材料科学与工程导论双语课件教学课件 ppt 作者 陈克正 王玮 刘春延 等编Chapter 2.ppt

* * * * * * * * * * * * * * HCP结构的配位数=12 * Suppose that the edge length of the unit cell is a and the radius of an atom is R. Then how to calculate Vs and Vc? * * * * * * * * * * * * * * * * * * * * * * * * Solid solution Substitutional solid solution Atomic radii: 0.128 0.125 nm Structures: FCC Electronegativities: 1.9 1.8 Cu Ni Substitutional alloy (e.g., Cu in Ni) Interstitial alloy (e.g., C in Fe) Solid solution Interstitial solid solution the maximum concentration of interstitial impurity atoms is low (less than 10%). even very small impurity atoms are larger than the interstitial voids. ? a linear or 1D defect around which some of the atoms are misaligned ? 2 types 2.3.2 Dislocation—— Linear Defects Edge dislocation caused by an extra half-plane of atoms Screw dislocation caused by shear stress 2.3.3 Interfacial Defects Separate regions that have different crystal structures and/or orientations External Surfaces Grain Boundaries Twin Boundaries Others Mirror lattice symmetry Surfaces atoms are not bonded to the maximum # of nearest neighbors Boundaries between grains Stacking faults Phase boundaries Ferromagnetic domain walls External Surfaces Compared with the atoms below the surface area, external surface atoms are much more active. Surface energy —— excess energy compared with that of the planes below surface area In order to reduce the surface energy, materials tend to minimize the total surface area. Grain Boundaries Atoms are bonded irregularly, thus highly active Accompanied with rain boundary energy Twin Boundaries A special type of grain boundary Atoms on one side of the boundary are located in mirror image positions of the atoms on the other side Miscellaneous Interfacial Defects Stacking faults: an interruption in the stacking sequence of crystal planes Phase boundaries: exist in multiphase materials across which there is a sudden change in physical and/or chemical characteristics. ? Metallic crystal structure types ? How to Index a