Alloys

a combination of two or more metals or a metal(s) with a metalloid (Si, C)

• improved mechanical properties (EL, FS, hardness) • improved corrosion resistance • lower melting point than individual metal

• steel - burs, instruments • gold alloy - inlays, crowns, bridges, partial dentures, wires • cobalt chrome - partial dentures • nickel-titanium - wires • nickel chromium - crowns, bridges, wires • amalgam - restorative material

physically distinct homogenous structure (can have more than one somponent) individual grains of metal A and B - two phases

a homogenous mixture at an atomic scale metals A and B in a homogenous mixture - one phase - solid solution

• be insoluble - no common lattice and exists as two separate phases • form an intermetallic compound with specific chemical formulation (e.g. Ag3Sn) • be soluble and form a solid solution - common lattice and exist as one phase

atoms of one metal replace the other metal in the crystal lattice/grain in a: * random way - requires atoms to be similar in size, valency and crystal structure * ordered way - in a regular lattice arrangement with similar size, valency and crystal structure

atoms of remarkedly different sizes with smaller atoms located in spaces in the lattice/grain structure of larger atoms

• the alloy is cooled from its molten state where both metals are not in a lattice • crystallisation begins at TL • the first few metal atoms are cooled to form nuclei of crystallisation • the two metals have different melting points and crystallise when these are reached • there is a change of temperature when the alloy crystallises from TL to TS

• metals crystallise at one temperature • alloys crystallise over a temperatre range

• the TL and TS values for each alloy are different • if you change the ratio of alloys, it produces different cooling curves due to their different melting points • platinum has a higher melting point than gold

• allows metal atoms to diffuse through lattice and find their equilibrium position • ensures the grain composition is homogenous • BUT results in large grains forming - undesirable

• each point on the liquidus represents the temperature that crystallisation started for the alloy • each point on the solidus represents the temperature that crystallisation ends for the alloy • liquidus: alloy in liquid state • solidus: alloy in solid state • between the curves, the solid is partly liquid and partly solid

• ensures grains are small and that there are many grain boundaries which will impede dislocations • alloy with composition X begins to crystalise at TL and a tie line is drawn horizontally and vertically to find the first grain composition (85%A, 15%B) • when the temperature is dropped, the grain formed is composed of 75%A and 25%B • when the temperature is dropped again, the grain formed is composed of 65%A and 35%B • the temperature of the alloy X cools to the temperature of the solidus and the last grain to form is composed of 20%A and 80%B • rapid cooling/quenching results in non-homogeneous grains

• prevents atoms diffusing through lattice • causes coring as composition varies throughout grain • improves mechanical properties • may reduce corrosion resistance of solid form of alloy

• fast cooling of liquid state (drop into cold water) • liquidus and solidus must be separated and determines extent of coring

**homogenising anneal ** * reheat the non-homogenous solid to allow the metal atoms to diffuse and cause the grain composition to overcome the concentration gradient and homogenise * keep the alloy below its recrystallisation temperature, otherwise its grains will be altered

• when alloys that form a solid solution consist of two or more metals of different atomic sizes arrange in a lattice, they have a distorted grain • the distorted lattice impedes dislocation movement so improves the mechanical properties (EL, UTS, hardness) • heat alloy below its recrystallisation temperature and cool slowly

• metal - defects roll over atoms with little energy to move along the slip plane until grain boundary • alloy - defects fall into spaces between large and small atom, requiring more energy to overcome different sized atoms until grain boundary (more fracture resistant as more energy needed)

• alloys that form a ordered solid solution have metal atoms distributed in a regular lattice but still have a distorted grain structure as lattice impedes dislocation movement • the resistance to dislocation gives improved mechanical properties (EL, UTS, hardness) • heat alloy below its recrystallisation temperature and cool slowly

• the liquidus and solidus are quite close together so the extent of the coring is limited • not much of a concentration gradient when rapidly cooled (quenched) • the area under the graph shows that there are two possible ordered solutions - AuCu3 and AuCu • these ordered solutions need to be annealed to remove coring • heat alloy below its recrystallisation temperature (424 degrees) and cool slowly

a homogeneous mixture of two or more substances, that melts or solidifies at a single, sharp temperature, lower than the melting point of any individual constituent

• liquidus is defined by point a, c, e • solidus is defined by point b, c, d • liquidus and solidus coincide at point c which is the eutectic point/composition where the crystallisation process occurs at a single temperature and grains of individual metals are formed simultaneously • on cooling rapidly, a eutectic alloy will form grains of individual metals (non-homogenous) in 2 phases

soldering parts together