Understanding Acoustic Emission Testing- Reading 1 Part B-A

Martensitic Transformation: Mysterious Properties Explained
The difference between austenite and martensite is, in some ways, quite
small: while the unit cell of austenite is a perfect cube, in the transformation to
martensite this cube is distorted so that it's slightly longer than before in one
dimension and shorter in the other two. The mathematical description of the
two structures is quite different, for reasons of symmetry, but the chemical
bonding remains very similar. Unlike cementite, which has bonding
reminiscent of ceramic materials, the hardness of martensite is difficult to
explain in chemical terms. The explanation hinges on the crystal's subtle
change in dimension, and the speed of the martensitic transformation.
Austenite is transformed to martensite on quenching at approximately the
speed of sound - too fast for the carbon atoms to come out of solution in the
crystal lattice. The resulting distortion of the unit cell results in countless
lattice dislocations in each crystal, which consists of millions of unit cells.
These dislocations make the crystal structure extremely resistant to shear
stress - which means, simply that it can't be easily dented and scratched.
Picture the difference between shearing a deck of cards (no dislocations,
perfect layers of atoms) and shearing a brick wall (even without the mortar).
Charlie Chong/ Fion Zhang
http://www.threeplanes.net/martensite.html