Abstract
Quench-ageing of iron-nitrogen alloys containing about 0.02% of nitrogen was studied by electrical resistivity measurement and transmission electron microscopy. Electrical resistivity measurement of Fe–0.014% N alloy during the 10 deg–15 min isochronal annealing after quenching from 720°C revealed that the metastable iron nitride, α″–Fe16N2, precipitated around 120°C initially and redissolved completely at 230°C prior to the precipitation of stable iron nitride γ′–Fe4N. Resistivity measurement during the course of isothermal annealing of Fe–0.022% N alloy also confirmed the reversion of the α″ phase at 242°C. Transmission electron microscopy gave direct evidence that the α″ phase initially disappeared by annealing at about 230°C. The reversion phenomenon mentioned above is explained in terms of the solubility limit of the metastable iron nitride and by the fact thet the α″ phase does not act as a nucleus for the γ′ phase.
