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Metals and Alloys

Metals

• There are several hundreds of different types of metals.

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Metals

Metals involved in the model of a 19th century steam traction engine consist of:

• Mild steel (wheels and frames) –low strength (200MPa); easy to cut, bend or machine to shape; cheap.

• Medium/high-carbon steels, low-alloy steels (gear wheel teeth, drive shafts) – hard (400MPa).

• Quenched steels (sharp teeth of gears) – very hard (1GPa).

Metals

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Metals

• Stainless steels are hard, tough, stiff, and corrosion resistant.

• But they are expensive.• Cast iron is brittle, fine

for low-stressed components like the cylinder block.

• Easy to make in shape (low melting point, good boundary lubricant of graphite particles); cheap.

Metals

Copper: boiler; piping systems, etc. –suitable for precision machining; expensive.

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Bronze is known for its high hardness(compared to other copper alloys), low melting point (compared to steels; easy processing, low cost…) , and high corrosion resistance.

Hard bronze is brittle

Marble vs Bronze

Metals

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Metals

• Each atom consists of the nucleus and the electrons.

• The number of the electrons determines the element.

Metals

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Metals

• Metal: an element with one, two, or three valence electrons.

• Metallic atoms bond with each other through metallic bonds.

• Usually metals are in crystalline form.

• Unit cell: the basic structural unit of a crystal.

• Lattice: regular structure of a crystal built from unit cells.

Atomic structure of CuAl2.

Metals

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Metals

Metals

• If the lattice is perfect, the theoretical value of the strength of a steel is around 1000GPa (the stress required to overcome the metallic bonds), which is 1000 times higher than the actual value!

• The reason: ordinary metal crystals will never be perfect. There are different types of defects.

• Point defects – missing atoms.

• Line defects (rows of missing atoms) – dislocations.

• Area defects – grain boundaries.

• Volume defects – voids and cavities.

• These defects, especially the dislocations, lower the strength of the crystals to the measured value.

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Metals

Metals

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Metals

• Usually, metals are not in the form single crystals. They are made from a number of misoriented small crystals (grains).

• The grain size is usually in the range of 10-100m.

• The grain structure should not be confused with the lattice structure (~1nm): each grain is a (randomly) oriented, defected lattice.

• The grain boundary is a thin layer of displaced atoms (due to the misfit of the two lattices at the both sides of it), often with the thickness of few atomic planes.

Metals

The grains are formed when crystals are formed at multiple sites in the liquid phase.

Grain boundaries are the borders of the crystals growing from these nucleation sites.

It is similar to the formation of soap bubbles.

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Metals

Metals

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Metals

Fracture across a grain boundary

Metals

Grain boundary sheared apart when there is a misfit of the fracture planes at the both sides.

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Metals

Schematic diagram of the fracture across a grain boundary

Steel

• Iron-based alloys are collectively called steel.

• The most important parameter of steel is the carbon content (C). If C < 0.025% the material can be considered as pure iron; if C > ~3% the material is cast iron; if C > 6.7% the material becomes carbide (Fe3C), a ceramic material.

• Steel is used wildly in construction industry as load-bearing material;(1) structural steel(2) reinforcing steel (rebars)(3) other uses (frames, pipes, ...)

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: “Neat” iron phase: Solid solution of carbon in iron matrixFe3C: carbide – a ceramic material

Carbide (Fe3C)

Phase Diagram of Steel

Cast IronSteel

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Annealed: Ferrite (white) + pearlite (dark) Quenched: Austenite Martensite

Steel

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Steel

Steel

• The carbon atoms can exist in the iron base in different forms:(1) in carbide (Fe3C) (the “pure” iron matrix is called phase)(2) in iron matrix as solution ( phase)

• Through high-temperature process (heat treatment), the form of carbon atoms can be changed, leading to significant change of the mechanical properties (strength and toughness):(1) annealing – heating the metal 10oC higher than the austenite line and holding it for several hours, following by slow cooling (pearlite and ferrite or cementite form; refine the grain; soften the steel; increase ductility; remove residual stresses; remove gases)(2) hardening – heating/quenching (austenite is formed; hardening)(3) tempering – reheating of hardened steel to 600-700oC and then quenching (strength and ductility are increased).

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Steel

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Steels

• Structural steels: used in hot-rolled structural shapes, plates, and bars.

• Carbon content: 0.15-0.27%; always contains copper.

• Six grades: (1) A36 – does not contain manganese; Y = 250MPa(2) A529 – Y = 290MPa (3) A572(4) A242(5) A588(6) A514.

• Choice of materials: high initial cost vs. high maintenance cost

Corrosion-resistant; low-alloy

High-strength

Most widely used

Steels

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Steels

• Reinforcing steel: improves tensile behavior of concrete-based structures.

• Rebars can be (1) plain bars – plain and without surface deformation;(2) deformed bars – have protruded (deformed) patterns on the surface(3) plain/deformed wire fabrics – flat sheets where wires pass each other by a certain angle.

• Prestressed rebars can apply residual compressive stresses in the concrete matrix, and thus the actual stress in the concrete can still be compressive (which concrete is good at) when the applied load is tensile (which concrete is not good at).

Steels

Tensile test of steel samples

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Steels

= E

= P/A

=l/l0

Necking

Steels

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Steels

Steel

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Steels

Steel

This is usually used to measure dynamic response of materials (total fracture work – brittle to ductile transition)

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Steel

• A convenient way to estimate the yield strength Y is hardness test.

• A indenter with specific shape is forced into the surface of the material (diamond cone – Rockwell hardness test)

• The size/depth of the indentation is proportional to Y.

• This method is also used to study coating materials or materials with small length scale structure.

Constant load; constant loading rate

Aluminum

• Aluminum is the most plentiful metal on Earth (8% of its crust).• However, since pure aluminum is very reactive, processing techniques

were developed successfully not until the 19th century.• Usually Aluminum exists in the form of oxides. • The very high bonding energy of aluminum atoms and impurities

makes production of pure aluminum from recycling impossible.• However, the low-end 25% of aluminum alloys can be obtained

through recycled materials.• Usually, the term aluminum is used to refer to aluminum alloys. • Pure aluminum is very soft.• Aluminum is used widely as architectural materials (20%), containers

(20%), conductors (10%), highway/air vehicles (10%), etc.

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Aluminum

• Properties of Al alloys:(1) has 1/3 the density/stiffness of steel(2) has 1/3-2/3 the strength of steel(3) good thermal/electrical conductivity(4) corrosion resistant(5) machinable, castable, formable…(6) compatible with surface treatment (coating)(7) nontoxic, nonmganetic, high reflectivity…

• But expensive…(3% of the electricity is used in the aluminum production!)

Aluminum

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Aluminum

Aluminum is usually evaluated through tensile test.

The yield strength is determined through the 0.2% strain offset line.

There is a strong strain hardening effect.

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Aluminum

• Some of the aluminum alloys are non-heat-treatable.

• Others are heat-treatable.• The heat treatment method consists of annealing,

hardening, and tempering.• Aluminum parts can be either welded or fastened

together. • Welding may cause pre-existing flaws; fastening

will lead to stress concentration around the fastening holes.

Other Light Alloys

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Other Light Alloys