Solute interactions in nitrided iron alloys

Abstract

Using the gas-metal equilibration method of constant activity ageing, high densities of fine, homogeneous nitride precipitates or substitutionalinterstitial solute-atom clusters are produced in low alloy steels in the temperature range 350-7500C under appropriate nitriding conditions. In Fe-Ti alloys, hardness and lattice parameter measurements, together with electron microscopy, show the formation of very fine disc-shaped zones. The tweed contrast of electron micrographs and the diffuse electron and-X-ray scattering effects observed are attributed to the partial ordering of the zones which consist of equatomic Ti-N monolayer plates on matrix cube planes. Excess nitrogen equivalent to twice the nitrogen content of the "TiN" plate is accommodated as part of the cluster but is removed by ageing in hydrogen. A model is suggested for the zone structure which accounts for its limiting compositions "Fe4TiN3" and "Fe2TiN" and for the occurrence of nitrogen in two different chemical environments with different stabilities. The "TiN" zone stoichiometry and morphology is stable even at high temperatures and overageing of the structure, accompanied by softening, takes several hours at 800°C. The strengthening which occurs in Fe-Ti alloys nitrided at intermediate temperatures is due to a particle shear mechanism. For Fe-Mo alloys nitrided under similar conditions the microstructures are much coarser. Tweed contrast is not observed but other electron and X-ray diffraction effects are similar to those for Fe-Ti alloys. Zones containing both titanium and molybdenum are formed in nitrided Fe-Mo-Ti alloys nitrided under similar conditions and the properties are determined mainly by the titanium present in the alloy; the most pronounced effect of the molybdenum is observed in the nitriding kinetics. A model proposed for the Mo-Ti-N zone structure accounts for the difference between the observed properties and those expected from a simple summation of the separate Mo-N and Ti-N interactions.