04 Metals and Alloys

Why metals and alloys are adaptable

Metals are widely used because they combine strength and toughness with ductility, thermal and electrical conductivity, and the ability to be shaped and joined. An Alloy consists of a metal and one or more additional elements. Alloying and processing allow engineers to tailor properties to a particular job.

Crystal structures and phases

Most metals are crystalline: their atoms form repeating arrangements. Common structures include Body-centered cubic (BCC), Face-centered cubic (FCC), and Hexagonal close-packed (HCP). Room-temperature iron has a BCC structure; aluminum and copper have FCC structures; magnesium has an HCP structure.

Crystal structure influences how readily a metal deforms. The arrangement and availability of slip systems help determine ductility. Alloying atoms may replace host atoms, forming a Substitutional solid solution, or fit into spaces between them, forming an Interstitial solid solution.

Some alloys contain multiple phases, meaning regions with different structures or compositions. The phases present depend on composition, temperature, and processing, and strongly affect properties. Steel is based on iron alloyed with carbon; stainless steels add chromium for improved corrosion resistance.

Properties and strengthening mechanisms

Important engineering properties include yield strength, tensile strength, hardness, ductility, toughness, fatigue resistance, density, conductivity, and corrosion resistance. These properties are linked: strengthening often reduces ductility, and alloying that raises strength can reduce electrical conductivity. Material selection therefore depends on the conditions and performance required.

Plastic deformation in metals occurs mainly as Dislocations move through the crystal. Strengthening methods make their movement more difficult, and several mechanisms may operate together in one alloy:

• Work (strain) hardening occurs when plastic deformation, such as cold rolling or drawing, increases dislocation density. Dislocations obstruct one another, raising strength and hardness while usually reducing ductility.

• Solid-solution strengthening occurs when dissolved alloy atoms distort the crystal lattice and impede dislocations.

• Grain refinement strengthens metal because grain boundaries obstruct dislocation motion; finer grains generally increase strength.

• Precipitation or particle strengthening uses fine particles within the metal to obstruct dislocations. Particle size, spacing, and distribution matter.

• Transformation strengthening can occur in certain steels when rapid cooling forms hard martensite, a phase that resists deformation.

Heat treatment and process choices

Heat treatment uses controlled heating, holding, and cooling to change a metal's microstructure and properties. The correct cycle depends on the alloy: a treatment that strengthens one alloy may have little effect or cause damage in another.

• Annealing commonly softens metal, relieves stresses, and improves ductility or formability. Full annealing of steel typically involves heating into an appropriate temperature range and cooling slowly.

• Normalizing is a steel treatment involving heating followed by air cooling. Compared with full annealing, it generally produces a finer structure and can help make properties more uniform.

• Quenching and tempering are often used together for steel. Heating first creates austenite; rapid cooling, or quenching, can produce hard martensite. Reheating to a lower temperature, or tempering, reduces brittleness and improves toughness while retaining useful strength.

• Solution treatment and aging strengthen many aluminum alloys and some other nonferrous alloys. Heating dissolves alloying elements into a solid solution; cooling retains a supersaturated solution, and subsequent aging forms fine strengthening precipitates.

A quench is not automatically the best choice. Cooling rate, section size, distortion, cracking risk, and the alloy's hardenability must be considered.

Applications and material selection

Different metal families serve different applications, and grade, processing, service conditions, and the balance of required properties determine suitability.

• Carbon and low-alloy steels are used for structural frames, machinery, shafts, and tools. Composition and heat treatment can balance strength, toughness, cost, and manufacturability.

• Stainless steels are used in food-processing equipment, chemical-processing systems, and other environments where corrosion resistance matters. Their resistance comes from a chromium-rich protective surface film, but this does not prevent every form of corrosion in every environment.

• Aluminum alloys are used in transportation and infrastructure components where low mass, useful strength, and corrosion resistance are advantageous. Some grades are strengthened by aging.

• Copper and copper alloys are used for electrical conductors and connectors, as well as heat-transfer equipment, because copper has high electrical and thermal conductivity.

• Titanium alloys are used in selected aerospace and biomedical components where high strength relative to mass or corrosion resistance is valuable. Grade, processing, and service conditions determine suitability.

Overall, metal performance depends on the relationship between composition, crystal structure, microstructure, processing, and properties. Alloying and strengthening methods can improve performance but often involve trade-offs. Heat treatment is alloy-specific: it can soften metal for forming, harden steel, or strengthen selected nonferrous alloys through precipitation. Material choice should match the required mechanical, physical, environmental, and manufacturing conditions.