CH 7 - Deformation & Strengthening Mechanisms Flashcards
How does plastic deformation occur in metals?
slips (edge dislocations)
What directions do dislocations move in?
- moves along slip plane in a slip direction perpendicular to the dislocation line
- slip direction is the same as the burgers vector direction
What is a slip plane?
- plane on which easiest slippage occurs
- highest planar densities
- large interplanar spacing
What is a slip direction?
- directions of movement
- highest linear densities
What is the slip system for FCC?
- {111} planes in directions
- total of 12 slip systems
What is resolved shear stress?
results from applied tensile stresses
What is critical resolved shear stress?
- condition for dislocation motion: resolved shear > critical
- ease of dislocation motion depends on crystallographic orientation
Why are polycrystals stronger than single crystals?
grain boundaries are barriers to dislocation motion
The grain with the largest ______ yields first
resolved shear stress
What are the four strategies for strengthening?
- reduce grain size
- form solid solutions
- precipitation strengthening
- cold working
How does reducing grain size strengthen a material?
- grain boundaries are barriers to slip
- barrier “strength” increases with increasing angle of misorientation
- smaller grain size = more barriers
How does forming solid solutions strengthen a material?
- impurity atoms distort the lattice & generate lattice strains
- strains act as barriers to dislocation motion
- small impurities tend to concentrate at dislocations; reduces mobility of dislocations and increases strength
How does precipitation strengthening strengthen a material?
- large shear stress needed to move dislocation toward precipitate and shear it
- dislocations “advance” but precipitates act as “pinning” sites
How does cold working strengthen a material?
- deformation at room temp
- reduce cross-sectional area
- dislocations entangle with one another making dislocation motion more difficult
- dislocation density increases
What are some benefits of cold working?
- yield and tensile strength increase
- ductility decreases
What are the three stages during heat treatment?
(1) Recovery
(2) Recrystallization
(3) Grain Growth
What happens during recovery (from heat treatment)?
Scenario 1:
- atoms diffuse to regions of tension
- dislocations annihilate and form a perfect atomic plane
Scenario 2:
-dislocations “blocked”
-atoms leave by vacancy diffusion allowing dislocations to “climb”
-“climbed” dislocations can now move on new slip plane
- opposite dislocations meet and annihilate
What happens during recrystallization (from heat treatment)?
new grains are formed that:
- have low dislocation densities
- are small in size
- consume and replace parent cold-worked grains
What happens during grain growth (from heat treatment)?
- at longer times, average grain size increases
- small grains shrink & eventually disappear
- large grains continue to grow
What is the recrystallization temperature?
temp at which recrystallization just reaches completion in one hour
What is the relationship between hot/cold work and the recrystallization temp?
Hot working = deformation above Tr
cold working = deformation below Tr
What is the influence of grain size on mechanical properties?
- metals with small grains are relatively strong and tough at low temps
- metals with large grains have good creep resistance at relatively high temps
