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ASM Metals Handbook - Desk Edition (ASM_ 1998) WW part 2

Tham khảo tài liệu 'asm metals handbook - desk edition (asm_ 1998) ww part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | dislocation at the lower edge of the incomplete plane of atoms. Interstitial atoms usually cluster in regions where tensile stresses make more room for them as in the lower central part of Fig. 11. o o o o o o o 0 o o o 0 o o o 0 o 0 o o o o o o o o 4 o o o o o o o o o o o 0 0 o 0 Q o o Qa p of 0 0 0-1-0 o o C-t-O 0 0 o o o 0 o o 0 o olo o o o o o o o o 0 1 0 Q o 0 o o o o o 0 o 0 o 0 o 0 0 0 o o o 0 o o o Fig. 11 Crystal containing an edge dislocation indicating qualitatively the stress shown by the direction of the arrows at four positions around the dislocation Individual crystal grains which have different lattice orientations are separated by large-angle boundaries grain boundaries . In addition the individual grains are separated by small-angle boundaries subboundaries into subgrains that differ very little in orientation. These subboundaries may be considered as arrays of dislocations tilt boundaries are arrays of edge dislocations twist boundaries are arrays of screw dislocations. A tilt boundary is represented in Fig. 12 by the series of edge dislocations in a vertical row. Compared with large-angle boundaries small-angle boundaries are less severe defects obstruct plastic flow less and are less effective as regions for chemical attack and segregation of alloying constituents. In general mixed types of grain-boundary defects are common. All grain boundaries are sinks into which vacancies and dislocations can disappear and may also serve as sources of these defects they are important factors in creep deformation. nũ oo Ũ C O 00 000 0 0 OOOoS OOOOOO OOOOOq r- OOo092.2 0oOon nOOOOOOOOooooo o o o o 0 o Ooooooj oooooo Oofoooo oo QO 2 n oSS o o ooooooOQ oOQ0000000qq o o o o Fig. 12 Small-angle boundary subboundary of the tilt type which consists of a vertical array of edge dislocations Stacking faults are two-dimensional defects that are planes where there is an error in the normal sequence of stacking of atom layers. Stacking faults may be formed during the .