INTERNAL NITRIDING OF TUNGSTEN-BASE ALLOYS CONTAINING HAFNIUM

It is shown that W, W-Re, and W-Re-Mo alloys containing as much as 3 at. % Hf in solid solution are subject to internal nitriding when exposed to nitrogen gas at temperatures above about 1500°C. The hafnium nitride particles which are formed as a result of nitriding precipitate as plates on {001} matrix planes and this plate-like habit is retained up to at least 2000°C. At 1 atm N2 pressure and temperatures between 1500 and 2000°C, the kinetics of internal nitriding are found to be controlled exclusively by the diffusion of nitrogen in the base metal or alloy matrix. The simple theoretical model developed by Wagner for the diffusion-controlled kinetics of internal oxidation is shown to provide an adequate description of the internal nitriding behavior. Permeation constants for nitrogen in W derived from the measured internal nitriding rates are found to be in good agreement with values obtained from direct steady-state permeation measurements. Using published solubility data, the diffusion coefficient for nitrogen in tungsten, D = D0 exp ( -QD/RT), has been evaluated from the relation P = ND, where P is the permeation constant and N the solubility at 1 atm pressure. The results yield D0≅ 1.2 × 10-2 cm2/sec and QD ≅ 32.2 kcal/mole. © 1969.