06 Metals and Alloys

Metallic bonding and crystal structure

Metals are widely used in engineering because they can combine strength, toughness, ductility, thermal and electrical conductivity, and manufacturability. An alloy is a metal containing one or more added elements; alloying and processing let engineers tailor properties to a particular job.

Metallic bonding consists of positive atomic cores held together by mobile, delocalized electrons. These electrons help explain metals’ electrical and thermal conductivity. Because the bonding is not strongly directional, atomic planes can often slide past one another without the material breaking immediately, giving many metals their characteristic ductility and malleability.

Grains, deformation, and properties

Most engineering metals are polycrystalline: they contain many small crystals, or grains, separated by grain boundaries. Common crystal structures include:

• Face-centered cubic (FCC): Found in aluminum, copper, nickel, and austenitic iron. FCC metals are generally ductile because their structure offers many directions for slip.

• Body-centered cubic (BCC): Found in ferritic iron and many steels. Deformation behavior in BCC metals varies with temperature and alloy composition.

• Hexagonal close-packed (HCP): Found in magnesium, titanium, and zinc. HCP metals often have fewer easily activated slip systems at room temperature, which can limit their formability.

In pure metals, dislocations—line defects in the crystal—move when the metal plastically deforms. Alloying atoms, precipitates, grain boundaries, and other obstacles can impede dislocation motion, increasing strength or hardness, often at the expense of ductility. Work hardening, grain refinement, solid-solution strengthening, and precipitation strengthening are common ways to alter this balance.

Engineering properties

Engineering properties describe how a metal performs under particular conditions. Strength is resistance to deformation or failure; ductility is the capacity for plastic deformation before fracture; toughness is the ability to absorb energy before breaking; and hardness measures resistance to indentation or scratching. Corrosion resistance, fatigue life, density, temperature capability, and weldability also matter.

Properties depend not only on chemical composition but also on grain size, defects, microstructure, and processing. The same broad alloy family can therefore have substantially different properties depending on grade, product form, and processing history.

Alloy families and uses

Different alloy families offer combinations of properties suited to different uses. These are broad tendencies, not guarantees; properties vary substantially by grade, product form, and processing history.

• Steels are iron-based alloys whose carbon content and other alloying elements are controlled for different uses. Low-carbon steels are widely used in buildings, vehicle bodies, and machinery because they are relatively economical, formable, and weldable. Higher-strength, tool, and stainless steels serve applications requiring greater strength, wear resistance, or corrosion resistance. Chromium, nickel, molybdenum, manganese, and other additions help tailor steel performance.

• Cast irons contain more carbon than steels and are commonly cast into complex shapes. Many grades offer good castability and wear or vibration-damping characteristics, though some are comparatively brittle.

• Aluminum alloys combine low density with useful corrosion resistance and formability. They are common in transportation, structures, and packaging; some are strengthened by precipitation heat treatment.

• Copper alloys, including brasses and bronzes, are valued for conductivity, corrosion resistance, and, in selected grades, wear resistance. Copper is used in electrical and thermal components.

• Titanium and nickel alloys are used where strength-to-weight ratio, corrosion resistance, or high-temperature performance justifies their greater cost. Examples include aircraft structures and hot-section components.

Phase diagrams and cooling

A phase diagram maps which phases are stable at equilibrium for specified temperatures and compositions. Engineers use it to anticipate melting and solidification behavior, solubility limits, and transformations during heat treatment. In a two-phase region of a binary diagram, a horizontal tie line identifies the compositions of the coexisting phases, and the lever rule can estimate their relative amounts.

A phase diagram describes equilibrium; by itself, it does not show how quickly a transformation occurs or exactly what forms during rapid cooling.

The iron–carbon system

The iron–carbon diagram relates temperature and carbon content to phases such as ferrite, austenite, and cementite under equilibrium conditions. Cooling rate changes the resulting microstructure: slow cooling can produce ferrite and carbide-containing structures, while rapid quenching can form hard martensite. Time–temperature–transformation and continuous-cooling-transformation diagrams help predict transformations at practical cooling rates.

Processing and heat treatment

Metals are shaped by casting, rolling, forging, extrusion, and drawing. Casting solidifies molten metal in a mold. Rolling and forging deform solid metal to make plate, bar, or components. Extrusion pushes material through a die, while drawing pulls it through a die to reduce its cross-section. Machining, welding, and additive manufacturing can further shape or join parts. Processing influences grain structure, defects, surface condition, and final properties.

Heat treatment changes properties by controlling temperature, time, and cooling. Exact procedures depend on the alloy and part geometry; unsuitable conditions can cause distortion, cracking, or unwanted microstructures.

• Annealing generally softens a metal, improves ductility, and relieves some residual stress. In steels, the treatment and cooling schedule are selected for the particular composition and purpose.

• Normalizing heats steel into the austenite range and cools it in air. Compared with full annealing, it commonly produces a finer, more uniform structure.

• Quenching and tempering is used for many steels. Steel is austenitized and rapidly cooled to form hard martensite, then reheated below the transformation range. Tempering reduces brittleness and adjusts the strength–toughness balance.

• Precipitation hardening is used for selected aluminum, nickel, copper, and stainless-steel alloys. Solution treatment and quenching create a supersaturated solid solution; subsequent aging forms fine precipitates that obstruct dislocation motion.

• Stress relief reduces residual stresses from operations such as welding or machining, generally without aiming to produce the same transformation as hardening treatments.

Selecting a metal for service

Material selection begins with the service requirements: loads, temperature, environment, expected lifetime, and manufacturing route. Compare candidate alloys for strength, stiffness, toughness, fatigue and corrosion resistance, density, weldability, formability, availability, and cost. Also consider whether heat treatment or protective coatings are needed and whether the final component can be inspected and repaired.

A lightweight aircraft part may favor an aluminum or titanium alloy, whereas a low-cost welded building frame may favor structural steel. The best choice is the alloy and processing route that meet performance requirements reliably—not simply the material with the highest strength.

Metal properties arise from the interaction of crystal structure, bonding, composition, defects, and microstructure. Alloying, forming, and heat treatment can control strength, ductility, toughness, corrosion behavior, and manufacturability. Phase diagrams guide equilibrium predictions, while transformation diagrams help account for cooling-rate effects; effective selection balances service performance with processing, durability, and cost.