Autoradiographic evidence of carbon clustering in Fe-Ni-C martensite

The distribution of carbon in the microstructure of the twinned martensite of Fe-23 Ni-0.4 C, has been studied by microautoradiography of radioactive carbon 14. A statistical analysis of the autoradiographs, observed in the scanning electron microscope for massive specimens or in the transmission electron microscope for thin foils, show that: (i) clustering of carbon in the twin boundaries of the martensite plates occurs rapidly by ageing at room temperature; (ii) two twinning systems may be active sites for carbon clustering in the same plate; (iii) carbon segregation seems more pronounced in twin boundaries than in martensite-martensite, martensite-austenite, or mid-rib interfaces; (iv) the precipitation of ∈-carbide by ageing at 100 and 150° C is confirmed.