Displacements caused by martensitic transformation - hot stage microscopy

Source: Wirtschaftsvereinigung Stahl, Düsseldorf.
Hot-stage optical microscopy showing the displacements due to the disciplined motion of atoms during the athermal formation of martensite during the cooling of a steel. The commentary is in German, but translates roughly as follows:

The polished surface of a steel can be observed during cooling, in a microscope with a hot-stage. The sample is austenitised and then cooled. The polished surface is altered by the formation of martensite plates. At 380 C, only the austenite grain boundaries are visible due to thermal grooving, but the surface starts to change at about 360 C. Individual plates form at the speed of sound in the metal, and the quantity of martensite only increases with undercooling. There is no isothermal transformation in this case.

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