Question

In precipitation hardenable alloy, like duralumin, intermediate precipitates can form due to:

• A. Coherency strains
• B. Ease of diffusion
• C. Difficulty of the growth of the final precipitate
• D. Difficulty of nucleation of the final precipitate

Hint:

Coherency strains in a supersaturated solid solution drive the formation of intermediate precipitates, enhancing age-hardening effectiveness.

Answer 1

The Correct Option is A

Step 1: Understanding precipitation hardening Precipitation hardening involves: - Solution treatment, - Quenching, - Aging (to form fine precipitates that hinder dislocation motion). Step 2: Role of intermediate precipitates In alloys like duralumin (Al-Cu), intermediate precipitates such as $\theta''$ and $\theta'$ form before the stable $\theta$ phase. These intermediate precipitates: - Retain coherent or semi-coherent interfaces with the matrix, - Cause coherency strains due to the mismatch in atomic sizes and lattice parameters between the matrix and the precipitate. Step 3: Impact of coherency strains These strains: - Increase the driving force for nucleation of intermediate precipitates, - Make intermediate precipitates thermodynamically favorable, - Provide effective strengthening due to lattice distortion. Hence, the correct answer is Coherency strains.