Abstract
Evidence for a coherent bond at the interfaces between retained austenite and as-quenched martensite in FeNiC alloys and the relation of coherent interfaces to the abnormally high tetragonality of martensite are presented. The iron based alloys with 20 Ni-0.73 C, 28 Ni-0.2 C, 20 Ni-1.2 C, 25 Ni-0.7 C, 30 Ni-0.37 C, 33.5 Ni-0.01 C (in wt%) were studied by means of X-ray and neutron diffraction, Mössbauer spectroscopy, internal friction, electrical resistivity and magnetic susceptibility. It is shown that the break of coherency occurs during heating of the freshly formed twinned martensite in the temperature range of 100–200 K and it is accompanied by a relaxation of stresses in the retained austenite and a decrease of tetragonality. A new internal friction peak centered at 145 K was observed and attributed to movement of coherent interfaces and to the subsequent break of coherency. The relation between abnormally high tetragonality and coherency at the interface was confirmed in experiments with external deformation of virgin martensite at temperatures around 100 K. It is shown that the plate morphology of martensite is a necessary condition for a coherent bond at the interface. A possible role of atomic ordering of austenite in abnormally high tetragonality is discussed. Mössbauer measurements gave evidence for nickel-rich regions in initial austenite. During quenching the regions with the highest nickel content were assumed to remain austenitic inside the martensite plates. The conclusion about the coherency at the interface between the freshly formed martensite and the ordered regions in the retained austenite and at the interface along the martensite plates as a reason for high tetragonality of the FeNiC martensite with plate morphology is made.
Published Version
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