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Strengthening Mechanisms

Strengthening Mechanisms. • Increase Grain Boundaries : barriers to slip/dislocation motion. • Solid-Solution Strengthening : pinning (opposing stress fields) or impeding dislocation motion (obstacles). Work-hardening (or Cold-Working) :

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Strengthening Mechanisms

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  1. Strengthening Mechanisms • • Increase Grain Boundaries: • barriers to slip/dislocation motion. • • Solid-Solution Strengthening: • pinning (opposing stress fields) or impeding • dislocation motion (obstacles). • Work-hardening (or Cold-Working): • Increased dislocation density, more interactions interactions (increased stress fields and entanglements).

  2. Strengthening: 1- reduce grain size • Grain boundaries are barriers to slip. • Barrier "strength" increases with misorientation. • Smaller grain size: more barriers to slip. • Hall-Petch Equation: Adapted from Fig. 8.12. (from A Textbook of Materials Technology, by Van Vlack, Pearson Education, Inc., Upper Saddle River, NJ.)

  3. Example: Grain-Size Strengthening • 70wt%Cu-30wt%Zn brass alloy • Data: 0.75mm Fig. 4.11(c), Callister 6e.

  4. Strengthening: 2 - solid solutions E, F x c T • Impurity atoms distort the lattice & generate stress. • Stress can produce a barrier to dislocation motion. • Smaller substitutional generates local shear at A and B that opposes dislocation. with Tension, T Compression, C Here T inducing solute is on T side of slip, so it is first repulsive. Here, solute is above slip plane, so it is attracted then repulsed. With host is under T it reduces C if solute is above slip plane Host atom under tension, T

  5. solid solutions - cont. E, F c T • Larger substitutional impurity generates local shear at C and D that opposes dislocation. Here, C inducing solute is on T side of slip, so it is first attracted. x Here, big solute is above slip plane, so it is first repulsed. Large solute induces C and below slip plane reduces T. Host atom in compression, C Note that solutes always restrict motion of dislocation, only depends on magnitude, not sign, of size mismatch!

  6. Ex: solid-solution strengthening in Cu • Tensile strength & yield strength increase w/wt% Ni. Adapted from Fig. 8.14 (a) and (b) • Empirical relation: • Alloying increases σy and TS.

  7. Strengthening Metals: 3 - precipitation strengthening • Hard precipitates are difficult to shear. Ex: Ceramics in metals (SiC in Iron or Aluminum). • Result:

  8. 1.5mm Precipitation Strengthening Application • Internal wing structure on Boeing 767 Adapted from Fig. 11.0, Callister 5e. • Al is strengthened with precipitates formed by alloying. Adapted from Fig. 11.24, Callister 6e.

  9. Precipitation Strengthening Application • Aluminum is strengthened by ordered metastable Al3Li precipitates formed by alloying. Li Al • Addition of Li also decrease density (weight).

  10. Strengthening strategy: 4 - Cold-Working (%CW) • Room temperature deformation. • Forming operations change the cross sectional area: -Forging -Rolling Adapted from Fig. 11.7, Callister 6e. -Drawing -Extrusion

  11. Dislocations during Cold-Working • Ti alloy after cold working: • Dislocations entangle with one another during cold work. • Dislocation motion more difficult. Adapted from Fig. 4.6, Callister 6e. (Fig. 4.6 is courtesy of M.R. Plichta, Michigan Technological University.)

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