1. Precipitation or Age Hardening

2. Defects and Strengthening Mechanisms
Strain Hardening
Introduces Line Defects
Varied Strengths
Grain Size Refining
Surface Defects
Defects and Strengthening Mechanisms
Solid Solution Strengthening
Single Phase
Point Defects
Low Strengthening Effect
Exceed Solubility Limit
Precipitation Hardening
Two Phase, Coherent
Point and Surface Defects +
High Strengthening Effect
Dispersion Strengthening
Two Phase, Non-Coherent
Point and Surface Defect
Medium Strengthening Effect

3. Take a minute Write down everything you remember from last class
What was a muddy point…what did you not understand

4. Dispersion Strengthening
Exceeds solubility limit => two solid phases
Matrix
continuous
soft and ductile
Precipitate
discontinuous
strong
round
numerous

5. Non-Coherent vs Coherent Precipitates
What is the difference in these two precipitates? Which would be more effective at blocking slip?

6. Non-Coherent vs Coherent Precipitates
Discontinuous
Only blocks slip if precipitate lies directly in path of dislocation
Does not disrupt surrounding crystal structure
Coherent
Planes of precipitate atoms relate to or continuous with planes of atoms in lattice
Creates widespread disruption of lattice
Movement of dislocations impeded at great distances from precipitate
obtained via special heat treatments - aging

7. Age Hardening Three step heat treatment Solution treat Quench Age
Natural
Artificial
Forms coherent precipitates
Used most frequently for non-ferrous metals
composition

8. Age Hardening vs. Slow Cooling

9. Overaging

10. Growth of Precipitates
Guinier Preston Zones
GP-I
GP-II
Coherent precipitate, q’
Non-coherent precipitate, q

11. Growth of Precipitates
GP-Zones
q’ Precipitate
q Precipitate

12. Effect of Aging Temperature and Time

13. Requirements for Age Hardening
Display decreasing solid solubility with decreasing temperature
Matrix should be soft/ductile
Precipitate should be hard/brittle
Alloy must be quenchable
Coherent precipitate must form

14. Group Work – Turn In!
Compare and contrast age hardening with dispersion strengthening and solid solution strengthening. What is the same, what is different?
Using the phase diagram provided determine the compositions that might be suitable for effective age hardening , prescribe an age hardening process (temperatures for a particular composition of your choosing, describe the microstructure that forms at each step.

15. Try the same thing for this one!
Only compositions between blue lines can be age hardened, the rest can not be heated to form a single phase solid
If I picked 15 % Rh as my composition then I would use the following heat treatment:
Solution great to above solvus but below eutectic temp (1700 – 1900 C) to form single phase solid solution , a
Quench to form supersaturated solid solution a ss
Natural age at RT or artificial age by heating to below solvus temp ~ 1600 C, probably wouldn’t go that high though to form a a matrix with coherent g precipitates
Very small region that can be heated to form all b and then quenched and aged to form g in b