Metals and Their Properties
How bonding, grains and alloy condition affect the properties used to choose a metal.
Metal selection starts with the job the part must do
Metal selection starts with the job the part must do
A metal can carry a load, conduct current, transfer heat or resist wear. Those are different jobs. Start with the required behavior, then compare a particular alloy in a particular condition. An alloy combines a base metal with other elements to change its behavior.
Metals belong to the material-family foundations topic. Use Material Properties for the general property vocabulary; this page applies it to metals. Continue with Types of Steel, Nonferrous Metals, Metal Stock Forms and Purchasing and Heat Treating Metals.
Metallic bonding helps explain conduction and forming
Metallic bonding helps explain conduction and forming
In a metal, some electrons are shared throughout the structure rather than tied to one bond between two atoms. Their mobility helps metals conduct electricity and heat. Metallic bonding can remain intact while atoms rearrange during permanent deformation, which helps explain why many metals can be rolled or drawn.
Most engineering metals contain many tiny crystals called grains. Atoms have an ordered arrangement inside each grain; adjacent grains have different orientations. Grain size, alloy chemistry and processing history all influence properties.
Why two pieces of the same alloy can behave differently
Permanent deformation involves movement of crystal defects called dislocations. Grain boundaries, dissolved alloying atoms and small particles can obstruct that movement. Cold working and heat treatment change these obstacles. Composition alone therefore cannot tell you the strength of a finished piece.
Strength, hardness, ductility and toughness answer different questions
Strength, hardness, ductility and toughness answer different questions
| Property | Question it answers | Selection example |
|---|---|---|
| Strength | How much stress can the material withstand? Yield strength concerns the start of permanent deformation; tensile strength concerns the maximum stress in a tensile test. | A loaded bracket must resist permanent bending. |
| Hardness | How strongly does the surface resist local indentation? | A wear surface may need hardness, but hardness alone does not predict every kind of wear. |
| Ductility | How much permanent deformation can occur before fracture? | A formed part needs enough ductility to survive bending. |
| Toughness | How much energy can the material absorb before fracture? | A part struck repeatedly needs resistance to fracture, not simply a hard surface. |
Strong does not mean stiff. Stiffness describes elastic deflection. Hardening a steel can raise its strength substantially without a comparable change in its elastic stiffness. Geometry also affects how far a part bends.
Pull a tensile specimen: strength, stiffness, ductility and toughness on one curve; Interactive
Pull a tensile specimen: strength, stiffness, ductility and toughness on one curve
A tension test pulls a specimen of known size until it breaks and records force against stretch. Dividing by the original area and gauge length turns that record into an engineering stress–strain curve, and one curve answers four different questions: how stiff, how strong, how far it stretches and how much energy it absorbs.
The curves below are synthetic teaching curves, shaped to resemble common behaviour. Read the method from them, not the numbers for any real alloy.
Tensile test: one curve, four different properties. This fallback shows the same synthetic curves and calculations as the live lab.
A round specimen with a chosen gauge length L0 = 50 mm and diameter 12.5 mm (original area A0 = 122.7 mm²) is pulled by a moving crosshead. Engineering stress σ = F/A0 (N/mm² = MPa). Engineering strain ε = ΔL/L0 (mm/mm, shown here in %). Both use the original dimensions, so engineering stress falls after the neck forms even though the force is concentrated on a shrinking section.
- Stiffness: the elastic slope, Young’s modulus E = σ/ε = 200 MPa ÷ 0.001 = 200 000 MPa = 200 GPa.
- Yield strength (0.2% offset): draw a line with slope E starting at 0.2% strain; where it meets the curve, σy ≈ 250 MPa (at 0.325% strain). Specimen A then stretches along a level yield plateau (stress constant at 250 MPa up to 1.5% strain) before strain hardening makes the curve rise again.
- Ultimate tensile strength (UTS): the highest engineering stress, 400 MPa at 20% strain. A neck begins to form after this peak.
- Ductility: elongation after fracture ≈ εf − σf/E = 30% − 0.16% = 29.84%.
- Toughness (tensile): the area under the curve to fracture, ∫σ dε ≈ 107 MJ/m³ (1 MPa × 1 = 1 MJ/m³ = 1 J/cm³) of work per original volume. For this gauge volume that is about 659 J. This is not fracture toughness KIC, which is measured on a cracked specimen.
Worked example: unloading leaves permanent set. Stop at ε = 10.0%, where σ = 371 MPa. Removing the load follows a straight line parallel to the elastic slope, so the elastic part σ/E = 371/200 000 = 0.185% springs back. Permanent set = 10.0% − 0.185% = 9.815%, or 4.91 mm on the 50 mm gauge length. Below the yield point the same rule gives zero set: at 0.1% strain and 200 MPa, 0.1% − 200/200 000 = 0.
Before loading. Gauge length 50.0 mm (original L₀ = 50.0 mm). Drawn to scale.
At the peak (UTS): stretched uniformly, no neck yet. Gauge length 60.0 mm (original L₀ = 50.0 mm). Drawn to scale.
After the peak: deformation concentrates in a neck. Gauge length 63.5 mm (original L₀ = 50.0 mm). Drawn to scale; neck shape illustrative.
Fractured; pieces fitted together. Broken pieces fitted back together: gauge length 64.9 mm (original L₀ = 50.0 mm). Drawn to scale; neck shape illustrative.
Compare the four synthetic specimens. Each ranks differently on each property: the stiffest is not always the strongest, and the most ductile is not the toughest.
- A Ductile, low-carbon-steel-like
- B Aluminium-alloy-like
- C Brittle, gray-iron-like
- D Polymer-like (cold drawing)
| Specimen | Stiffness E (GPa) | 0.2% offset yield (MPa) | UTS (MPa) | Elongation after fracture (%) | Area to fracture (MJ/m³) |
|---|---|---|---|---|---|
| A: Ductile, low-carbon-steel-like | 200 | 250 | 400 | 29.8 | 107 |
| B: Aluminium-alloy-like | 70 | 266 | 310 | 12.6 | 38 |
| C: Brittle, gray-iron-like | 100 | none: fractures first | 215 | 0.1 | 0.42 |
| D: Polymer-like (cold drawing) | 2.5 | 36.7 | 48.0 | 53.2 | 21 |
Specimen C fractures before the 0.2% offset line reaches its curve, so it has no offset yield strength in this test: the value is undefined, not zero. Specimen D (polymer-like) reaches its peak stress early and then draws a neck along the specimen; its offset value is shown only for comparison.
Model, assumptions and sources
The four curves are synthetic: piecewise cubic curves through chosen points, shaped to resemble common behaviour. They are not measurements, grades, certification values or design allowables. Real results depend on composition, processing, specimen geometry, test speed and temperature, none of which are modelled.
Unloading and reloading follow a straight line with the initial elastic slope E (an ideal elastic approximation, no hysteresis). The load never goes below zero: once the unload line reaches zero load the specimen is slack. Permanent set = ε − σ/E, never negative. Elongation after fracture uses the same rule at the fracture point.
The specimen length is drawn to scale. Its thinning before the peak keeps volume constant; the neck and the fracture are drawn only to show where deformation concentrates, not calculated from a necking or fracture law. True stress, strain rate, temperature and hardness are outside this lab.
The area under the curve is work per original volume in this tension test (MPa = MJ/m³). Impact and fracture-toughness tests measure related but different behaviour.
Sources: ASTM E8/E8M, Standard Test Methods for Tension Testing of Metallic Materials (the test method and its definitions; consult the current edition). W. D. Callister Jr. and D. G. Rethwisch, Materials Science and Engineering: An Introduction, chapter “Mechanical Properties of Metals” (engineering stress and strain, modulus, 0.2% offset yield, tensile strength and necking, ductility, toughness as area under the curve, elastic recovery on unloading).
Density, conductivity and corrosion can decide the choice
Density, conductivity and corrosion can decide the choice
Density is mass per volume. Compare equal volumes when comparing densities; a large aluminum part can still weigh more than a small steel part. Use Geometry, Mass, and Force and Dimensional Analysis for mass, weight and unit calculations.
Conductivity describes how readily heat or current travels through a material. Metals vary widely: choosing a good conductor is different from choosing a high-strength alloy.
Corrosion resistance depends on the metal and its surroundings. Stainless steel and aluminum can corrode. Water, salts, trapped moisture and contact with other metals can change performance. A protective surface film helps only while the environment allows it to remain effective.
Why connecting two different metals can change corrosion
When dissimilar metals are electrically connected in a conducting liquid, such as salty water, galvanic corrosion can accelerate attack on the more active metal. Material pairing, exposed areas and the environment matter. Isolation and drainage can be as important as the alloy choice.
Ferrous means iron-based; a magnet does not identify the grade; Practice
Ferrous means iron-based; a magnet does not identify the grade
Ferrous metals have iron as their base: steels and cast irons belong here. Nonferrous metals have another base, such as aluminum, copper or titanium. Nonferrous does not mean that no trace of iron can be present.
Magnetic response is a clue, not a material certificate. Some stainless steels are weakly magnetic or effectively nonmagnetic in certain conditions. Appearance and a magnet cannot establish an alloy, its temper or its strength.