martensite: a standalone construct

E. C. Bain's 1924 study The nature of martensite gave the first crystallographic account of the hard constituent that forms when steel is cooled so rapidly (quenched) that its carbon atoms cannot move: he showed the iron lattice is frozen into a body-centered tetragonal form, a slightly stretched version of the ordinary body-centered cubic structure, with the carbon trapped in place. The constituent had been named around 1895 by the French metallurgist Floris Osmond in honor of Adolf Martens, a German pioneer of iron and steel microscopy, and the name has marked this phase ever since. Christian, Olson, and Cohen (1995) set the general meaning now used across metallurgy: a martensitic transformation is diffusionless and displacive, meaning the crystal changes structure by a coordinated shear of the parent lattice, accompanied by a change of shape, rather than by atoms diffusing to new positions, so the product keeps the exact composition of the phase it came from. Olson and Cohen (1976) modeled how such a transformation begins, identifying the lattice defects at which the new phase first nucleates. What makes martensite a landmark is its reach: the same diffusionless, shear-based mechanism that hardens quenched steel also defines a whole class of transformations seen in shape-memory alloys, ceramics, and other metals, so the word names both a specific phase in steel and a general mode of solid-state change.[1][2][3]

Unit

A crystalline phase of a metal, one microstructural constituent of steel and the product of a broader class of diffusionless transformations

Recovery means

Largely irreversible in steel and softened by tempering; in some alloys the transformation reverses on heating, the basis of the shape-memory effect

Measured by

Identified by optical and electron microscopy of its lath or plate structure, and confirmed by X-ray or electron diffraction of its body-centered tetragonal lattice and by hardness testing