1. Freedom is an elusive concept
Freedom is an elusive concept. Some men hold themselves prisoner even when they have the power to do as they please and go where they choose, while others are free in their hearts, even as shackles restrain them
-Zensunni wisdom

2. Dislocation interactions
With tiny precipitate particles
Why don't dislocs move thru precipitates?
a. complex structure - poorly defined slip planes
and/or
b. slip planes wrongly oriented

3. Solid solution strengthening
Solute atoms different size than solvent atoms
Crystal structure distorted – slip planes less well-defined
Disloc movement restricted
Cu solid solutions

4. Strengthening methods - summary
Work hardening
Solution hardening
Precipitation hardening
Grain size reduction

5. Dislocations
Oh, yeah….one more thing about dislocs
Ease of disloc motion depends on slip plane
orientation wrt applied stress direction

6. Dislocations
Slip occurs only as a result of shear & only along slip plane in a slip direction
The greater the no. of slip systems, the greater the probability some slip plane & slip direction correctly oriented

7. Dislocations
Slip system = slip plane + slip direction
ccp slip systems hcp - 3 slip systems
"bcc" slip systems

8. Example - 718 Ni-base superalloy
Composition:
52.5Ni 19Cr Mo 19Fe 0.4Al Nb-1Ti 0.04C

9. Example - 718 Ni-base superalloy

10. Example - 718 Ni-base superalloy

11. Example - 718 Ni-base superalloy
Possibilities for strengthening 718:

12. Example - 718 Ni-base superalloy

13. Example - 718 Ni-base superalloy
52.5Ni Provides friendly matrix for alloying elements
19Cr Oxidation and corrosion resistance, solid solution strengthening
3Mo Solid solution strengthening, stabilizes chromium oxide film,
carbide formation for Hi T strength
19Fe Solid solution strengthening
0.4Al Increases oxidation resistance, forms strengthening pptates
5Nb-1Ti Form strengthening pptates
0.04C Carbide formation for creep resistance

14. Dislocations
The primary strengthening phase for alloy 718 is " with some contribution from '. The " is coherent and has a body-centered-tetragonal crystal structure, while ' has an ordered face-centered-cubic crystal structure. The stoichiometry of both can be represented as Ni3(Nb,Ti,Al). The " and ' phases in alloy 718 are both metastable. The stable precipitate phase is , which has an incoherent orthorhombic crystal structure of Ni3(Nb,Ti). It forms as intergranular lathes along (111) planes and/or at grain boundaries.
ccp slip systems hcp - 3 slip systems

15. Dislocations
718 (Ni-18.5Fe-18Cr-5.3Nb-3Mo-0.9Ti-0.5Al-0.03C)
ccp slip systems hcp - 3 slip systems