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How do solutes strengthen metals (the two effects)?
- 1. Alloying elements can create lattice strain: interstitial atoms create strain fields which interact with dislocations, increases the work done by the dislocation as it moves.
- 2. Segregation of alloying elements to dislocations, creates dislocation atmospheres which pin dislocations. Two parts:
- A) Large solutes segregate to tensile strain fields of edge dislocations. Small solutes segregate to compressive strain fields of dislocations.
- B)When solutes with smaller shear moduli than the solvent (i.e.,Gsolute < Gsolvent), the energy of the dislocations will be reduced. This causes an attraction between the solutes and the dislocations.
What are the ways metals can be strengthened? Know all four
strengthening mechanisms. Since strengthening is merely providing obstacles to dislocation motion, what are the obstacles in each type of strengthening?
- 1. Strain hardening: Plastic deformation, or work hardening, of metals increases the dislocation density. Dense dislocation 'tangles' can form, obstructing the movement of other dislocations.
- Dislocation barrier: Other dislocations
- 2. Solute hardening: Alloying elements can create lattice strain, modulus hardening and can 'pin' dislocations, thereby strengthening the metal.
- Dislocation barrier: Solute atoms
- 3. Second Phase hardening: Fine, closely dispersed precipitates can significantly increase the strength of metals.
- Dislocation barrier: Precipitates, second phases
- 4. Grain size hardening: Reducing the grain size increases metal strength according to the Hall-Petch relationship.
- Dislocation barrier: Grain boundaries
Describe lattice resistance to plastic
- Crystals have a lattice resistance, f, against dislocation motion (stemming from the charge repulsion of metallic ion cores in the metallic crystal.
- When F_ul > f, the dislocation moves.
- F_ul= f= τb so the shear yield strength is τ_y=f/b.
Memorize the equations for lattice resistance
- f = lattice resistance
- b = magnitude of the burgers vector
- τ_y = shear yield strength
What is work hardening? Note that only
cold work is effective in producing work hardening. Why is it a strengthening mechanism?
Sigma_y_0 = undeformed yield strength
When you deform at room temperature, eleastic strain is recovered but the plastic deformation is not. TO deform again we need to start from sigma_y_1. See slide 10.
%CW = (t_0 - t_f)/t_0 * 100
Why is it a
strengthening mechanism? How do we generate and multiply dislocations during plastic deformation? What are the three ways in which dislocations interact and increase shear stress for dislocation motion?
Proportional to dislocation density.
- Dislocation generation: dislocations are inherent in all crystals. Dislocations are also generated during plastic deformation from
- frank-read sources.
Dislocation multiplication: Dislocations multiply during plastic deformation through double-cross-slip.