The kinetic behavior of a Fe–24 Ni–0.4 C (weight percent) martensitic steel during aging at room temperature has been investigated by transmission electron microscopy. Electron diffraction, coupled with imaging techniques in a transmission electron microscope, including high resolution studies, have been used at various stages during the aging process, in order to characterize the microstructural features of the analyzed samples.

Rapid cooling (quenching) in liquid nitrogen of the austenitic state of the above-mentioned material leads to the formation of a martensitic phase which, at room temperature (RT), begins to transform, through a process called spinodal decomposition. As a result of this process, a modulated structure is formed, in which carbon-rich regions (precipitates) occur in a periodic manner throughout the matrix, leading to the presence of diffuse streaks or satellite spots around each fundamental (matrix) reflection on the electron diffraction patterns, see Figure1. This process is thus accompanied by a reduction of the tetragonality of the martensitic phase (bct), which evolves towards the formation of a cubic structure (bcc), see Figure2, corresponding to alpha-iron (ferrite). After long ageing times, Fe3C is observed. The presence and structure of intermediate carbides is also studied.  

The microstructure of the carbon-rich phase is also analyzed by electron diffraction. The evolution of the tetragonality with time, translated as the c/a ratio, which is also function of the carbon composition, is studied. Moreover, the variation of the distance between the carbon-rich precipitates (in fact, the periodicity of the modulated structure), along with the evolution of their width and length, are also observed.

Thanks are due to the Clym ( Centre Lyonnais de Microscopie) for the access to the TEM 2010F.

Figures:

TEM bright field images showing the decomposed modulated structure of martensite after ageing at RT for: a) 46 hours and b) 142 hours.

Evolution of the c/a ratio of the bct phase with time, indicating a continuous transformation of the bct phase to a bcc phase (ferrite).

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EMC Abstracts - https://emc-proceedings.com/abstract/kinetic-behavior-of-fe-ni-c-martensitic-steels-during-aging-at-room-temperature/