What Are Metals?

Metals are a class of materials characterized by high electrical and thermal conductivity, optical reflectivity, mechanical ductility, and the capacity to form positive ions in solution. These properties arise from metallic bonding, in which valence electrons are delocalized across a lattice of positive ion cores rather than localized in covalent or ionic bonds. The free-electron model captures the essential physics: mobile electrons carry charge and heat efficiently, and the non-directional bonding allows atomic planes to slide past each other without fracture, giving metals their characteristic formability.

Metals constitute a large fraction of the periodic table and span an enormous range of properties. Engineers draw on this range to select materials optimized for strength, conductivity, corrosion resistance, weight, or cost in applications from structural frameworks to semiconductor interconnects.

Ferrous Metals: Iron and Steel

Iron is the most widely used engineering metal, primarily through its alloys. Steel, an alloy of iron and carbon containing less than about 2.1 percent carbon by weight, achieves its wide range of properties through heat treatment and alloying additions. Low-carbon steels are ductile and weldable; high-carbon and tool steels are hard and wear-resistant; stainless steels incorporate chromium to form a passive oxide layer that resists corrosion.

Processing routes determine microstructure and therefore mechanical properties. Hot rolling, cold drawing, forging, and powder metallurgy each produce distinct grain structures and residual stress states. Advances in thermomechanical processing and microalloying have produced high-strength low-alloy (HSLA) steels that reduce structural weight without sacrificing safety. NIST's materials science resources on steel properties and standards support the property databases that engineers use to select and qualify structural steels.

Nonferrous Metals: Aluminum, Copper, and Titanium

Aluminum alloys combine low density (roughly one-third that of steel) with good strength, corrosion resistance, and electrical conductivity. The 2000, 6000, and 7000 series alloys, strengthened by precipitation hardening, dominate aerospace structural applications. Aluminum's conductivity makes it the metal of choice for high-voltage transmission lines, where its weight advantage over copper outweighs its lower conductivity per unit cross-section.

Copper is unmatched among common metals for electrical conductivity and is the standard material for wire, motor windings, printed circuit board traces, and heat exchangers. Its conductivity, second only to silver among practical materials, makes it irreplaceable in applications where resistive losses matter. Copper alloys such as brass and bronze add strength and corrosion resistance for bearings, fittings, and marine hardware.

Titanium and its alloys offer an exceptional combination of high strength, low density, and outstanding corrosion resistance, including resistance to seawater and many acids. Ti-6Al-4V, the most widely used titanium alloy, appears in airframes, jet engine compressor blades, and biomedical implants. The high cost of titanium sponge production and the difficulty of machining the metal constrain its use to applications where performance justifies the premium. Research from ASME on titanium alloy fatigue behavior examines how additive manufacturing of titanium components compares with wrought product in terms of defect population and fatigue life.

Metal Processing and Corrosion

Metal processing covers the operations that transform raw metal into usable components: casting, forming, machining, joining, heat treatment, and surface finishing. Each operation affects microstructure, residual stress, and surface integrity, and therefore the mechanical and corrosion performance of the finished part.

Corrosion is the electrochemical degradation of metals through reaction with their environment. Galvanic corrosion occurs when dissimilar metals are coupled in an electrolyte; crevice and pitting corrosion attack passive films in confined geometries. Protective strategies include coatings, anodizing, cathodic protection, and alloy selection. IEEE Xplore publications on corrosion monitoring sensors cover electrochemical impedance spectroscopy and wireless sensor networks deployed to track corrosion rates in bridges, pipelines, and offshore structures.

Applications

• Structural steel framing in buildings, bridges, and offshore platforms
• Aluminum alloy airframes and aluminum conductor cables in power transmission
• Copper windings in electric motors, transformers, and power distribution equipment
• Titanium implants in orthopedic and dental surgery requiring biocompatibility and durability
• Specialty alloys in turbine blades operating at high temperature and stress
• Thin metal films in semiconductor interconnects and photovoltaic contacts