Metals & Alloys 2

a combination/mixture of • two or more metals • a metal/s with a metallloid (Si, C)

Advantages: improved mechanical (FS, EL, hardness) properties, corrosion resistance, lower melting point Dental uses: steels, gold, cobalt-chromium, nickel-titanium, nickel-chromium, amalgam

Phase: physically distinct, homogeneous structure that can be composed of one or more components. Solution: homogeneous mixture at atomic scale

Homogeneous mixture of metals A and B in each grain = one phase (solid solution)

two phases

Solubility refers to the ability of metallic elements in an alloy to dissolve into each other in the solid state. There are three cases: Complete solid solubility - Forming a single phase solid solution (metals can completely dissolve into each other in the solid state) No solid solubility - The metals are completely insoluble in each other in the solid state. Each metal forms its own distinct grains/phases (euctetic) Partial solid solubility - Two solid solution phases form, with solubility limits in solid state (amalgam)

when they are molten

Atoms of one metal replace atoms of the other metal in the crystal lattice/grain Can be: a) Random - Metal atoms are similar in size, valency, crystal structure (FCC) but not in consistent locations b) Ordered - Metal atoms are arranged in a regular pattern in the lattice, with uniform gaps

Smaller atoms fit into the spaces/interstices in the lattice of larger atoms Requires metals with significantly different atomic sizes

When a pure molten metal is cooled, it crystallizes at a single temperature The cooling curve shows a flat arrest during crystallization as heat is released

For alloys, crystallization occurs over a temperature range rather than a single point The cooling curve shows two arrests between which both liquid and solid phases coexist

For a solid solution alloy, both metals crystallize together forming a homogeneous solid solution phase For an insoluble alloy system like a eutectic, each metal forms separate solid grains as it crystallizes The proximity of TL and TS depends on the solid solubility - they converge for insoluble systems like eutectics

Liquidus temperature (TL) - The temperature at which crystallization begins Solidus temperature (TS) - The temperature at which crystallization completes

allows metal atoms to diffuse through lattice ENSURES grain composition is homogeneous BUT this results in LARGE GRAINS

generates MANY SMALL grains which impede dislocation movement, improving its MECHANICAL PROPERTIES prevents atoms diffusing through lattice causing CORING (composition varies throughout grain.)

fast cooling of liquid state; LIQUIDUS and SOLIDUS must be separated (see Phase Diagram), larger separation = more coring reduces corrosion resistance of solid alloy

once solid cored alloy formed REHEAT to allow atoms to diffuse and so cause grain composition to become homogeneous NOTE: keep below recrystallisation temperature, otherwise grains altered resolves coring

distorted grain structure which IMPEDES dislocation movement and so improves mechanical properties (EL, UTS, hardness) defect does not “roll” over the lattice plane. Instead it falls into the spaces between the large & small atom. More energy/force is needed to move any dislocations present in a solid solution making alloys inherently more fracture resistant (ie stronger) than a metal with better mechanical properties

Defect “rolls” over the atoms in the lattice plane, little energy/force is needed for defect to move along slip plane. easier to move dislocation (less mechanical properties)

distorted grain structure atoms are arranged without any particular order. Dislocations can move more easily through this random arrangement because there are no consistent barriers to their motion. Dislocations can glide more freely between the atoms without needing to overcome specific obstacles.

Metals insoluble in solid state, form separate grain phases. Crysalisation process occurs at single temperatue Eutectic composition has lowest melting point Hard but brittle, poor corrosion resistance

• Partially soluble Alloys always produce two distinct grains, however, both grains will have both metals, but one grain will be richer in one metal and vice versa for the other grain. • Annealing of a Partially soluble alloy will make the alloy undergo Precipitation Hardening - which makes the alloy stronger and its surface harder.