Metals: Properties, Structure, and Applications in Construction

Posted on Dec 17, 2024 in Chemistry

Introduction to Metals

Metals are materials composed of metallic elements, characterized by atoms held together by metallic bonds. These bonds influence their properties, allowing ion layers to displace without rupture, determining plasticity. Metals are known for their electrical and thermal conductivity. In nature, metals are often combined with other elements. Metallurgy is the process of extracting metals by studying their properties. Iron extraction leads to steel.

Ferrous Metals

Ferrous metals, primarily composed of iron, are known for high tensile strength and hardness. Alloys are created with tin, carbon, silver, platinum, manganese, vanadium, and titanium. Steel (iron and carbon alloy) is widely used in construction.

Nonferrous Metals

Nonferrous metals generally have lower tensile strength and hardness than ferrous metals but offer higher corrosion resistance. Although initially costly, improved extraction and refining techniques have reduced costs, increasing their competitiveness. Common nonferrous metals in construction include aluminum, copper, nickel, lead, and zinc. They are also used in alloys like bronze (copper, lead, tin) and brass (copper, zinc).

Structure and Characteristics of Metals

The properties of metallic materials are determined by their structural characteristics.

Constitution

The chemical elements and compounds forming a metal are crucial in determining its properties. Binding forces between atoms are influenced by the type of atoms and their valences.

Crystal Structure

The crystal structure significantly affects material properties. For example, graphite and diamond, both pure carbon, have different properties due to their crystalline structures. Solid materials can be crystalline or amorphous. Crystalline structures have atoms in an orderly arrangement, while amorphous structures do not. Metals in their solid state typically have crystalline structures. Polymorphous materials can have different crystal structures. Common structures include:

  • Body-centered cubic (BCC): Iron at room temperature (alpha iron/ferrite), chromium, molybdenum.
  • Face-centered cubic (FCC): Iron at 910 ºC (gamma iron/austenite), aluminum, silver, copper, gold, nickel, lead, platinum. These metals are more ductile.
  • Hexagonal close-packed (HCP): Beryllium, cadmium, magnesium, zinc, titanium. This structure allows malleability and ductility but is fragile.

Grain Structure

Grain size, influenced by cooling rate and heating, affects metal properties. When liquid metal cools, crystals form dendritic structures. Grain boundaries form where these structures meet. Smaller grains result in harder, more ductile materials, while larger grains are prone to fracture. Metallurgical microscopes are used to determine grain size.

Alloys

Adding alloying elements to pure metals modifies the original lattice, enhancing mechanical, thermal, and durability properties. Alloys are solutions where alloying elements are the solute and the pure metal is the solvent. Steel (iron-carbon alloy) is a key example.

Properties of Metals

Metals have many interesting properties, but we will focus on the following:

Tensile Strength

The tensile test is crucial for assessing metal resistance. It involves stretching a metal cylinder to break, measuring deformations as a function of applied stress. Key results include:

  • Yield (fy): Maximum stress a metal can withstand elastically.
  • Limit Break (fs): Maximum tensile stress a metal can withstand before breaking.
  • Modulus of Elasticity: Slope of the line defining the elastic behavior of the metal. For steel, it is 2.1 * 106 Kp/cm2.

Deformity

Metal deformation during breaking includes:

  • Elasticity: Material deforms proportionally to applied stress and recovers fully once stress is removed.
  • Plasticity: Material deforms permanently beyond a threshold voltage. Ductility is related to plasticity, referring to the ability of metals to be drawn into wires or deform under tensile stress without breaking. Brittleness is the opposite of ductility. Malleability is the ability to form films. Sharpness and toughness are also related to deformation. Sharpness is the property of metals to increase their endurance by increasing plastic deformation. Toughness is the energy required to break a material.

Hardness

Hardness is another important property, related to mechanical strength. Methods include:

  • Scratch Hardness: Resistance to being scratched, more applicable to stone.
  • Indentation Hardness: Resistance to permanent deformation by applying stress. Tests include Brinell, Vickers, and Rockwell. The Brinell test involves pressing a 10 mm ball on the metal with a 3000 kg load.
  • Impact Hardness: Measuring the rebound of a mass impacting the metal surface. The Shore hardness test is a common example.
  • Wild Hardness: Primarily applied to metals.

Welding

Weldability is the joining of two metal pieces by heating, friction, or pressure. Welding has advanced significantly since Egyptian times, with major developments during the Industrial Revolution and World Wars. It is an economical, reliable, and quick method to join metals.

Electrical Conductivity

All metals are conductive, with copper and aluminum being the most common for electrical cables. Resistance is proportional to length and inversely proportional to the cross-sectional area. Resistivity is a property inherent to each material. Electrical conductivity is the inverse of electrical resistivity.

Thermal Properties

Thermal properties include:

  • Thermal Conductivity: Metals conduct thermal energy well, depending on conditions and the metal.
  • Thermal Expansion: Solids expand when heated and contract when cooled. Linear expansion is described by the equation: Δl = α * l * ΔT.
  • Fire Resistance: Steel is noncombustible but loses resistance at high temperatures. Heavy gauge steel is more resistant to fire than light sections. Steel can “burn” at very high temperatures (800-900 ºC), becoming corrosive and unusable. Cast iron pillars can break when heated and cooled rapidly.

Durability

Factors affecting durability include:

  • Fatigue: Metals can break at lower voltages under dynamic stress. Fatigue is studied through cyclical testing.
  • Oxidation: Slow process caused by contact with atmospheric oxygen.
  • Corrosion: Accelerated oxidation, often caused by water. Galvanic corrosion occurs when two metals with different galvanic potentials are in contact.

Treatments

Ferrous products in construction undergo thermal and mechanical treatments to improve bearing capacity or ductility.

Mechanical Treatment

  • Wrought: Heating or shocking the base metal.
  • Rolling: Deforming a piece through rollers, done hot or cold.
  • Moulding: Melting and pouring metal into a mold.
  • Machining: Polishing, cutting, drilling, etc.

Heat Treatments

Heat treatment involves heating, retaining, and cooling metal to modify its properties. Variables include heating rate, temperature, residence time, and cooling rate. Common treatments include:

  • Annealing: Heating and slow cooling.
  • Standardisation: Heating and cooling in calm air.
  • Tempering: Heating followed by rapid cooling.
  • Tempering: Heat treatment to improve ductility and reduce residual stresses.
  • Patented: Heat treatment for wires and strips in prestressed concrete.
  • Trefilado: Stretching steel by passing it through holes.

Metals in Construction

20% of metals used in industry are non-ferrous alloys, selected for their tensile strength, corrosion resistance, electrical conductivity, and machinability. Selection depends on mechanical tests, production volume, cost, and aesthetics. Non-ferrous metals are generally more corrosion-resistant than ferrous metals.

Aluminum

Aluminum is obtained from bauxite. It is abundant, soft, white, lightweight, ductile, malleable, and a good conductor of electricity. It is used in high-voltage wires, alloys, transportation, decks, carpentry, and light elements. It is recyclable.

Zinc

Zinc is obtained from zinc sulfide concentrates. It is gray-white or bright blue, with few mechanical properties, soft, and highly corrosion-resistant. It is used for roofing, downspouts, gutters, and to protect metals from corrosion through galvanization.

Copper

Copper is obtained from chalcopyrite. It is expensive, red, malleable, tenacious, and corrosion-resistant. It is used in electric wires, coils, and alloys.

Lead

Lead is obtained from galena. It is bluish-white, with few mechanical properties, very ductile, soft, heavy, and resistant to acids and pure water. It is used in red lead paints for corrosion protection.

Iron

Iron is bright white, does not harden to the temple, and is obtained from oxides, carbonates, silicates, and sulfides. Its mechanical properties are not very good, but they improve with carbon. It is mainly used in alloys, especially steel.

Steel

Steel is the main product in steel construction, with carbon steel being the most important. Carbon steel is an alloy of iron and carbon, with other elements. Increasing carbon content increases tensile strength but decreases toughness and ductility. There are many types of steels with different properties. Alloy steels contain other elements to improve key features. Steels for metallic structures are generally hot-rolled products of non-alloy steel.

Galvanized Steel

Galvanized steel is surface-treated with zinc to protect it from corrosion. Methods include hot-dip galvanizing, cold galvanizing (electrolytic zinc, bonding with zinc, sherardization, zinc dust paints), and cathodic protection.

Stainless Steel

Stainless steels are iron-chromium alloys with a minimum of 11% chromium. They are corrosion-resistant due to a thin film of chromium oxide. They are easy to transform and have various aesthetic appearances. Stainless steels are not indestructible but are highly resistant to corrosion with proper care.