Question

The characteristics of martensite formation does not include

• A. Athermal transformation
• B. No change in chemical composition
• C. Nucleation and growth process
• D. Martensite formation occurs in a range of temperature

Hint:

Remember that martensite formation is a military transformation (atoms move in a disciplined, cooperative shear), while pearlite/bainite formation is a civilian transformation (atoms move individually via diffusion, i.e., nucleation and growth).

Answer 1

The Correct Option is C

Step 1: Recall the defining feature of martensite formation.
Martensite forms by a cooperative, almost instantaneous shear of the parent lattice, atoms shift by less than one atomic spacing relative to their neighbors, and there is no long range diffusion of carbon or iron atoms at all.
Step 2: Contrast with a genuinely diffusional mechanism.
A classic nucleation and growth process, the kind seen in pearlite formation, needs small stable nuclei of the new phase to first appear and then slowly grow larger as atoms diffuse toward them over time. This is inherently time dependent, held longer at a given temperature, more of the transformation completes.
Step 3: Testing each statement against martensite's real behaviour.
Martensite is athermal, its fraction depends only on how far below $M_s$ you cool, matching statement (A). Because there is no diffusion, the martensite plate has exactly the same composition as the parent austenite, matching statement (B). It also forms progressively over a temperature span from $M_s$ down to $M_f$, matching statement (D). But it certainly does not grow slowly over time at a fixed temperature the way a nucleation and growth product would; individual plates instead form in fractions of a microsecond.
Step 4: Picking the odd one out.
Since "nucleation and growth" describes a time dependent, diffusion driven mechanism that is the opposite of how martensite actually forms, it is the one characteristic that does NOT belong to martensite formation.
\[ \boxed{\text{Nucleation and growth process}} \]