CHE 333 Class 4. Phase Diagrams. Why Phases?. Few materials used in pure state – gold, copper, platinum etc for electrical properties or coatings.
properties or coatings.
zinc in a binary two component system. Another is steel which is a binary system of carbon and iron.
three and four elements or components in them. Best example of a
ternary system is stainless steels, containing iron, chromium and nickel.
the chemical and physical properties.
for example casting, soldering, hardened
steel, injection molding, slip casting.
Phase Changes – Pure copper goes from solid to liquid at its melting point of 1084C. Pure
nickel goes from solid to liquid at its melting point of 1455C. These are singular points in that
all solid goes to all liquid at a unique temperature. Therefore on the phase diagram they can be
represented by a single point. What happens in between?
Add copper to nickel, say 47 gms of copper and 53 gms of nickel. This is then the “average composition”.
If this block of material is analyzed it will show 47 gms of copper and 53 gms of nickel. What happens on
heating . The phase diagram is a map of the phases present. Up to 1275C solid solution a is present
which is a mixture of copper in nickel. At 1275 the dashed line showing the composition intersects a
curved line joining the two single melting points. This line is called the solidus line and represents a phase
boundary where solid a changes to two phases of solid a +liquid. Some of the a has melted. Below the
solidus only one phase is present so the copper is completely soluble in the nickel or complete solid solubility
Consider a 90%Pb10%Sn average composition at 330C, it will be all liquid. At 300C it will have crossed
the liquidus and so will be a mixture of solid and liquid. At 275 it will cross the solidus and become
a single phase a solid. As the temperature decreases, another phase boundary occurs, the solvus
where a second new solid phase forms, so at room temperature two solid phases, a and b exist. In
This case, tin has limited solid solubilty in lead at room temperature as shown by the presence of the
Consider the composition of 61.9% Sn38.9% Pb At 250c it is all liquid and at 182C it is all solid
but with two phases, a and b, present. At 183C all the liquid goes to all solid, but the solid is in two
phases. This is called a “Eutectic” reaction. Liquid Solid 1 + Solid 2.
Consider composition 60%Pb 40% Sn. At temperature a, it is all solid; at b some solid will form
and at c it is clearly in a two phase solid a + liquid region. The solid will have a composition
which follows the solidus while the liquid will have a composition that follows the liquidus. So at
183C any liquid remaining will have the eutectic composition and will become solid of two phases
a and b. On further cooling the solid a and b will follow the solvus lines for their phases for composition.
At room temperature there will be two phases, a and b.