Question

Which of the following alloys cannot be strengthened by heat treatment?

• A. Al-Cu-Mg
• B. Cu-Be
• C. 70-30 Brass (Cu-Zn)
• D. Al-Zn-Mg-Cu

Hint:

For an alloy to be strengthenable by precipitation hardening (aging), it must exhibit a decreasing solid-solubility limit with decreasing temperature on its phase diagram.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks to identify the alloy among the choices that cannot be strengthened by thermal processing (heat treatment), such as precipitation hardening or martensitic transformation.

Step 2: Detailed Explanation:

• Precipitation Hardening Alloys: Alloys like Al-Cu-Mg (Option A, duralumin), Cu-Be (Option B, beryllium copper), and Al-Zn-Mg-Cu (Option D, 7000-series aerospace aluminium) are classic age-hardenable alloys.
These alloys are strengthened by solutionizing (heating to form a single-phase solid solution), quenching to form a supersaturated solid solution, and aging (heating to precipitate fine, coherent secondary phases that block dislocation motion).

• Structure of 70-30 Brass: 70-30 Brass (Option C, cartridge brass) is a single-phase alpha brass containing \( 70\% \) Copper and \( 30\% \) Zinc.
The copper-zinc binary phase diagram shows that zinc remains completely dissolved in the alpha (fcc) solid solution at all temperatures below \( 400^\circ\text{C} \) for this composition.
Since there is no change in solid solubility with decreasing temperature, second-phase precipitation cannot occur.
Furthermore, brass does not undergo any shear-induced martensitic phase transformations.

• Strengthening of Brass: Consequently, single-phase 70-30 brass can only be strengthened by cold working (strain hardening) or solid-solution strengthening.
Heat treatment of cold-worked brass only causes recovery, recrystallization, and grain growth, which softens the material rather than strengthening it.


Step 3: Final Answer:
Thus, 70-30 Brass (Cu-Zn) cannot be strengthened by heat treatment, corresponding to Option (C).