Metals are elements or metallic materials characterised by bonding and electronic structures that commonly permit high electrical and thermal conductivity, metallic lustre, plastic deformation and a range of useful mechanical properties. Metallic bonding involves delocalised valence electrons interacting with positively charged atomic cores, although real metals are better described using quantum mechanical electronic structures than by a simple sea of electrons model. Most engineering metals are crystalline under ordinary conditions, but metallic materials can also be amorphous or nanocrystalline.
Important elemental metals include iron, aluminium, copper, titanium, nickel, magnesium, zinc and many others. Metallic behaviour varies substantially because atomic size, electron configuration, crystal structure, bonding strength, phase stability and defects differ among elements. Metals are fundamental engineering materials because their properties can often be adjusted through alloying, heat treatment, deformation, grain refinement and other microstructural controls.
Metallic crystal structures strongly influence their properties. Common structures include body centred cubic, face centred cubic and hexagonal close packed arrangements. Body centred cubic metals often have good strength and can exhibit temperature dependent ductility, while face centred cubic metals generally possess many readily activated slip systems and can show substantial ductility.
Hexagonal close packed metals have fewer easily activated independent slip systems under some conditions and may therefore display more pronounced anisotropy or limited room temperature ductility depending on composition and microstructure. The crystal structure of a metal can change with temperature or pressure, as demonstrated by polymorphic metals such as iron and titanium. These transformations affect solubility, diffusion, mechanical behaviour and processing response. Grain size, crystallographic texture, phase distribution and defect density further modify the macroscopic properties of polycrystalline metals.
Mechanical behaviour in metals is strongly connected to dislocation motion. A dislocation is a line defect that permits plastic deformation to occur at stresses much lower than those required for simultaneous movement of an entire atomic plane. Slip occurs when dislocations move along particular crystallographic planes and directions. Strengthening mechanisms work largely by impeding dislocation motion. Grain refinement increases the number of grain boundaries that dislocations encounter and can produce Hall Petch strengthening over appropriate grain size ranges.
Solid solution strengthening arises when solute atoms distort the lattice and interfere with dislocation movement. Precipitation hardening uses finely distributed secondary phases to obstruct dislocations, while work hardening increases defect density through plastic deformation. These mechanisms can operate simultaneously, allowing engineers to balance strength, ductility, toughness and manufacturability.
Metals conduct electricity and heat because their electronic structures permit mobile charge carriers and efficient energy transport. Electrical conductivity varies greatly among metals and is affected by temperature, impurities, defects and microstructure. Copper and aluminium are widely used as electrical conductors because they combine good conductivity with practical processing characteristics, while other metals are selected where strength, corrosion resistance or temperature capability is more important. Thermal conductivity is also significant in heat exchangers, electronic thermal management and cooking equipment.
Optical reflectivity is another characteristic of many metals and results from their interaction with electromagnetic radiation. Magnetic behaviour varies considerably. Iron, cobalt and nickel can exhibit ferromagnetism under suitable conditions, while many other metals are paramagnetic, diamagnetic or exhibit more complex magnetic behaviour. Thus metallicity is not synonymous with any single electrical, thermal, magnetic or optical property.
Corrosion is a major consideration for metallic materials because many metals are thermodynamically capable of reacting with their environments. Corrosion can involve electrochemical oxidation, dissolution, galvanic coupling, localised attack or high temperature reactions. Some metals develop protective passive films that significantly slow further corrosion. Aluminium forms a thin oxide layer that contributes to its corrosion resistance, while chromium is important in producing passivation in stainless steels.
Corrosion behaviour depends on composition, microstructure, environment, temperature, stress, surface condition and electrical contact with other materials. Forms of corrosion include uniform corrosion, pitting, crevice corrosion, galvanic corrosion, intergranular corrosion, stress corrosion cracking and erosion corrosion. Engineering practice therefore considers not merely intrinsic metal properties but also environmental compatibility, surface treatments, coatings, cathodic protection and appropriate material selection.
Metal processing includes casting, forging, rolling, extrusion, machining, welding, powder metallurgy, additive manufacturing and numerous specialised techniques. Processing changes microstructure through mechanisms such as solidification, grain growth, recrystallisation, phase transformation and precipitation. Heat treatment is particularly important for steels and many non ferrous alloys. Annealing can reduce residual stress and restore ductility, while quenching can create metastable structures and increase hardness in suitable alloys. Tempering can modify the properties of quenched steels by controlled thermal treatment.
Cold working increases strength but may reduce ductility, whereas subsequent annealing can produce recovery, recrystallisation and grain growth. Manufacturing route therefore becomes part of the material specification because two components made from nominally identical compositions can have substantially different properties due to different processing histories.
Metals have historically supported infrastructure, transportation, manufacturing, energy generation and electronic technology. Steel provides structural strength in buildings, bridges, machinery and vehicles. Aluminium is important where low density and corrosion resistance are advantageous. Copper is widely used for electrical systems and heat transfer. Titanium is valued for its combination of low density, high specific strength and corrosion resistance, including applications in aerospace and biomedical devices.
Nickel based superalloys retain useful mechanical properties at elevated temperatures and are therefore used in demanding turbine environments. Precious metals can be selected for chemical stability, electrical contacts or specialised applications. The limitations of metals include density in some cases, corrosion susceptibility, fatigue, wear, thermal limitations and raw material or processing costs. Material selection consequently requires balancing mechanical, thermal, chemical, electrical, manufacturing, environmental and economic requirements.
Modern metallic materials research includes high entropy alloys, metallic glasses, nanostructured metals, ultrafine grained materials, metal matrix composites and advanced additively manufactured alloys. Computational materials science increasingly links electronic structure, thermodynamics, phase diagrams, microstructure evolution and mechanical behaviour across length scales. Sustainable metallurgy also addresses energy consumption, recycling, critical raw materials and reduction of greenhouse gas emissions associated with extraction and processing.
Recycling is particularly valuable for metals because many metallic materials can be recovered and remelted without losing their fundamental metallic identity, although contamination and alloy separation can complicate closed loop recycling. Future metallic material development therefore involves not simply increasing strength but controlling combinations of strength, ductility, fatigue resistance, corrosion resistance, manufacturability, recyclability and cost.
