ch 7 prequiz stuff

Dislocation

A linear crystalline defect around which there is

atomic misalignment.

Edge dislocation

A linear crystalline defect associated with the

lattice distortion produced in the vicinity of the end of an

extra half-plane of atoms within a crystal.

Grain Boundary

The interface separating two adjoining grains

having different crystallographic orientations.

Linear Elastic Regime

Region where stress is directly proportional to strain (Hooke's Law) and deformation is fully reversible.

Plastic Strain

Deformation that is permanent or nonrecoverable after release of the applied load. It is accompanied by permanent atomic displacements.

Second Phase Particle

A precipitate or inclusion with a different composition/structure than the matrix, used for strengthening.

Stress

The instantaneous load applied to a

specimen divided by its cross-sectional area before any de-

formation.

Dislocation density

The total dislocation length per unit volume

of material, or the number of dislocations that

intersect a unit area of a random surface section.

Elastic-Perfectly plastic

Elastic (reversible) strain, then plastic (permanent) strain at constant yield stress.

Linear Density

The number of atoms per unit length along a specific crystallographic direction.

Normal stress

The intensity of internal force acting normal (perpendicular) to a cross-sectional area. (sigma = F/A)

Screw dislocation

A linear crystalline defect associated with the

lattice distortion created when normally parallel planes are

joined together to form a helical ramp.

Shear stress

The instantaneous applied shear load divided

by the original cross-sectional area across which it is applied.

Toughness

A mechanical characteristic that can be the measure of a material’s resistance to fracture when a crack or other stress-concentrating defect is present, the ability of a material to absorb energy and

plastically deform before fracturing, and the total area under the material’s tensile engineering stress–strain curve taken to fracture.

Dislocation Slip (Slip)

Plastic deformation as the result of dislocation motion, and

the shear displacement of two adjacent planes of atoms.

Grain

An individual crystal in a polycrystalline metal or ceramic.

Linear elastic

Behavior where stress is proportional to strain (Hooke's Law) and all deformation is fully reversible.

Planar Density

The number of atoms centered on a specific crystallographic plane per unit area of that plane.

Second Phase Boundary

The 2D interface (defect) separating a distinct second phase (like a precipitate) from the matrix.

Strain

 The change in gauge length of a speci-

men (in the direction of an applied stress) divided by its

original gauge length.

Ultimate strength

The maximum engineering stress that may be sustained without fracture.

Identify schematic images of screw dislocations

Look for a helical or spiral ramp distortion in the atomic planes, centered around the dislocation line.

Identify and sketch edge dislocations

Look for an extra half-plane of atoms ending in the lattice. Sketch this, marking the dislocation line (often shown with a ⊥ symbol) at its edge.

Describe the stresses near an edge dislocation

Compressive stress exists above the slip plane (where the extra half-plane is), and tensile stress exists below it.

Describe the formation of a step on the surface of a crystal by the motion of both an edge and screw dislocation

For an edge dislocation, the extra half-plane glides, exiting the crystal to form a step. For a screw dislocation, the helical defect shears the crystal, forming a step as it exits.