Dislocation and strengthening mechanisms Flashcards
Why is the actual yield strength much lower than the theoretical value
Due to dislocations and impurities
Discuss dislocations in metals
-Easiest to dislocate
+non directional bonding
+close packed directions for slips
Discuss dislocations in ceramics
- difficult to dislocate
+ (covalent) directional (angular) bonding
+ (ionic) need to avoid nearest neighbors of like sign (- and +)
Discuss dislocation in polymers
Fail in brittle manner before slip
-dislocation is minor
- large Vol of amorphous materials -> no dislocation
-Perma deformations: stretching, rotation, disentanglement of monomers
What is dislocation density
Number of dislocations/ unit volume
Range of values of different materials regarding dislocation density (mm^-2)
-Carefully solidified metal crystals: 10^3
-Heavily deformed metals: 10^9 - 10^10
-Ceramic materials: 10^2 - 10^4
-Silicon single crystals: 0.1 - 1
Define slip system
Combination of slip plane and slip direction
Define slip plane
Plane with high PD slips easily
Define slip direction:
Slip occurs most easily along direction with high LD
There will be question about a diagram with theta, tao, slip plane and normal to slip plane
Ok
How many potential slip systems in FCC, list them all
12 slip systems
(111) [1-10]
(-111) [01-1]
(1-11). [10-1]
(11-1)
Write expression for resolved shear stress
Tr = o’ * cos(tao) * cos(theta)
Unit: Gpa
What is the critical resolved shear stress and its range
When levels of RSS exceeds CRSS, slip occurs.
10^-4 - 10^-2 GPA
At which condition and the associated minimum stress to initiate yielding
Tr = Tcrss -> slip/ yielding occurs
Min stress:
o’ = 2Tcrss
Methods for strengthening materials:
-grain size reduction
-solid solution hardening
-Cold working
How does grain size reduction increase material strength
-increase grain boundary area
-more barriers to dislocation motion
-increase yield strength, tensile strength and hardness
How does solid solution strengthening improve strength
Impurity atoms distort lattice-> creates lattice strains
Lattice strains act as barriers to disc motion
How does cold working (strain hardening) improve strength
Plastically deforming most metals at room temp makes them harder and stronger
-increases in disc density due to reduction in volume
For edge, screw, mixed dislocations, cite the relation between app shear stress (apss) and the direction of disc line motion
Edge: disc line // apss
Screw: disc line perpendicular with apss
