Question
With increase in carbon content (up to 2 mass%) in Fe-C alloy, which one of the following statements is correct with respect to the lattice parameters ($c$ and $a$) of BCT martensite?
The correct answer is
$c$ increases but $a$ decreases
Fe-C Martensite Lattice Parameters Explained
Martensite formed in Fe-C alloys possesses a Body-Centered Tetragonal (BCT) crystal structure.
The distortion from the original Body-Centered Cubic (BCC) structure of iron is primarily caused by interstitial carbon atoms incorporated into the lattice.
Effect of Carbon Content on BCT Lattice Parameters
- As the carbon content increases in the Fe-C alloy (up to the specified limit of 2 mass%), more carbon atoms occupy interstitial sites within the martensite lattice.
- These interstitial carbon atoms cause a significant distortion. The lattice elongates preferentially along the $c$-axis. Consequently, the lattice parameter $c$ increases.
- Simultaneously, this distortion leads to a contraction along the axes in the basal plane ($a$-axis). Hence, the lattice parameter $a$ decreases.
Therefore, with increased carbon content in Fe-C alloys, the BCT martensite structure exhibits an increase in parameter $c$ and a decrease in parameter $a$.
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