Materials S2 : Ch2 - Defects

point defect

Localized disruption in a crystal structure (Include: Vacancies, Interstitial atoms, Substitional atoms

Schmid’s law

the value of at which slip occurs in a given material with specified dislocation density and purity is a constant, known as the critical resolved shear stress

Surface defect

imperfections on the surface of materials

A defect

a deviation from an ideal crystal

A vacancy

Facilitates the displacement of atoms. The density of vacancies

increases with temperature.

Interstitial defect

an extra atom of ion is inserted into the crystal, at position normally non occupied. (small atoms C or H or Fe). Modifies the surroundings.

Interstitial sights

Substitutional defects

One atom is replaced by another atom, can be larger or smaller than the original one, they deform the surrounding crystals

Dislocations

line imperfections in the perfect crystals

when are dislocations typically introduced?

during the solidification or when the solid is deformed permanently

What are dislocations useful for?

Predicting deformation and strengthening in metallic materials

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What are the types of dislocations?

screw dislocations, edge dislocations

screw dislocation

a dislocation formed by cutting halfway through a crystal and sliding those regions on each side of the cut parallel to the cut to create spiraling atom planes

What type of dislocation to we use the Bergers vector for?

screw, edge

edge dislocations

Formed when an additional half-plane of atoms gets inserted into the middle of a crystal

For this type of dislocation the Burgers vector and the dislocation line are perpendicular

Edge dislocation

Dislocation line

a one-dimensional defect in a crystal lattice that represents a discontinuity in the atomic arrangement

For this type of dislocation the Burgers vector and the dislocation line are parallel

Screw dislocation

Complex stress field

spatially varying distribution of internal forces (stresses) within a crystal, where the stress is not uniform and often changes in magnitude and direction across the material

Complex stress field is caused by

Dislocations disturbing the surrounding along the dislocation line

Elastic deformation

reversible process of atomic bond stretching (like a

spring)

Plastic deformation

irreversible process arising by some slip between

atomic bonds

Slip

The process by which a dislocation moves and causes deformation (permanent)

Slip direction

The direction along which the plane moves

What is the relation of the Bergers vector to the slip direction (for both types of dislocation)?

They are parallel

The direction along which the dislocation moves for the edge dislocation?

Parallel to the Burgers vector or to the slip direction

The direction along which the dislocation moves for the screw dislocation?

Perpendicular to the Burgers vector or the slip direction

Slip plane

A plane defined by the movement of the dislocation

Slip system

slip direction + slip plane

Peierls-Nabarro stress

the stress required to move the dislocation between the the initial set of surroundings to another set of surroundings

Which conditions will the dislocation move along?

The conditions minimising the expenditure of energy

Should slip direction have a small or large repeat distance?

Small

Should slip direction have high linear density or a low one?

High

Slip occurs more easily between planes of atoms that are rough or smooth?

Smooth

Slip occurs easier between planes of atoms that are close by or far apart?

Far apart

Interplanar spacing

Distance between the planes in the material

Slip planes are expected to be close or far packed planes

Close packed planes

What type or bonds makes it harder for the dislocations to move?

Covalent bonds

Surface defects

The boundaries, or planes that separate a material into regions, each region having same crystal structure but different orientations.

Material surface

The exterior surfaces of the material at which the crystal ends

Grain

Grain boundaries

Surfaces separating the grains

To what degree is the material organised in the grain boundaries?

Not organised at all; more like chaos

Is yield stress higher is the grains are bigger or smaller?

Smaller

Etching