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