Precipitation or Age Hardening

Strain Hardening –Introduces Line Defects –Varied Strengths Grain Size Refining –Surface Defects –Varied Strengths 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

Take a minute

• Write down everything you remember from last class • What was a muddy point…what did you not understand

Dispersion Strengthening

• Exceeds solubility limit => two solid phases – Matrix • continuous • soft and ductile – Precipitate • discontinuous • strong • round • numerous

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

Non-Coherent vs Coherent Precipitates • Non-Coherent – 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

Age Hardening

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

Age Hardening vs. Slow Cooling

Overaging

Growth of Precipitates

• Guinier Preston Zones – GP-I – GP-II • Coherent precipitate, q ’ • Non-coherent precipitate, q

Growth of Precipitates

GP-Zones q ’ Precipitate q Precipitate

Effect of Aging Temperature and Time

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

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.

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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