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

Alloys that respond to heat treatment are ones where a second phase can be dissolved into solution on heating and then coaxed back out as fine precipitates on cooling and ageing, duralumin, beryllium copper and the 7000 series aluminium alloys all behave this way. 70-30 brass is different because zinc stays fully dissolved inside the copper rich matrix across its whole practical temperature range at that composition, there is no second phase waiting to come out. Heating and cooling it will not create hardening particles, at best it just recrystallises, which softens the metal rather than strengthening it. The only real way to toughen this brass is by cold working. So the alloy that cannot be strengthened by heat treatment is 70-30 brass, option (C).