tailieunhanh - Kinetics of Materials - R. Balluff_ S. Allen_ W. Carter (Wiley_ 2005) Episode 8

Tham khảo tài liệu 'kinetics of materials - r. balluff_ s. allen_ w. carter (wiley_ 2005) episode 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | EXERCISES 299 A crystal growing from the vapor phase possesses a singular surface with two screw dislocations intersecting it. The dislocations are very close to one another relative to the dimensions of the surface and have opposite Burgers vectors. Describe the form of the step structure that is produced because of the presence of the two dislocations. Solution. Before any growth the two dislocations are associated with steps that may be as indicated in Fig. . During growth each dislocation rotates about its point of intersection to produce a spiral step as in Fig. . However the spirals will rotate in opposite directions and sections will annihilate one another when they meet as in a and b . The process will then continue as in c - f generating a potentially unlimited series of concentric steps. Figure Step structure generated on growing crystal surface at the intersections of two screw dislocations with opposite Burgers vectors. Suppose that the Burgers vectors of the two screw dislocations in Exercise are now the same. Describe the ledge structure that is produced. Solution. Start with the ledge structure in Fig. . The growth spirals of the two dislocations now rotate in the same direction as indicated. This will generate an interleaved double growth spiral of potentially unlimited extent as illustrated in Fig. . Figure Step structures generated on growing crystal surface at the intersections of two screw dislocations with the same Burgers vectors. Consider a ledge on a surface which on average lies parallel to a closely spaced row of atoms. The ledge contains N sites with spacing a where either a positive or a negative kink can exist. When traveling along a ledge positive and negative kinks displace the ledge in opposite directions. Show that the total number of kinks present in thermal equilibrium is p 2e-G fcT 12 34 300 CHAPTER 12 MOTION OF CRYSTALLINE SURFACES where is the kink free energy of formation .

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