Resistometric study of Fe-V and Fe-Mo nitrided by constant activity aging

Resistometry has been employed to study alloys of Fe containing 1.1, 2.2, and 3.3 at. pct V and 2.2 and 3.0 at. pct Mo nitrided at 500 to 600 °C in NH33/H2 gas atmospheres for up to 300 hours. The very large resistivity maxima are attributed to static displacements due to interstitial N, as proposed by Hoffman and Cohen for interstitial alloys.15 The platelet size increases with nitridation temperature, and platelet number density increases with substitutional solute concentration at fixed temperature. Platelets in Fe-V-N are smaller and more stable than in Fe-Mo-N. In Fe-Mo-N overaging commences prior to saturation with nitrogen and occurs at a more rapid rate and at lower temperature than for Fe-V-N. Fe-Mo-N samples that are fully nitrided and subsequently aged in pure H2 experience a rapid initial drop in resistivity, due to removal of N from the α-Fe matrix, but then age more slowly than samples in a nitridation atmosphere. Estimates of local atomic displacements around platelets are in approximate agreement with prior results obtained from TEM measurements.