NITROGEN TRAPPING TO CHROMIUM IN ALPHA-IRON STUDIED BY INTERNAL-FRICTION AND MAGNETIC AFTER-EFFECT TECHNIQUES

The effect of chromium on the behaviour of nitrogen in alpha iron has been studied by anelastic and magnetic relaxation measurements on dilu te Fe-Cr-N alloys. The presence of chromium is known to give rise to a n additional contribution to the Snoek relaxation at a higher temperat ure than the ordinary Snoek peak. When specimens quenched from a solut ion-treatment temperature are subjected to isothermal ageing at room t emperature, the high temperature component develops while the ordinary component decays. This observation can be explained in terms of redis tribution of nitrogen atoms: almost random distribution established at the solution-treatment temperature changes into a metastable distribu tion, in which a substantial number of nitrogen atoms are bound to chr omium atoms. The binding energy of a Cr-N pair has been estimated inde pendently from two experiments: 20+/-2 kJ/mol from the kinetics of the redistribution and 15 kJ/mol from the characteristics of the Snoek re laxation profile, which are in reasonable agreement with each other.