The structure of nitrided iron-titanium alloys

Abstract

The structure of gas-nitrided dilute Fe-Ti alloys has been investigated by electron microscopy, electron and X-ray diffraction, and measurements of nitrogen content. The observations are interpreted in terms of a periodic array of equiatomic Ti-N mono-layer plates on the cube planes of the matrix. The plates distort the surrounding matrix and allow the total accommodation of up to three times the nitrogen content appropriate to 'TIN' stoichiometry. The adsorption enthalpy is -25 kcal mol-1 and the excess nitrogen can be removed by hydrogen reduction at 575°C or heating in an inert atmosphere at 750°C and above. After hydrogen reduction the excess nitrogen can be replaced by re-nitriding. The structure is resistant to coarsening, and only after several hours at 825°C does appreciable softening and plate growth occur. The model of mono or multi-layer substitutional atom-nitrogen plates is considered to be a special case of a mixed substitutional-interstitial atom cluster, but it can be extended to other nitriding systems and is compatible with previous observations. © 1976.