Mechanical properties of some nitrided iron alloys

Abstract

When binary iron alloys containing strong nitride forming elements such as vanadium, niobium, titanium, molybdenum or chromium as the substitutional solute are nitrided under suitable conditions, a homogeneous dispersion of coherent, disc-shaped particles precipitateson the ferrite cube planes. Initially, the particles give rise to continuous streaking on electron diffraction patterns and, after subsequent heat treatment, overage to the equilibrium alloy nitride. Although the internal friction Snoek peak of the nitrided alloys is extremely small and substitutional- I interstitial interaction peaks are absent, relatively strong damping is observed at 150 - 2300C. Broad, assymmetric peaks occur indicating a distribution of relaxation times, the peak temperature being dependent on the associated microstracture. A qualitative interpretation for the origin of the peak is outlined based upon the movement of nitrogen atoms in the elastic strain fields surrounding the precipitated particles'. Nitrided alloys with as little as O. lat. % substitutional solute produce large strength increasesq the maXimUm strength being obtained at the smallest particle size. The Yield strength is proportional to the square root of the Volume fraction of precipitated phase and is strongly temperature dependent. When coherent particles are precipitated deformation occurs by particle shearing and electron micrographs of the deformed microstructures show that slip is predominantly on {110}α planes with dislocations having a Burgers vector of a/2<111> The major part of the high initial strengthening is thought to derive from the energy required for dislocations to shear the coherent particles. A change to an Orowan type deformation mechanism occurs when the particles become incoherent.