Materials Chapter 7
When do dislocations occur? 3 scenarios
1) during solidification 2) during plastic deformation 3) during rapid cooling (dislocations due to thermal stresses)
What are four strategies to strengthen metallic materials?
1) grain size reduction 2) precipitation strengthening 3) solid-solution alloying 4) strain hardening GPSS
Why does the grain boundary act as a barrier to dislocation motion? (2 reasons)
1) the slip plane is discontinuous from one grain to another 2) the dislocation must change its direction of motion
What is a slip system?
A combination of slip plane and slip direction
Read over slide 7, and describe dislocation annihilation
Aye aye captain
What is slip direction?
Direction of movement on the slip plane, most closely packed with atoms/higher linear density
Why are the number of dislocations present greatest in metals?
Dislocation motion is easiest in metals because of their a) non-directional bonding and b) close-packed directions for slip
True/False: the net plastic deformation for the motion of both edge and screw dislocations is DIFFERENT
FALSE- the deformation experienced is the SAME. See slide 5.
True/False: A coarse material is harder and stronger than a fine-grained counterpart
FALSE: Small grain = harder and stronger because of grain boundaries
True/False: In polycrystalline material dislocation, the grain with the largest resolved shear stress will yield LAST
FALSE: The grain with the largest resolved shear stress WILL YIELD FIRST
True/False: in polycrystalline material dislocation, grains are distorted and grain boundaries come apart/open
FALSE: grain boundaries do NOT come apart or open
What is solid-solution strengthening?
Impurity atoms (substitutional or interstitial) distory the lattice and generate strains on the surrounding host atoms. These strains act as barriers to dislocation motion.
Deformation and slip in polycrystalline materials are (more/less) complex due to the random crystallographic orientation of the numerous grains?
MORE
Is dislocation easy in covalent ceramics? Why/why not?
NO- motion in covalent ceramics is difficult because of directional/angular bonding
Is dislocation easy in ionic compounds like NaCl? Why/why not?
NO- motion in ionic compounds is difficult because they are charged- atoms avoid nearest neighbors with like signs (+/-), so generally stay put
Can ceramics and silicon be plastically deformed? Why/why not?
NO- their dislocation densities are too small (slide 6)
What is resolved shear stress?
No matter the applied stress is pure tensile or compressive, shear components exist in all directions EXCEPT parallel and perpendicular to the stress direction
What is the core principle for metal strengthening?
Restricting or hindering dislocations renders a material harder and stronger
On an FCC structure, which planes are most close-packed?
SEE SLIDE 8 for all Slip System information*****
_____________ grain size results in more grain boundaries and more barrier to slip
Smaller
Polycrystalline metals are __________ than their sing-crystal equivalents
Stronger
True/False: In polycrystalline materials, resolved shear stress will vary from one grain to another
TRUE
True/False: In polycrystalline materials, the direction of slip varies from one grain to another
TRUE
True/False: The direction of dislocation motion in mixed dislocations is neither parallel (as in edge dislocations) or perpendicular (as in screw dislocations). It is somewhere in between.
TRUE
True/False:Grain boundary/barrier "strength" increases with increasing angle of misorientation/atomic disorder
TRUE
What are slip planes?
The plane on which the easiest slip occurs
The alteration of grain shape causes ____________ in mechanical properties (for polycrystalline materials)
anisotrophy
What does resolved shear stress result from?
applied tensile stresss
Plastic deformation, molecularly, is due to a large number of ___________________
dislocations
For edge dislocation, the dislocation line (half-plane of atoms) moves __________________ to the stress direction
parallel
For screw dislocation, the direction of dislocation motion is __________________ to the stress direction
perpendicular'
Dislocations move in response to ______________________ stresses applied along a slip plane and in a slip direction
shear
If dislocations cannot move, plastic deformation can't occur in a material. Thus, we can ______________________ a material by limited dislocations
strengthen
In solid-solution strengthening, smaller substitutional impurities generate ___________ stress while larger substitutional impurities generate _____________ stress.
tensile, compressive
What is dislocation density?
the total dislocation length/unit volume
