Chap2_ Plastic deformation_Dislocation - Chap 2 Plastic Relationship between atomic structure and plastic behavior of materials Much of the fundamental

Chap2_ Plastic deformation_Dislocation - Chap 2 Plastic...

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Chap. 2 Plastic Deformation- Dislocation
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Relationship between atomic structure and plastic behavior of materials Much of the fundamental work on the plastic deformation of metals are performed with single crystal to eliminate the effect of grain boundary and restrains imposed by neighboring grains and second phase particles Plastic deformation and dislocation theory
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Crystal Geometry 3 Simple Cubic Structure Found in ionic crystals (NaCl) but not in metal
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Metals have either BCC (body centered cubic), FCC (face centered cubic) crystal structure or HCP (Hexagonal closed packed structure) 4
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FCC and HCP are closed packed structure 74% of the volume is occupied by atoms In contrast for: BCC ( 68 % Volume occupied by atoms) Simple Cubic Cell (52%) 7
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Plastic deformation is generally confined to the low index planes, which have a higher density of atoms than the high-index plane 9
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Deformation by Slip The usual method of plastic deformation of metals is by sliding of block of crystal over one another along defined crystallographic planes 10
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Slip occurs in specific direction on certain planes The slip plane is the plane of greatest atomic density and the slip direction is the closest packed direction within the slip plane Slip system is together the slip plane and the slip direction 13
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In HCP structure, there are 3 slip systems Limited number of slip systems is the raison for the extreme orientation dependence and low ductility in hcp crystals In FCC structure, { 1 1 1} and <1 1 0> are the closed packed systems. 4 sets of { 1 1 1} planes each contains three <1 1 0> directions. Therefore, 12 possible slip systems 14
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BCC is not a closed packed structure like FCC or HCP There is no one plane of predominant atomic density In BCC, slip is found to occur on the {110}, {112}, {123} planes and always in the closed packed <111> which is common to each of these planes Dislocation can readily move from one type of plane to another by cross sip giving rise to the irregular wavy slip bands 16
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Certain metals show additional slip systems with increased temperature Al deform on {110} plane at elevated temperature while in magnesium plays an important role in deformation slip above 225 Celsius. In all cases, the slip direction remains the same when the slip planes changes with temperature 17
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Slip in perfect lattice 18
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Shear stress and displacement can be estimated by: Hook’s law at small value of displacement 19
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20 Shear modulus for metals is in the range 20 to 150 Gpa Therefore this equation predict theoretical shear stress in the range (3 to 30 Gpa) Actual values of shear stress required to produce plastic deformation in metal single crystals are in the range of 0.5 to 10 MPa.
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