Question

The following elements forms an isomorphous phase diagram

• A. Cu-Ni
• B. Cu-Sn
• C. Pb-Sn
• D. Ni-Zn

Hint:

Copper-Nickel (monel/cupronickel) is the most famous example of a binary isomorphous system. Another common example is the Gold-Silver (Au-Ag) system.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks to identify which alloy system forms a completely isomorphous binary phase diagram.
An isomorphous system is one in which the two components are completely soluble in each other in both the liquid and solid states across the entire composition range.

Step 2: Detailed Explanation:

• Criteria for Solid Solubility: According to the Hume-Rothery rules, complete solid solubility between two elements requires:
1. The atomic radius difference must be less than \( 15\% \).
2. They must have the same crystal structure.
3. They should have similar electronegativities.
4. They must have similar valencies.

• The Copper-Nickel (Cu-Ni) System:
- Both Copper (Cu) and Nickel (Ni) have an FCC crystal structure.
- Their atomic radii are very similar (\( r_{\text{Cu}} = 0.128 \text{ nm} \) and \( r_{\text{Ni}} = 0.125 \text{ nm} \), a difference of only \( \sim 2.3\% \)).
- Their electronegativities and valencies are also very close.
- Consequently, the Cu-Ni system satisfies all Hume-Rothery rules and forms a classic, lens-shaped isomorphous phase diagram consisting of only liquid (\( L \)), solid (\( \alpha \)), and two-phase (\( L+\alpha \)) fields.

• Other Systems:
- Cu-Sn (Option B) and Ni-Zn (Option D) form complex phase diagrams with several intermetallic phases.
- Pb-Sn (Option C) is a classic eutectic system with limited solid solubility.


Step 3: Final Answer:
Therefore, the Cu-Ni system forms an isomorphous phase diagram, matching Option (A).