Materials Sem 2

interdiffusion

Interdiffusion is the mutual diffusion of atoms at high temps between two contacting materials, leading to compositional changes at the interface.

Vacancy diffusion

Vacancy diffusion is the movement of atoms through a crystal lattice by hopping into adjacent vacant lattice sites.

Interstitial diffusion

Atoms move from one interstitial site to another, Small atoms carbon moving through iron for steel, Usually faster than vacancy diffusion.

Austenite

The structure from which other structures are formed when the material cools.

Ferrite

Structure of pure iron. Soft and ductile, imparts these properties to steel, Very little carbon under 0.01%.

Cementite

Iron carbide Fe3C, Hard and brittle, increases hardness and strength, reduces ductility and toughness.

pearlite

Laminated structure of alternate layers of ferrite and cementite, Combines hardness and strength of both.

Martensite

Very hard needle-like structure of iron and carbon only formed by very rapid cooling from the austenitic structure. exceptional’ strength and hardness.

Bainite

Formed from Austenite at lower temperatures. Needle like particles of cementite in a matrix of ferrite. Fine microstructure. High strength, hardness and ductility.

Sphereoidite

Forms if pearlite or bainite are heated to a temperature of about 700°C for 18-24 hours. soft, ductile, and weak.

Tempered martensite.

Tempering relieves internal stresses and reduces brittleness. Metal is heated and air cooled.

Annealing.

Annealing is a heat treatment that softens metals, relieves internal stresses, and improves ductility. It is achieved by heating the material to 30-60 degrees above eutectic temperature, holding it, and then cooling it slowly.

Normalising

Normalising is a heat treatment that refines grain structure, improves mechanical properties, and relieves stresses. It is achieved by heating the steel to about 30–50 °C above its upper critical temperature, holding, and then air cooling to room temperature.

Case hardening

Selectively harden the surface via heat treatment for tough outside and soft inside.

Selective hardening.

Improves Wear resistance in critical regions, toughness elsewhere. Achieved through Flame hardening, Induction hardening, Laser/electron beam hardening.

Chemical hardening

Creates Hard, wear-resistant case and tough core by chemical diffusion of elements into surface at high temp. Achieved with Carburising, Nitriding, Carbonitriding, Boriding.

Hot working.

Hot working improves ductility and refines grain structure by shaping metals above their recrystallisation temperature. It is achieved through processes like rolling, forging, or extrusion while the metal is heated so new grains form during deformation.

Cold Working

Cold working strengthens metals by increasing dislocation density, improving hardness and strength but reducing ductility. It is achieved by deforming the metal below its recrystallisation temperature through processes like rolling, drawing, or pressing.

Cold rolling.

Cold rolling increases strength, hardness, and surface finish by compressing metal sheets at room temperature. It is achieved by passing the metal between rollers below the recrystallisation temperature, causing plastic deformation and strain hardening.

Welding and its impact on microstructure

Welding joins metals by locally melting and fusing them, creating a strong joint. It impacts microstructure by forming distinct zones: a fusion zone with coarse grains, a heat-affected zone (HAZ) with altered grain structure, and the unaffected base metal, which can lead to residual stresses and changes in hardness or toughness.

polymerisation

Polymerisation is a chemical process that joins small monomer molecules into long-chain polymers, forming materials with higher molecular weight. It is achieved through reactions like addition or condensation, often initiated by heat, light, or catalysts.

Addition Polymerization

Addition polymerisation is a process where monomers with double bonds join together without forming by-products, creating a polymer chain. It is achieved by initiating the reaction with heat, light, or a catalyst that opens the double bonds.

Condensation Polymerization

Condensation polymerisation is a process where monomers join to form a polymer, releasing small molecules like water or methanol as by-products. It is achieved by reacting monomers with two functional groups, forming covalent bonds and eliminating the small molecules.

thermoplastics

Thermoplastics are polymers that soften when heated and harden when cooled, allowing repeated reshaping. They are made of linear or lightly branched chains, which slide past each other when heated without chemical change.

Thermosets

Thermosets are polymers that form a rigid, three-dimensional network when cured and cannot be remelted. They are produced by chemical reactions that create strong covalent cross-links, giving high heat and chemical resistance.