Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 8M

Year: Dec 2014

Eutectoid phase diagram

1. Transformation from solid to solid state for Fe-C alloys. Allotropic transformation of γ-iron to α-iron, as well as decomposition of austenite into ferrite & cementite (cementite precipitates as an excess carbon
2. Line ‘GS’ indicates beginning of the decomposition of austenite and precipitation of ferrite
3. Critical points along line ‘GS’ are designated as Ac3 in heating and Ar3 in cooling
4. Line ‘SE’ indicates precipitation of excess carbon as cementite, known as secondary / pre-eutectoid cementite
5. Temperatures along line ‘SE’ are designated as Acm points
6. Line ‘SE’ indicates precipitation of excess carbon as cementite, known as secondary / pre-eutectoid cementite, which indicates reduction in the solubility of carbon in -iron (austenite) with decrease in temperature
7. Point ‘S’ corresponds to minimum temp. of 723 OC with 0.8 % carbon at which austenite exists in a state of equilibrium
8. Point ‘S’ is also referred as ‘Eutectoid’ point with Eutectoid Temp. of 723 OC & Eutectoid Comp. of 0.8 % carbon
9. At this point, austenite decomposes with simultaneous precipitation of ferrite and cementite (secondary cementite), i.e. eutectoid mixture known as Pearlite

10. Eutectoid Transformation / Reaction:

    γ(0.76 wt% C)

    ↓

    α (0.022 wt% C)

    +

    Fe3C

Isomorphous phase diagram

This figure contains the copper-nickel phase diagram. Its system is termed as being isomorphus.

1. A good interpretation of a binary phase diagram that is easy to understand and interpret is the Cu-Ni system.
2. This diagram has three different phase regions, the alpha region, the liquid region, and the alpha + liquid region, which are defined by specific compositions and temperatures as illustrated.
3. Both points A and B are located in the alpha and the alpha + liquid regions respectively. The phase boundaries are separated by two lines.
4. The line separating the liquid and the alpha + liquid regions is the liquidous line. The line separating the alpha and the alpha + liquid regions is the solidous line.
5. The intersection of these two lines signifies the melting temperatures of the two constituents individually.
6. The Cu-Ni system is especially noted for its complete liquid and solid solubility of its constituents, and is thus identified as an isomorphous system.