Fine structure of austenite produced by the reverse martensitic transformation

Abstract

The fine structure introduced into austenite by the reverse martensitic transformation was investigated by transmission electron microscopy in an iron-33.5 w/o nickel alloy. Direct martensitic transformation induced by cooling to − 195°C produces finely twinned martensite surrounded by retained austenite containing relatively uncomplicated dislocation arrangements. On rapid heating to 450°C, the reverse transformation replaces these structures with tangled and jogged dislocations in concentrations up to 10 11 per cm 2, and thereby accounts for the marked strength increases observed in reversed austenite. In some previously martensitic areas of the reversed austenite, twins up to 5 microns in width are observed with standard metallographic techniques. Selected-area electron diffraction confirms the twinned nature of these structures, and it is found that the twins are quite imperfect and contain a high concentration of tangled dislocations.