Aging of freshly formed Fe-based martensites at low temperatures

Aging of Fe-based martensites at subambient temperatures is studied by means of neutron and X-ray diffraction, Moessbauer spectroscopy, positron-lifetime, internal friction, electrical resistivity, magnetic susceptibility, and dilatometric measurements. Abnormally high or abnormally low tetragonality is observed in the freshly formed martensite alloyed with Ni or Mn in accordance with previous studies. It is shown that the high tetragonality is the result of stresses arising from the coherency at the interface between virgin martensite and retained austenite. The coherency is broken during aging in the temperature range of 100 to 200 K, and it is accompanied by a decrease of tetragonality. The new internal friction peak centered at 145 K corresponds to the movement of the coherent interfaces and the break of coherency. Correlation between short-range atomic ordering in austenite and the high tetragonality of the virgin martensite is evident. It is shown that aging at temperatures from about 170 to 270 K, defined as the second stage of aging, is controlled by the pinning of dislocations by C atoms and the third stage (mainly above 250 K) by the clustering of C atoms in a solid solution. By means of Moessbauer spectroscopy, electrical resistivity, and magnetic susceptibility measurements, a striking difference is shown between the redistribution of C and N atoms above 200 K. Evidence for the clustering of C and ordering of N is given and discussed.