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