Metals & Alloys

Example 1: palladium – silver (casting alloy) A multicomponent solid or liquid material whose primary component is a metal. Example 1: palladium – silver (casting alloy) Example 2: Fe – Cr – C (stainless steel) Example 3: Ag – Cu (dental solders)

Metals & alloys – crystalline in the solid Liquid metals and alloys are amorphous. In most circumstances it is not possible to solidify the liquid metal fast enough to suppress crystallization in the solid. Metals are usually polycrystalline solids.

Unit Cell: Parallelepiped element of a crystal structure, containing atoms within it, that when repeated through space will build up a crystal.

Only 14 types of unit cells are found in nature Most useful metals crystallize in one of just three crystal structures.

face-centered cubic unit cell body-centered cubic unit cell

FCC BCC ( Au, Ag, Cu, Ni ) ( Fe, Cr, W, β - Ti )

Hexagonal close-packed unit cell - HCP α - Ti, Co, Zn

metals & alloys - phases A phase is a volume of material that has the same structure, composition, and properties throughout. In principle, phases can be physically separated from one another.

Phase Diagram: A graphical representation of the pressures, temperatures, and compositions at which various phases are observed.

Equilibrium Phase Diagram: A phase diagram that shows the pressures, temperatures, and compositions at which various phases are observed when the system is at equilibrium. At equilibrium, all reactions have ceased and no further changes will occur with time.

Metallurgical Phase Diagrams: Usually only two components (binary) Atmospheric pressure is assumed X-axis is composition (wt. % usual; atomic % sometimes) Y-axis is temperature

Alloys: Metals melts at a single temperature. Alloys usually melt over a range of temperatures.

all other compositions melt over a range of temperatures Pure Pd melts at a single temperature. all other compositions melt over a range of temperatures liquid + solid Pure Ag melts at a single temperature.

metal crystals liquid metal

A polycrystalline material: Since crystals are randomly oriented with respect to one another when they nucleate in the liquid, they remain randomly oriented after they grow together to form a solid polycrystalline material. grain boundary – the surface that separates two grains grain – a single crystal within a polycrystalline material

Solid Solution: A single solid phase made up of two or more components. A solid solution alloy is an alloy which is a solid solution at all compositions.

liquid + solid Note: Ag-Pd is an example of an alloy system that is a solid solution at all compositions. That is, Ag-Pd is a “solid solution” alloy.

Approximately 10% Ag dissolved in Pd Palladium Silver Approximately 10% Ag dissolved in Pd

Solute: The component or components of a solution that are present in the lesser amount. The solute is dissolved in the solvent.

Solvent: The component of a solution that are present in over 50% of the solution amount. The solute is dissolved in the solvent.

Approximately 50% Ag dissolved in Pd Palladium Silver Approximately 50% Ag dissolved in Pd

Solid solutions: conditions that favor: similar atomic sizes similar electronegativities similar valences similar crystal structures

Within this region two solid phases co-exist, α Ag and α Cu. more α Ag less α Ag

1 2 3 β α α + β Temperature % B 1 2 3

Precipitation hardening fast cooling 2 - supersaturated x1 α α + β Temperature x2 3 - heating heating β x3 % B

The End