Alloys: uses, compositions and evaluation guide
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Definition and purpose of alloys
An alloy is a deliberately made mixture of a base metal with other elements that modifies the base metal's properties. The added elements occupy lattice sites or interstitial positions, which blocks dislocation motion and changes mechanical strength, hardness and ductility. Alloying causes trade-offs: increased strength often reduces malleability, while added elements can increase corrosion resistance but raise cost. Selection of an alloy therefore balances desired properties and limiting factors such as weight, cost and resistance to chemical attack.
Bronze: composition and use
Bronze is an alloy mainly of copper with tin (typically a few percent) and sometimes small additions of phosphorus or aluminium. Tin atoms substitute for copper in the lattice, increasing hardness and reducing corrosion compared with pure copper. Bronze is used for bearings, sculptures and ship fittings because increased hardness reduces wear and improved corrosion resistance extends service life. Limiting factors include cost compared with other alloys and lower tensile strength than some steels.
Brass: composition and use
Brass is an alloy of copper and zinc, with zinc content typically varying from about 5% to 40%. Zinc atoms change the crystal structure and improve strength and malleability; higher zinc increases strength but reduces corrosion resistance and cold workability. Brass is used for decorative fittings, musical instruments and plumbing components because of good corrosion resistance, attractive colour and ease of machining. Limiting factors include dezincification in certain environments and lower high-temperature strength than steels.
Gold alloys: composition and use
Pure gold is soft; alloying with silver, copper or other metals increases hardness and adjusts colour. Carat rating (e.g., 18 ct) specifies the fraction of gold by mass; lower carat increases hardness and wear resistance but reduces gold content and value. Gold alloys are used for jewellery and electrical connectors because gold retains conductivity and resistance to corrosion while alloying supplies required strength. Limiting factors include cost and potential tarnishing from specific alloying elements.
Steels: composition, types and use
Steel is an iron–carbon alloy with carbon content typically up to about 2% and other elements (chromium, nickel, manganese) added to produce stainless, low-alloy and high-strength steels. Carbon increases hardness by forming iron carbides and hindering dislocation motion; other alloying elements provide corrosion resistance (chromium) or toughness (nickel). Different steels suit different uses: low-carbon steel for car body panels (malleable and easy to shape), high-carbon steel for cutting tools (very hard), and stainless steel for cutlery and chemical plant (corrosion resistance). Limitations include increased brittleness at high carbon contents and higher cost for stainless alloys.
Aluminium alloys: composition and use
Aluminium alloys contain elements such as copper, magnesium, silicon and zinc that increase strength while keeping low density. Alloying forms second-phase particles and solid-solution strengthening, which raise tensile strength and fatigue resistance. Aluminium alloys are used for aircraft structures, car components and packaging because low density reduces weight while alloys maintain strength. Constraints include lower melting point than steels and reduced stiffness, requiring design adjustments for load-bearing parts.
Interpreting alloy composition
Composition percentages indicate how much each element contributes to properties: small additions of strong carbide-forming elements dramatically increase hardness, while corrosion-resistant elements like chromium at sufficient percentages form protective oxide layers. Cause → effect reasoning links specific element additions to property changes. Evaluation of composition requires comparing element percentages to known thresholds (for example, chromium above ~10.5% produces stainless behaviour in steels) and considering interactions between elements that produce synergistic or antagonistic effects.
Evaluating alloy choice for a use
Evaluation of an alloy choice uses required properties (strength, ductility, corrosion resistance), operating environment (temperature, exposure to chemicals), cost and manufacturability. Cause → effect logic links the operating environment to the need for specific properties and therefore to particular alloying elements. A reasoned evaluation compares candidate alloys by matching their dominant properties to application demands, noting trade-offs such as weight versus strength or cost versus corrosion resistance, and highlighting limiting factors like availability and fabrication constraints.
Key notes
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