Materials Science II

Phase Diagrams

1. The silver copper system has a eutectic at 71.9wt% silver. At the eutectic temperature   (780°C) 7.9wt% silver dissolves in copper and 8.8wt% copper dissolves in silver whereas at room temperature the solubilities are 2.2wt% and 1.2wt% respectively. The melting points of pure copper and silver are 1080° C and 950° C respectively. Sketch the phase diagram.
2. Mark on the phase diagram from question 1 the lines corresponding to liquidus, solidus and solvus.
3. An alloy containing 30% silver and 70% copper is slowly heated from room temperature. Describe the microstructural changes which occur answering the following questions.

   (a) At what temperature does the first liquid phase form?

   (b) What is the composition of this liquid phase?

   (c) At what temperature does complete melting of the alloy occur?

   (d) What is the composition of the last solid remaining prior to complete melting?

4. What is meant by a eutectic reaction? Is it described by a line, a point or an area on the phase diagram?
5. A 50wt%Ni-50wt% copper alloy is slowly cooled from 1400° C. Describe the development of microstructure on cooling answering the following questions.

   (a) At what temperature does the first solid phase form?

   (b) What is the composition of this solid phase?

   (c) What are the proportions of solid and liquid at 1300° C

6. For a lead-tin alloy of composition 80wt% Sn-20wt% Pb and at 180° C determine the following:-

   (a) The mass fractions of the a and b phase

   (b) The mass fractions of primary b and eutectic mixture

   (c) The mass fraction of b in the eutectic mixture.

 [All the necessary phase diagrams are on HO2]