Strengthening Materials/Heat Treatment Flashcards
What are at a broader glance the 4 materials strengthening mechanisms?
grain size effects solid solution strengthening strain hardening precipitate hardening
What are the three stages of heat treating materials
recovery, recrystallization, grain growth
Atoms below the dislocation plane are generally in
tension
Atoms above the dislocation plane are generally in
compression (because think of the extra plane of atoms we have to fit)
What are some pros and cons of strengthening
Pros: strengthening allows for TS or yield stress to increase by inhibiting the sliding of dislocations Cons: material is usually a lot more brittle and more stiff LITTLE TO NO INFLUENCE ON ELASTIC MODULUS
What is strain hardening/cold working?
We deform the metal into the plastic regime and allow for plastic deformation and dislocations to form. We achieve an increase in yield stress because of the repulsive nature of large quantities of dislocations.
What is grain size reduction?
By having the dislocation motion change direction at grain boundaries, we form barriers to prevent slip. A smaller grain size means more barriers to slip
What is solid-solution strengthening?
We strengthen metals by introducing impurities, as high purity metals are typically softer than their alloys or impure counterparts.
For solid solution strengthening, small substitutional impurities
add tension and segregate above the slip plane to cancel compressive strain of energy
For solid solution strengthening, larger substitutional impurities
Add compression and segregate below the slip plane to cancel tensile strain of edge dislocations
What is precipitate hardening?
We insert precipitates of other phases (strong ones) such as inter-metallics, covalent precipitates, and ionic precipitates. These strong precipitates act as a dislocation motion blockade.
Describe the heat treatment phase of recovery
Recovery is the reduction of dislocation density by annihilation -> that is, we rearrange the dislocations into lower energy configurations These dislocations combine to reduce overall strain energy in the lattice
Describe the heat treatment phase of recrystallization
Strain still exists in grains, but small unstrained grains nucleate into larger grains. These larger grains consume the smaller strained grains, reducing the overall strain energy
Describe the heat treatment phase of grain growth
Grain growth is when the smaller grains disappear and the larger grains grow to reduce grain boundary energy. Larger grains have fewer dislocations, which leads to lower strength but improved ductility
Draw the relationship between Ductility, tensile strength, and annealing temperature
