CHAPTER 7: DISLOCATIONS AND STRENGTHENING

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1 CHAPTER 7: DISLOCATIONS AND STRENGTHENING ISSUES TO ADDRESS... Why are dislocations observed primarily in metals and alloys? Mech Notes 7 1

2 DISLOCATION MOTION Produces plastic deformation, in crystalline solids Depends on incrementally breaking bonds. Plastically stretched zinc single crystal. If dislocations don't move, plastic deformation doesn't happen! Mech Notes 7 2

3 DISLOCATIONS & PLASTIC DEFORMATION DISLOCATIONS are very important to mechanical properties. They make metals weaker than they should be, BUT also allow metals to be deformed (I.e. allow plastic deformation). Movement of dislocations under applied stress is called SLIP. Direction of dislocation movement is SLIP DIRECTION. Plane swept out by the burgers vector and the dislocation is called the SLIP PLANE. Combination of direction and plane = SLIP SYSTEM Mech Notes 7 3

4 Mech Notes 7 4

5 Mech Notes 7 5

6 When a dislocation moves, less atomic bonds have to break at a time than if all the bonds on the plane break at once. So lower stress required to deform material than to break it apart. E.g. Instead of pulling whole carpet across floor, make ruck and push across; or movement of caterpillars. Visible plastic deformation is due to movement of many dislocations under applied shear stresses. Virtually all crystalline materials contain some dislocations. The number of dislocations is expressed as Total dislocation length per unit volume (mm/mm 3 which becomes mm -2 ). Can be as low as 10 3 mm -2 but if metal is worked (heavily deformed) can reach mm -2. I.e. more dislocations are created during deformation. Mech Notes 7 6

7 Mech Notes 7 7

8 CHARACTERISTICS OF DISLOCATIONS When metal is plastically deformed, most of energy is dissipated as heat but approx. 5% is stored internally mostly as strain energy associated with dislocations. Distortions of lattice around dislocation line causes Lattice Strains. E.g. in +ve edge dislocation, tensile strain below dislocation, and compressive strain above (atoms bunched together). In screw dislocations - shear strains. Strain fields of dislocations can affect other dislocations. Same sign dislocations on same plane repulse each other whereas opposite signs will attract and annihilate on meeting. Mech Notes 7 8

9 When there are many dislocations the repulsive force between like dislocations makes slip more difficult this make plastic deformation more difficult and your material is stronger. Mech Notes 7 9

10 SLIP SYSTEMS Slip system comprises: slip direction & slip plane Slip occurs in systems that minimize atomic distortion during dislocation movement. Each crystal system has preferred slip planes and directions: Favoured planes are those planes that are most closely packed (highest planar density). These are smoother planes (with a high interplanar spacing d). Slip occurs more easily in closest packed directions (highest linear density). E.g. FCC crystals, {111} planes are all close packed. And < 110 > type direction is close packed direction on these planes. Thus {111}<110> represents slip systems for FCC. Mech Notes 7 10

11 Mech Notes 7 11

12 SLIP SYSTEMS cont. Independent slip systems made up of different combinations: 4 unique {111} type planes and 3 directions on each = 12 FCC and BCC metals tend to have high numbers of slip systems ( 12). Some slip system is nearly always oriented favourably for slip so these metals tend to be ductile - deformable. HCP metals have few active systems so are normally brittle. Dislocations do not move easily in materials with covalent bonds (very strong and directional bonds). Usually break before slip occurs. Materials with ionic bonds are resistant to slip (due to disruptions in charge balance). Usually break before slip occurs. Mech Notes 7 12

13 DISLOCATIONS & MATERIALS CLASSES Metals: Disl. motion easier. -non-directional bonding -close-packed directions for slip. electron cloud ion cores Covalent Ceramics (Si, diamond): Motion hard. -directional (angular) bonding Ionic Ceramics (NaCl): Motion hard. -need to avoid ++ and -- neighbors Mech Notes 7 13

14 STRESS AND DISLOCATION MOTION Crystals slip due to a resolved shear stress, τ R. Applied tension can produce such a stress. Applied tensile stress: σ = F/A slip direction F A F Resolved shear stress: τr=fs/as slip plane normal, n s τr slip direction Fs τr As τ R = σcos λcos φ Relation between σ and τr τ R =Fs/As Fcosλ F slip direction λ Fs A/cosφ nsφ A As Mech Notes 7 14

15 CRITICAL RESOLVED SHEAR STRESS Condition for dislocation motion: τ >τ = σ 2 Crystal orientation can make it easy or hard to move disl. R CRSS y σ τ R =σcos λcos φ σ σ τ R = 0 λ=90 τ R = σ/2 λ=45 φ=45 τ R = 0 φ=90 Mech Notes 7 15

16 DISL. MOTION IN POLYCRYSTALS Slip planes & directions (λ, φ) change from one crystal to another. τ R will vary from one crystal to another. The crystal with the largest τ R yields first. σ Other (less favorably oriented) crystals yield later. 300 μm Mech Notes 7 16

17 SUMMARY Dislocations are observed primarily in metals and alloys. Slip occurs by shear on preferred planes in preferred directions. Strength is increased by making dislocation motion difficult. Mech Notes 7 17

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