二十七页，编辑于星期日：二十一点 三十二分

19、。Grain boundaryGrain boundaryGrainGrainGrainExcessvacanciesComplex concentration gradient between the grain center and the boundaryNon-equilibrium segregation mechanismSupersaturatedcomplexesComplexesdecomposeVacancyannihilationSoluteatomsThermal equilibriumvacanciesThermal equilibriumvacanciesCompl

20、ex diffusion to the boundaryNon-equilibrium grain boundary segregation 第28页/共47页第二十八页，编辑于星期日：二十一点 三十二分。solute atomsSolute concentrationDistance from the boundarymatrix levelNESpoint defect-solute complexesESSegregation mechanismsFor diffusion of complexes, see the paper: S.-H. Song and L.-Q. Weng, M

21、aterials Science and Technology 21 (2005) 305. 第29页/共47页第二十九页，编辑于星期日：二十一点 三十二分。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 (255 (1998) 189.) High tempe

22、rature plastic deformation-induced non-equilibrium segregation (S.-H. Song, Q. Zhang and L.-Q. Weng, Materials Science and Engineering A 473 (2008) 226.)第30页/共47页第三十页，编辑于星期日：二十一点 三十二分。Twin Boundaries A special type of grain boundary across which there is a mirror lattice symmetry: atoms on one side

23、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 energy第31页/共47页第三十一页，编辑于星期日：二十一点 三十二分。Stacking Faults In close-packed crystal structures, fcc and hcp, the stacking sequence of th

24、e close-packed plane (111)-type for fcc and (0001)-type for hcp) is ABCABC. for fcc structures and ABABAB. for hcp structures 第32页/共47页第三十二页，编辑于星期日：二十一点 三十二分。 FCC structure (111)-type第33页/共47页第三十三页，编辑于星期日：二十一点 三十二分。 FCC structure (111)-type第34页/共47页第三十四页，编辑于星期日：二十一点 三十二分。 HCP structure (0001)-type第3

25、5页/共47页第三十五页，编辑于星期日：二十一点 三十二分。Stacking Faults If there is an interruption in the stacking sequence, such as ABCACABC for fcc or ABAABAB for hcpStacking faultStacking fault第36页/共47页第三十六页，编辑于星期日：二十一点 三十二分。Phase Boundaries Boundaries between different phases 第37页/共47页第三十七页，编辑于星期日：二十一点 三十二分。Phase Bounda

26、ries 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: l

27、attice sites on the boundary are not shared by both phases. Normally, the second phase is stable (equilibrium phase)第38页/共47页第三十八页，编辑于星期日：二十一点 三十二分。第39页/共47页第三十九页，编辑于星期日：二十一点 三十二分。第40页/共47页第四十页，编辑于星期日：二十一点 三十二分。第41页/共47页第四十一页，编辑于星期日：二十一点 三十二分。GP zone Cu enriched area, disc shaped, coherent with the matrix (i