An atom probe study of the aging of iron-nickel-carbon martensite

Abstract

The atom probe field ion microscope has been used to study the redistribution of carbon in iron-nickel-carbon martensites aged in the temperature range 22 to 250 °C. The Ms temperature of the alloy used was −50 °C. During all stages of aging a considerable amount of redistribution of carbon was detected in the form of carbon segregation to twin boundaries and other lattice defects and in the formation of small clusters and carbides. Small carbon-rich areas were detected at 40 °C, and disc-shaped carbide particles were found after 24 hours at 100 °C. Thin cementite lamellae were observed after one hour at 250 °C. It is evident that a close similarity exists between the sequence of reactions occurring during the tempering process and the sequence of precipitation reactions observed in age-hardening alloys. The main additional complications that arise in the case of martensites are due to the high densities of lattice defects that are present and to the strong interaction between carbon atoms and such defects.