Question

For a diffusional transformation (i.e., growth of \( \beta \) precipitates in an \( \alpha \) matrix), which of the following is/are true with increasing degree of undercooling?

• A. Rate of transformation first increases and then decreases
• B. Rate of transformation first decreases and then increases
• C. Thermodynamic driving force increases monotonically
• D. Mobility of atoms in \( \alpha \) matrix remains unchanged

Hint:

Diffusional transformations balance two factors: increasing driving force and decreasing diffusion rate. Always consider both when analyzing transformation kinetics.

Answer 1

The Correct Option is A, C

For diffusional phase transformations, the kinetics depend on two competing factors: the thermodynamic driving force and atomic mobility.
1. Driving Force: As undercooling increases, the free energy difference between phases increases. Thus, the thermodynamic driving force for transformation \textit{always increases monotonically}. This makes option (C) correct.
2. Diffusion Mobility: As temperature decreases due to undercooling, atomic mobility reduces because diffusion slows down exponentially. Initially, undercooling increases the driving force significantly, so the transformation rate rises. But at high undercooling, diffusion becomes too slow, reducing the transformation rate. Hence, the rate first increases and then decreases. This makes option (A) correct.
3. Mobility of atoms: Mobility does not remain constant—it decreases significantly with undercooling. Thus, (D) is incorrect.
4. Option (B): The rate does not first decrease; it increases first. Hence (B) is incorrect.