Martensite crystal structure of nickel steel at cryogenic temperatures

The crystalline structure changes of fresh martensite (FM), formed from the austenite phase in Fe–25.3 wt.% Ni–0.5 wt.% C steel single crystals after successive γ–α–γ transformations, have been studied by X-ray diffraction at low temperatures. Diffusion-mediated redistribution of carbon within the α-crystal lattice and loss of carbon out of it are the driving processes which determine the structural change of FM at cryogenic temperatures. The decomposition of transformation-hardened martensite at low temperatures can be described in terms of coherent stress relaxation and carbon reordering in the crystal lattice.