Aging of Iron-Based Martensites at Low Temperatures

In the present work aging of iron-based martensites, defined as all the phenomena preceding the first stage of tempering, is studied using various methods. In some alloys aging was observed to start at very low temperatures, even near 4 K. Abnormally high or abnormally low tetragonality is observed in freshly formed martensite alloyed with Ni or Mn, respectively. The reason for high tetragonality obviously lies in the coherency at the interface between the martensite and the retained austenite. It is shown in the present work that the coherency is broken during aging in the temperature range of 100 - 200 K and it is accompanied by a decrease of tetragonality (first stage of aging). The new internal friction peak centered at 145 K corresponds to the movement of the coherent interface and the break of coherency. Some evidence for correlation between short range atomic ordering in austenite and the high tetragonality of the virgin martensite is given and a contribution of nontransformed ordered austenitic regions inside martensite plates to tetragonality is discussed. It is shown that the second stage of aging at temperatures about 200 - 270 K is controlled by the pinning of the dislocations by carbon atoms and the third stage (mainly above 250 K) by the clustering of carbon atoms in a solid solution. By means of Mössbauer spectroscopy, electrical resistivity and magnetic susceptibility measurements, a striking difference is shown between the redistribution of carbon and nitrogen atoms above 200 K. Evidence for the clustering of carbon and ordering of nitrogen is given and discussed. The dynamics of clustering was studied by means of small angle neutron scattering. Deformation of martensite at subambient temperatures reduced the tetragonality of martensite to cubic and changed the redistribution of carbon atoms. According to different measurements, practically no aging was observed below room temperature in the deformed martensite