Formation of eta carbide in ferrous martensite by room temperature aging

For several decades, the formation of carbon(C)-rich domains upon room temperature aging of supersaturated martensite has been a matter of debate. C-rich tweed-like patterns are observed to form after short aging times at room temperature and coarsen upon further aging. Here, we present a systematic atomic-scale investigation of carbide formation in Fe-15Ni-1C (wt.%) martensite after two to three years of isothermal room temperature aging by a combination of atom probe tomography and transmission electron microscopy. Owing to the sub-zero martensite start temperature of -25 degrees C, a fully austenitic microstructure is maintained at room temperature and the martensitic phase transformation is initiated during quenching in liquid nitrogen. In this way, any diffusion and redistribution of C in martensite is suppressed until heating up the specimen and holding it at room temperature. The microstructural changes that accompany the rearrangement of C atoms have been systematically investigated under controlled isothermal conditions. Our results show that after prolonged room temperature aging nanometer-sized, plate-shaped eta-Fe2C carbides form with a macroscopic martensite habit plane close to {521}. The orientation relationship between the eta-Fe2C carbides and the parent martensite grain (alpha') follows [001](x')//[001](eta), ((1) over bar 10) alpha'//(020)(eta). The observation of eta-Fe2C-carbide formation at room temperature is particularly interesting, as transition carbides have so far only been reported to form above 100 degrees C. After three years of room temperature aging a depletion of Fe is observed in the eta carbide while Ni remains distributed homogenously. This implies that the substitutional element Fe can diffuse several nanometers in martensite at room temperature within three years. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.