PROPERTIES OF METALS

ELASTIC DEFORMTION

The degree at which a structure deforms or strains depends on the magnitude of the imposed stress Most metals are stressed at relatively low tension. At relatively low levels, stress and strain are directly

proportional: σ=Eϵ Deformation in which stress and strain are proportional is ELASTIC DEFORMATION

- Elastic deformation is not permanent When applied load is release, the material returns to its original size.

PLASTIC DEFORMATION

Where stress is not proportional to strain Permanent Transition from elastic to plastic is gradual On atomic level:

- Plastic deformation corresponds to the breaking of bonds with original atom neighbours and then reforming bonds with new neighbours

TENSILE PROPERTIES

Most structures are designed to ensure that only elastic deformation occurs. YIELDING: where plastic deformation begins YIELD STRENGTH: the stress at which yielding occurs

- it is found by offsetting the curve by 0.002 and drawing a linear line. The intersection is the yield strength.

TENSILE STRENGTH: the maximum strength the material can undergo (if this stress is maintains, fracture will result)

DUCTILITY

The measure of the degree of plastic deformation that has been sustained at fracture. A metal that has sustained little or no plastic deformation is very brittle. Measured by % elongation/%reduction in area

- %EL=lf−lolo

- %RA=Ao−A fAo

MALLEABILITY

Refers to deformation under a compressive force ** different to ductility which is deformation under tensile force

TOUGHNESS

The capacity of a material to absorb energy before fracture It is the area under the entire curve up to the point of fracture

HARDNESS

Measure of a material’s resistance to plastic deformation. Hardness of a metal is directly proportional to the tensile strength of the metal.

IRON-CARBON STEEL ALLOYS

Ferrite (α-iron) has an BCC structure where there are carbon atoms interstitially dissolved in the iron. It is relatively soft.

Austenite (γ-iron) has an FCC structure which is the result of ferrite transformation with increased temperature.

σ-ferrite has a BCC structure which is produced by further heating of austenite.

Cementite (Fe3C), an iron carbide compound is formed when there is >6.70 wt% of carbon.

Martensite is formed when austenitised iron-carbon alloys are rapidly cooled to a relatively low temperature. It is a nonequilibrium single phase structure that results from diffusionless transformation of austenite. Has a body-centred-tetragonal structure.

Pearlite is made out of alternating layers of ferrite and cementite. It is harder than ferrite.

STRENGTHENING MECHANISMS FOR METALS

SOLID SOLUTION STRENGTHENING

High purity metals are generally softer and weaker than alloys ↑ concentration of impurity atoms = ↑ in yield and tensile strength Impurity atoms impose lattice strains on surrounding host atoms which field interactions between

dislocations → dislocation movement is restricted.

SOLUBILITY IN METAL ALLOYS

Complete Solid Solubility implies complete solubility (mixing and interchangeability) of the atoms of the alloying species. The two (or more) types of atoms have to be able to exist in the same crystal structure.- Atomic size, electronegativity must be similar- Must have same crystal structure- Valency of the solute is preferably higher.

Partial Solid Solubility results when there is saturation limit for one of the alloying species in the other, and this saturation limit depends on temperature. So, as the temperature is lowered and solubility decreases a two-phase material forms from the initial single phase.

Insolubility means that the materials are so different in nature (atomic size, valence electron structure, etc.) that they are insoluble – ie. Separate into two distinct elemental phases.

PRECIPITATION HARDENING:

Strength and hardness of the metal is enhanced by the formation of extremely small uniformly dispersed particles of a second phase within the original phase matrix

Steps for precipitation hardening

1. Solution heat treatment → heated to temperature within the A phase field. All solute atoms are dissolved to form a single phase solid solution (so that all the B phase is completely dissolved so that only A phase is present)

2. Rapid cooling or quenching → prevents any B phase from forming.3. Results in a non-equilibrium situation where only A phase solid solution is supersaturated with B atoms

at this temperature 4. The supersaturated A phase solid solution is then heated to an intermediate temperature in the A+B

phase region where diffusion rates are appreciable.5. The B precipitate phase starts to form as finely dispersed particles (aging).6. At appropriate ageing time at the intermediate temperature, the alloy is cooled to room temperature.7. With increasing time the particles grow bigger. Overaging → particles become excessively large and will

not provide as many barriers.

ANNEALING:

A heat treatment Material is exposed to an elevated temperature for an extended period of time and then slowly cooled Purpose of annealing:

1. Relieve stresses2. Increase softness, ductility, and toughness3. Produce a specific microstructure

NORMALISING:

Annealing of ferrous alloys Steels that have been plastically deformed consist of grains of pearlites which are irregularly shaped and

relatively large, but vary in size. Normalising is used to refine the grains (i.e. to decrease the average grain size) and produce a more uniform

ad desirable size distribution → fine-grained pearlitic steels are tougher than coarse-grained ones.

HOT WORKING:

Deformation is achieved at a temperature above that at which recrystallization occurs. Advantages: - large deformations are possible as material is soft and ductile

- deformation energy requirements are less than that of cold working. Disadvantages: - metals may experience some surface oxidation

→ material loss and poor final surface finish.

COLD WORKING (strain hardening):

Deformation at recrystallization temperature. Advantages: - produces an increase in strength (but with decreasing ductility)

→ greater amount of strains in the metal which increases the hardness.- higher-quality finish.- better mechanical properties.

SOLDERING:

Solders are metal alloys that are used to bond or join two or more components (usually other metal alloys). Usually made out of alloys with preferably lower melting points – the melting point with a composition near

the eutectic temperature.→ having the solder of eutectic composition will give the lowest melting temperature with the existence of a liquid phase.

WELDING:

A fabrication technique Two or more metal parts are joined together to form a single piece with one-part fabrication is expensive or

inconvenient. Joining bond is metallurgical – involves diffusion.

HARDENABILITY

The ability of an alloy to be hardened by the formation of martensite. Rate at which an alloy is quenched affects the hardenability. A steel alloy that has a high hardenability is one that hardens, or forms martensite, at a large degree

throughout the entire interior. Jominy-Bar test is used to determine the hardenability of a metal

POLYMERS

Four classes of polymers:

Thermosets Thermoplastics Elastomers Natural

THERMOPLASTICS

Can be softened with application of heat – secondary bonds breaking Considered linear, can be branched (but not crosslinked) Only contains van der Waals forces They do not have a definite melting point due to:

- Range of molecular weights (varying chain lengths)- Varying packing states (part crystalline/part amorphous)

THERMOSETS

Produced by chemical reaction on mixing two components (resin and hardener) Forms heavily cross-linked polymer Also known as network polymers Harder and stronger

ELASTOMERS

Almost linear polymers with occasional cross-links. Therefore, elastic memory. For a polymer to be elastomeric:

1. It must not easily crystallise; they are amorphous, having molecular chains that are naturally coiled and kinked in an unstressed state

2. Chain bond rotations must be relatively free for the coiled chains to readily respond to an applied force3. In order to experience large deformations, it must have crosslinks in order to prevent chain slippage

NATURAL

Derived from plants and animals

POLYMER CRYSTALS

Crystallinity in polymers can be induced by deformation or in the manufacturing process Can improve strength but lowers toughness Liquid crystal polymers – crystal formation promoted in the liquid state. (“self-reinforced” plastics)

CRYSTALLINITY IN POLYMERS

Crystallinity due to the ordered arrangement of molecular chains. Polymers are rarely 100% crystalline – difficult for all regions of chains to be aligned. Heat treatment can cause crystalline regions to grow.

SEMICRYSTALLINE POLYMERS

Spherulite strucures- Alternating region of crystallites and amorphous regions.

TACTICITY

Arrangement of side grounds on polymer chains- Isotactic – side groups on one side- Syndiotactic – regular alternating arrangement - Atactic – random arrangement

Affects staking capability of polymer chains Regularity (iso, syndio) – allows high degree of packing – increased density with an increased tendency to

crystallise – stiffness and strength increases Irregularity (atactic) – maintains low density, amorphous nature.

POLYMER FAILURES

CROSSLINKED/NETWORK: Brittle failure – no plastic deformation SEMICRYSTALLINE/PLASTIC POLYMERS: amorphous regions elongate, crystalline regions align. ELASTOMERS: amorphous chains are naturally kinked and has some crosslinking. Chains become straighter

and still crosslinked.

MELTING AND GLASS TRANSITION TEMPERATURES

Melting and glass temp increases with increasing stiffness Dependent on:

- Bulky side groups- Polar groups or side groups- Chain double bonds and aromatic chain groups

Regularity of repeat unit arrangements only affects melting point. Glass transition temperature – shown by the steep drop in the modulus (due to secondary bonds breaking)

PRE-DEFORMATION BY DRAWING

Drawing stretches out the polymer prior to use and aligns chains in the stretching direction Results of drawing:

- Increases the elastic modulus- Increases tensile strength- Decreases ductility

Annealing after drawing decreases chain alignment and reverses the effects of drawing.

EFFECT OF TEMPERATURE ON STRESS-STRAIN CURVE

Decreasing temperature:- Increases E- Increases TS- Decreases %EL

CERAMICS AND GLASSES

CERAMICS

A compound of a metallic and non-metallic elements, in which the interatomic bonding is ionic (predominant) or covalent. The atomic structure is ordered or crystallised

Have greatest hardness and melting point in nature → diamond

GLASSES

A combination of metallic and non-metallic elements, in which the interatomic bonding is ionic or covalent. The atomic structure is random or amorphous (usually silicate based)

TYPES OF CERAMICS

Natural Ceramics:- Rocks, sand, ice

Vitreous Ceramics:- Porcelain, pottery, brick

Engineering Ceramics- Alumina, PSZ, sialon

ENGINEERING CERAMICS

Ionic ceramics – typically compounds of metal with non-metal (e.g. MgO) Covalent ceramics – typically compounds of metalloid or non-metals (e.g. SiO2)

PROPERTIES OF CERAMICS

Have high Young’s modulus Brittle

FAILURE PROBABILITY OF CERAMICS

Size and distribution of flow significantly affects the strength of ceramics. larger volume, there is a greater chance of flaw Increased strength is observed for a smaller piece Therefore statistical analysis is required.

COMPOSITE MATERIALS

Composite materials are made from the three primary classifications of materials (i.e. metals and alloys, polymers, glasses and ceramics).

A mixture of two or more distinct constituents or phases where:- Both constituents are present in reasonable proportions (>5%)- Constituent phases must have different properties- In man-made composite materials – the phases are sourced separately and be mixed together (i.e. one

phase is not formed from the original matrix) Produced in order to increase stiffness in low modulus material and increase toughness of a brittle material

CONSTITUENTS:

Matrix:- largest volume of composite material which are made out of polymer matrices (thermoplastic and

thermosets)- supports and transmits loads to the fibres and provides ductility and toughness

Reinforcement - natural fibres, synthetic organic fibres… etc- fibres provide strength and carries most of the load

GEOMETRY:

Fibrous Particulate Lamella Natural Structural

PARTICLE REINFORCED COMPOSITES

Reinforced with randomly shaped and dispersed particles E.g. concrete

FIBRE REINFORCED

Reinforced with randomly shaped and dispersed fibres GFRP (fibre glass) and CFRP (carbon glass)

CONCRETE

Compound of:- Cement paste- Aggregate- Water- Pores

Multiphase component. Manufactured from clay and limestone Two stages to produce concrete:

1. Plastic stage – ease of deformation and forming into various shapes2. Formation of hard, rigid structure – can withstand many severe enviroments

CORROSION

CREVICE CORROSION

Local corrosion is caused by the production of EMF due to the difference in concentration of one or more of the reactants of the electrolyte solution

The localised corrosion results from a crevice formed between two surfaces, one at least of which is a metal

FRETTING CORROSION

Localised deterioration at the interface between two contacting surfaces accelerated by relative motion of sufficient amplitude between them to produce slip.

GALVANIC CORROSION

Corrosion associated with the current resulting from the coupling of dissimilar electrodes in an electrolyte.

HIGH TEMPERTURE CORROSION

Corrosion associated with the effect of atmospheric conditions, various gases, molten materials and salts at high temperatures

PITTING CORROSION

Localised corrosion in which appreciable penetration into the metal occurs, resulting in the formation of cavities.

STRESS CORROSION CRACKING

Premature cracking of metals produced by the combined effects of a corrosive environment and surface tensile stress.

MICROBIAL CORROSION

Deterioration of materials caused directly or indirectly by bacteria, moulds of fungi or in combination.