An interpretation of the carbon redistribution Process during Aging of High Carbon Martensite

Abstract

Aging of high carbon martensite in the range from room temperature to 360 K often produces the tweedlike contrasts in transmission electron microscopy (TEM) and the diffuse streaks on electron diffraction spots in four equivalent 〈0l2〉 directions. These phenomena have been interpreted so far in terms of the carbon-modulated planar structure formation normal to {012} planes,i.e., the spinodal decomposition of martensite. The close examination of TEM images and the corresponding selected area electron diffraction patterns, however, indicates that the tweedlike contrasts are definitely not those of the {012} planes but of the lines parallel to the 〈l21〉 directions. This suggests that the parallel lines contrasts at about 1-nm spacing arise from the dislocations which were introduced during martensitic transformation, rearranged by glide motion into energetically more stable configuration, and decorated by carbon atoms. The density of these line defects estimated from the image contrasts are in the range between 10¹² and 5 × 10¹² cm^-2. A model explaining the diffuse streaks is also proposed.