Alloy

Definition and Classification of Alloys

• An alloy is a mixture of chemical elements, with at least one being a metal.
• Alloys retain the properties of metals, such as electrical conductivity and luster.
• Alloys may have different properties than the pure metals, such as increased strength or hardness.
• Some alloys can reduce the overall cost of a material while preserving important properties.
• Metallic bonding joins the atoms in an alloy, unlike covalent bonds found in chemical compounds.
• Alloys are classified as substitutional or interstitial, based on the atomic arrangement.
• They can be homogeneous (single phase) or heterogeneous (multiple phases).
• Alloys can be solid solutions of metal elements or mixtures of metallic phases.
• The constituents of alloys are measured by mass percentage or atomic fraction.
• Intermetallic alloys have a specific atomic arrangement and properties.

Examples and Applications of Alloys

• Red gold (gold and copper) and white gold (gold and silver) are examples of alloys.
• Sterling silver is an alloy of silver and copper.
• Steel and silicon steel are alloys of iron with carbon or silicon, respectively.
• Solder, brass, pewter, duralumin, bronze, and amalgams are also alloys.
• Alloys like beryllium-copper are used for non-sparking tools.
• Steel alloys are used in buildings, automobiles, and surgical tools.
• Titanium alloys are used in the aerospace industry.
• Beryllium-copper alloys are used for non-sparking tools.
• Alloys like duralumin find applications in the aviation industry.
• Exotic alloys like Inconel are used in high-temperature environments.

Alloy Formation, Impurities, and Properties

• Alloys are formed by mixing two or more elements, with one being a metal.
• The mechanical properties of alloys can be different from their individual constituents.
• Some alloys form solid solutions, while others have a heterogeneous microstructure.
• Impurities in alloys can have detrimental effects on their properties.
• Care is taken during alloying to remove excess impurities using various methods.
• Alloys are strong solvents that can dissolve most metals and elements.
• They readily absorb gases like oxygen and can burn in the presence of nitrogen.
• Alloys must be melted in vacuum induction-heating and special, water-cooled, copper crucibles to avoid contamination.
• Alloying can be done with one or more constituents in a gaseous state or in the solid state.
• By adding another element to a metal, internal stresses are created in the lattice of metallic crystals, enhancing its properties.

Alloying Elements and Heat Treatment

• Alloying elements are added to a base metal to induce desired properties.
• Most metals and alloys can be work hardened by creating defects in their crystal structure.
• Alloys can have their properties altered by heat treatment.
• Many alloys of aluminum, copper, magnesium, titanium, and nickel can be strengthened by heat treatment.
• Steel undergoes a change in the arrangement of its atoms when alloyed with carbon.
• Allotropes of iron, such as alpha iron and gamma iron, show differences in atomic arrangement.
• Alloying elements can be dissolved in iron, forming a homogeneous phase called austenite.
• Slow cooling of steel allows carbon atoms to precipitate out, forming cementite and ferrite phases.
• Rapid cooling of steel traps carbon atoms within iron crystals, forming a hard but brittle phase called martensite.
• Precipitation hardening alloys soften when quenched and then harden over time.

History and Examples of Alloys

• Alloys have been used by humans since ancient times.
• The use of alloys started with meteoric iron, a naturally occurring alloy of nickel and iron.
• Bronze and brass are examples of substitutional alloys.
• Steel is an example of an interstitial alloy.
• Stainless steel is a combination of interstitial and substitutional alloys.
• Meteoric iron was used to make tools, weapons, and nails.
• Bronze is an alloy of copper and tin, while brass is an alloy of copper and zinc.
• Amalgams are alloys in a soft paste or liquid form at ambient temperature.
• Gold was often alloyed with copper or iron for aesthetic purposes.
• Pewter is an alloy primarily consisting of tin.

Alloy Data Sources