Material Science

Module 3 - 5

Crystalline defect

Lattice irregularity on one or more of its dimensions on the order of an atomic diameter

Imperfections include:

Point defects

Linear defects

Interfacial defects

Vacancy and Self-interstitial

2 types of point defects.

Vacancy

A vacant lattice site. These are formed during solidification, or the result of vibrations displacing atoms from their atomic sites.

Self-interstitial

• An atom from the crystal, crowded into an interstitial site (a void space otherwise unoccupied) between atoms.

• Causes relatively large distortions in the surrounding lattice due to the atom being substantially larger than the interstitial space.

Alloys

• A combination of two or more metals/elements. 

• Impurity atoms (solute) are added to the host atoms (solvent) to impart specific characteristics.

Solid solution

• Addition of impurity atoms results to the formation of a:

• Random distribution of point defects.

• Two or more elements dispersed in a single phase.

3 Mechanisms of alloying

1. When components are insoluble in each other in the solid state.

2. When components are soluble in each other in the solid state.

3. When components form an intermediate compound.

Formation of a new phase

More likely as the concentration of B increases.

1. Atomic size

2. Crystal structure

3. Electronegativity

4. Valence

Determines the degree the solute and the solvent dissolve in each other.

Atomic size

must be within 15%, otherwise substantial lattice distortions will occur, and a new phase will form

Crystal Structure

Must be the same for the constituent metals.

Electronegativity

The greater the difference in – of the component metals will more likely result in the formation of an intermetallic compound.

Valence

a metal has a higher tendency to dissolve in a metal of a higher – than of a lower one

Primary

Has the structure of the solvent material

Secondary

Characterized by a different crystal structure from that of the component elements.

Types of solid solutions

1. Substitutional solid solution

2. Interstitial solid solution

Composition

The amount of the impurity (b) in the host system (a)

Dislocation

a linear, one-dimensional defect around which some of the atoms are misaligned

Edge dislocation

• A half-plane of atoms that terminate within the crystal.

• There is distortion around the dislocation line, tension on one side, compression on the other.

Screw dislocation

• Caused by shearing force

• Looks like a spiral ramp

• Portion of crystal is shifted by one atom

• Distortion is parallel to the dislocation line.

Mixed dislocation

• Features both edge and screw dislocations. 

• Type of linear defects

1. External surfaces

2. Grain boundaries

3. Twin boundaries

• Anything that deviates from a perfect crystal packing.

• Interfacial Defects

External surfaces

The external atoms are not surrounded by all possible nearest neighbors, therefore they are at a higher energy.

Grain boundaries

• There is a mismatch of the atoms where the grains come together. 

• These can be low angle or high angle (angle of misalignment)

Twin boundaries

• A special type of grain boundary. 

• A mirror lattice symmetry. 

• Occurs along specific crystal planes.

Miscellaneous interfacial defects

• Stacking faults

• Phase boundaries

Bulk or Volume Defects

• Other defects exist in all solid materials that are much larger than those previously discussed.

• Includes pores, cracks, foreign inclusions, etc.

• Normally introduced during processing and fabrication steps.

Structure

• Atomic bonding 

• Crystal structure 

• Microstructure 

• Macrostructure

Mechanical Properties

• Ductility 

• Brittleness 

• Elasticity 

• Malleability

Performance

• Stress 

• Loading 

• Temperature

Stress

The application of a load on an area.

Strain

Amount of deformation from an applied force.

Tensile stress

Produces linear deformation strain (elongation)

Compressive stress

Produces negative linear deformation strain (contraction)

Torsional strain

Variation of pure shear.

Shear strain

Stress applied parallel to the area.

Elastic Deformation

• When a force is applied to a member, it deforms by a certain amount proportional to the applied force. 

• When the applied force is released, the piece returns to its original shape.

• Non-permanent and reversible deformation.

Plastic Deformation

When the amount of strain is no longer proportional to the applied force, permanent, non-recoverable deformation occurs.

Hooke’s Law

The greater the elastic modulus, the stiffer the material, and the smaller the elastic strain that results from a given application of force.

Anelasticity

• It has been assumed that an applied stress causes an instantaneous strain and is constant over the period of time the stress is maintained.

• Time-dependent elastic behavior.

Poisson’s Ratio

Ratio of the lateral and axial strains.

Yield Point

• The point in tensile properties at which the strain deviates from being proportional to the applied stress.

• Called proportional limit.

Tensile Strength

• The maximum point on the stress-strain curve after the yielding.

• If stress is applied (and maintained), a fracture will result at the neck.

Ductility

• The measure of the degree of plastic deformation that has been sustained at fracture.

• Materials that have little or no plastic deformation upon fracture is termed brittle.

Less than 5%

Brittle materials have a percent elongation of –.

Resilience

The capacity of a material to absorb energy when it is deformed elastically, upon unloading, to have this energy recovered.

Modulus of resilience (U)

Computed as the area under the stress-strain curve taken at yielding.

Resilient materials

Those materials with high yield strengths and low moduli of elasticity.

Toughness

The ability of the material to absorb energy up to fracture.

Dynamic loading conditions

• High strain rate

• Notch toughness is assessed using an impact test

Static situation

• Low strain rate

• Toughness is assessed similar to resilience but up to the point of fracture.

Strength and Ductility

For a material to be tough, it must display both –.

Hardness

The resistance of a material to localized plastic deformation (dent or scratch).

Indenter

is forced onto the surface of the specimen, under controlled loading conditions.

Hardness tests

are performed more frequently than other mechanical tests for several reasons.

Reasons why hardness tests are done.

1. Simple and inexpensive

2. Non-destructive

3. Other mechanical properties can be estimated from hardness data

Ductile

materials are capable of being drawn into wires without necking down

Brittle

Stiff materials have a high modulus of elasticity

Flexible

materials will bend considerably without rupture

Elastic

materials return to its original shape when straining forces are removed

Plastic

materials retain a permanent deformation after straining forces are removed

Tough

materials withstand heavy shocks or absorb a large amount of energy

Malleable

materials can be hammered into thin sheets without rupture

Hard

Materials offer high resistance to scratching or denting. Materials have little or no plasticity.

Deformation

This is plastic deformation which corresponds to the motion of a large number of dislocations.

Shear stress

An edge dislocation moves in response to _____ applied perpendicular to its line.

Metals

This type of material has easier dislocation motions, non-directional bonding, and has closed-packed directions for slip.

Covalent ceramics (diamond)

This type of material has harder dislocation motions and an angular bonding.

Ionic ceramics (NaCl)

This type of material also has harder dislocation motions but has uniform bonding.

Slip Systems

This system has no movement on dislocations and has the same degree of ease on all crystallographic planes of atoms and in all directions.

Slip Plane

Called a preferred plane.

Slip Direction

Called a preferred direction.

larger

Metals with FCC or BCC crystal structures have a relatively — number of slip systems

plastic deformation

Metals are quite ductile because — is possible along various systems.

HCP crystals

Have few slip systems and are quite brittle.

Resolved shear stresses (tR)

• Shear components exist at all but parallel or perpendicular alignments to the stress direction.

• Magnitudes of these shear stresses depend on both applied stress and the orientation of the slip plane and direction.

Critical Resolved Shear Stress

The minimum shear stress required to initiate slip on the most favorably oriented slip system.

Discontinuous

Crystallographic directions and planes are —

because of the fractured crystal structure.

Greatest τR

Crystal with the — will yield first.

Stronger

Polycrystalline metals are — than their single crystal counterparts since slip is constrained to individual grains.

adjacent

Polycrystalline metals can’t deform unless it is — and less favorably oriented grain is capable of slipping.

Dislocations to move

The ability of a materials to plastically deform depends on the ability of –

Harder and stronger

Restricting dislocation motion renders a material –.

• Grain-size reduction

• Solid-solution alloying

• Strain hardening (Cold Working)

Three techniques of strengthening mechanisms

Adjacent grains

Have different crystallographic orientations and a common grain boundary

Move across

When plastic deformation happens, the slip must — this boundary.

• Slip from one grain must change direction.

• Boundary acts as a discontinuity in slip planes.

2 reasons why the boundary acts as a barrier to dislocation motion.

Greater total grain boundary area

Smaller grains have – to impede dislocation motion.

High-angle grain boundaries

More effective in interfering with the slip process than small-angle grain boundaries.

High-purity metals

Almost always softer and weaker than alloys.

Impurity atoms

Added substitutionally/interstitially to strengthen the lattice.

Overlapping stress fields

Produce a barrier to dislocation motion.

Increases

Moving a dislocation past an impurity – the strain energy.

Pinning point

The impurity acts as a –

Strain hardening

Achieved by deforming the metal through work hardening.

Cold Working

Temperature at which metals are worked is “cold” relative to its melting temperature.

More, increases

When the material is being worked at, – dislocations appear and the dislocation density –.

prevent

Overlapping stresses – movement of dislocations.

Enhanced strength, becomes brittle

Net effect on the material are: