Lecture 7 Flashcards
How do you shear a perfect crystal?
You slide atoms above one plane leading to deformation
At what point is the shear stress maximum at? What happens at the point?
The saddle point. Atoms shift to a new position causing plastic deformation.
What is the relationship between the maximum theoretical shear strength and the materials shear modulus?
The theoretical shear strength is approximately 1/10th of the shear modulus
Define yield stress.
The critical stress for the transition from elastic to plastic deformation of real materials
What causes the large discrepancy between yield stress in real materials and ideal shear strength in perfect materials?
Because dislocation gliding dominates plastic deformation in real materials
What is an edge dislocation?
A defect in a crystal lattice where an extra half-plane of atoms is inserted
Where is the dislocation line l in an edge dislocation?
Bottom of the extra half-plane
Where is burgers vector for an edge dislocation? Where is it pointing to?
Perpendicular to the dislocation line. It points in the direction that the crystal must be shifted to restore perfect order.
What is a slip plane in edge dislocation?
Plane along which the dislocation moves when shear stress is applied.
What is the dislocation in an edge dislocation?
It is the dislocation line, not the extra half-plane itself.
What are the steps of an edge dislocation?
- extra half-plane creates stress
2 & 3. dislocation moves - creates a step the size of b (Burgers vector) on the surface
What is dislocation slip or glide in an edge dislocation?
Each time the dislocation moves ( the extra half-plane moves one Burgers vector b)
What do the dislocation slip activities during tensile loadings result in?
Much smaller yield stress or critical shear stress than the ideal shear strength
What does more deformation lead to?
- Increased displacement in one slip plane
- Activates more slip planes
Which planes are slip planes?
The ones with the highest density of atoms such as close-packed atoms
Which direction are slip directions?
The ones with the smallest atom spacing such as closed packed directions
What is critical resolved shear stress (CRSS)?
Minimum stress required to produce a slip
What are CRSS for fcc and hcp metals?
typically low (soft)
What are CRSS for bcc metals?
very high (strong)
