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530.352 Materials Selection

530.352 Materials Selection. Lecture #17; Strengthening Mechanisms Monday October 24 th , 2005. Strengthening Mechanisms :. Lattice Friction Solid Solution Strengthening Precipitation Hardening Strain Hardening and Recovery Grain Boundary Strengthening. Lattice Friction:. FCC.  H.

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530.352 Materials Selection

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  1. 530.352 Materials Selection Lecture #17; Strengthening MechanismsMonday October 24th, 2005

  2. Strengthening Mechanisms : • Lattice Friction • Solid Solution Strengthening • Precipitation Hardening • Strain Hardening and Recovery • Grain Boundary Strengthening

  3. Lattice Friction: FCC H Easy BCC H Hard

  4. Refractory Metals : • Tungsten, molybdenum, tantalum, niobium, columbium • High Tmelting • Very good high T strength !!! • Limited RT ductility -- hard to form • Poor oxidation resistance !!!

  5. Lattice Friction: Crystal Structure Slip systems Friction FCC 12- {111}<110> very low BCC 12- {110}<111> high Diamond cubic 12- {111}-like very high(w covalent bonding) HCP basal slip 2- (0001)<1120] very low othersvery high

  6. H Cu Zn Zn Not as easy Solid Solution Strengthening: y C1/2

  7. Examples of solute strengthening

  8. Precipitation strengthening: Solution,Quench,Heat treat: - forms  precipitates in the  matrix L 700 600 500 400 300 1.   Temperature  3. 2. Al % Cu

  9. Nucleation and Growth: Atoms in Nucleation More nucleation Growth of solution of precipitates until exhausted precipitates

  10. Surface Energy drives growth: For a fixed volume fraction: L ? L L Most surface area -> Least surface area Higher energy-> Lower energy

  11. The mechanical properties: cutting bowing precipitates get too big,i.e. spacing (L) too large. 50 40 30 no precipitates y (ksi) precipitates form and grow y Gb / L Heat Treating Time

  12. Cutting precipitates: When too small - dislocations can cut them:

  13. Bowing: dislocations bow past precipitates precipitates slow dislocations y Gb / L

  14. Yielding: • Single crystalsy b = Ffriction + Fsol’n + Fprec + Fwork hardening • Polycrystals • Yielding starts in some grains before others • General yielding occurs aty = 3y • Grain boundaries add strength

  15. Grain Boundary Strengthening: • dislocations pile up at boundary. • stress builds up. • dislocation motion initiated in neighboring grain. Hall - Petch relation: y = i + ky d -1/2

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