6 Metals and Alloys

Composition and alloy families

Metals are often grouped according to their main base element. Ferrous metals are iron-based, while nonferrous metals are metals and alloys in which iron is not the principal constituent. This distinction describes composition, not quality: either group can be strong, durable, or corrosion-resistant depending on its alloy and processing.

An alloy combines a metal with one or more other elements to tailor properties. Adding carbon and other elements to iron produces steels with a wide range of strength and hardness. Chromium in stainless steels helps improve corrosion resistance. Brass is mainly copper and zinc, while bronze is traditionally copper and tin; these copper alloys are engineered for particular uses.

Crystal structure and properties

Most metals are crystalline: their atoms form repeating arrangements, and engineering metals usually contain many small crystals called grains. Common crystal arrangements include body-centered cubic (BCC), face-centered cubic (FCC), and hexagonal close-packed (HCP).

A metal's crystal structure is one factor in its behavior, not a standalone guarantee of a particular property. Crystal structure, alloy composition, grain structure, and processing together influence strength, ductility, hardness, conductivity, and corrosion resistance.

Iron changes crystal structure with temperature. In steel, the BCC phase is called ferrite, and the FCC phase is called austenite. Aluminum and copper are examples of metals with FCC crystal structures. Heat treatment can transform the microstructure and alter the balance of strength, hardness, and ductility.

Typical uses and trade-offs

Ferrous metals are often strong, tough, and widely available, and many grades can be heat-treated. Some grades need coatings or alloying to resist corrosion. Carbon and alloy steels are used in buildings, tools, machinery, vehicles, and pipelines. Stainless steels are used in cutlery, food-processing equipment, and chemical equipment where corrosion resistance is needed. Cast irons are used in engine blocks, pipes, and machine bases, where their casting behavior and compressive strength are useful.

Nonferrous metals are often selected for low weight, corrosion resistance, or electrical and thermal conductivity, but their properties vary widely by metal and alloy. Aluminum alloys are used in aircraft, vehicles, buildings, and packaging because they combine low density with useful strength. Copper is used in electrical wiring and heat exchangers for its high conductivity; copper alloys such as brass are used in fittings and connectors. Titanium alloys are used in aerospace and other demanding applications where high strength relative to weight and corrosion resistance are valuable.

These characteristics are general tendencies, not rules for every grade. Alloying and heat treatment can make steel very strong, while pure aluminum is relatively soft. Copper alloys usually trade some conductivity for greater strength or other useful properties.

Selecting a material for service

Material selection depends on the specific alloy and condition, not just a label such as “steel” or “aluminum.” Relevant factors include required strength and ductility, density, the corrosion environment, operating temperature, conductivity, manufacturing method, availability, and cost. Property values can differ significantly among grades and tempers, so engineering designs rely on data for the specified material rather than broad group averages.

For a component that must carry electrical current, copper is a common choice when conductivity is a priority. If lower weight is especially important, an aluminum conductor may be preferable, with design adjustments to account for its different properties. For a structural beam, steel may be selected for strength and stiffness, while an aluminum alloy may reduce weight where its cost and design requirements are acceptable. In corrosive environments, stainless steel, aluminum, or an appropriate copper alloy may be considered; suitability depends on the specific environment and grade.

In general, ferrous alloys are based primarily on iron, while nonferrous alloys are based on other metals. Crystal structures and microstructures, alloying elements, and processing determine properties. Steel is widely used for strength and structural applications; aluminum alloys are valued where low weight matters; and copper and its alloys are common where conductivity, corrosion resistance, or formability is important. The particular grade should meet the actual service requirements.