Metals (Phase Diagrams)

Classification of Metals

Ferrous (those based on iron)

Steels

Cast Irons

Nonferrous (all other metals)

Aluminium, magnesium, copper

Nickel, titanium, zinc, lead, tin

Molybdenum, tungsten, gold, silver, platinum and others

Superalloys

Metal and Alloys

An Alloy = A metal composed of two or more elements (at least one element is metallic)

Enhanced properties vs pure metals

Strength

Hardness

Corrosion resistance

Two main categories

Solid Solutions

Intermediate phases

Alloys

Intermediate phases

Inter-metallic Compound

Metallic Compounds

Solid Solutions

Substitutional

Interstitial

Solid Solutions

An alloy in which one element is dissolved in another to form a single-phase structure

Base element is metallic (solvent)

Dissolved element, metallic or non-metal

What is a phase? (in a material structure)

A phase = any homogeneous mass of material, such as a metal, in which the grains all have the same crystal lattice structure

Intermediate phases

there are usually limits to the solubility of one element in another

when the amount of the dissolving element in the alloy exceeds the solid solubility limit of the base metal, a second phase forms in the alloy

the term intermediate phase is used to describe it because its chemical composition is intermediate between the two pure elements

its crystalline structure is also different from those of the pure metals

Types of intermediate phases

Metallic Compounds
( consist of a metal and nonmetal, such as FE3C )

Intermetallic Compounds
( two metals that form a compound, such as Mg2Pb )

in some alloy compositions, the intermediate phase is mixed with the primary solid solution to form a two-phase structure

some two-phase alloys are important because they can be heat treated for much higher strength than solid solutions

Phase Diagrams

A phase diagram for an alloy system consisting of two elements at atmospheric pressure is called a binary phase diagram

Composition is plotted on the horizontal axis and temperature on the vertical axis

any point in the diagram indicates the overall composition and the phase or phases present at the given temperature under equilibrium conditions

Chemical Compositions of Phases

the overall composition of the alloy is given by its position along the horizontal axis

however, the compositions of liquid and solid phase are not the same

these compositions can be found by drawing a horizontal line at the temperature of interest

where the line intersects the solidus and liquidus indicates the compositions of solid and liquid phases , respectively,.. We use the Inverse Lever Rule to find the compositions