Thermal expansion and phase transformations of nitrogen-expanded austenite studied within situsynchrotron X-ray diffraction

Abstract

Nitrogen-expanded austenite, γN, with high and low nitrogen contents was produced from AISI 316 grade stainless steel powder by gaseous nitriding in ammonia/hydrogen gas mixtures.In situsynchrotron X-ray diffraction was applied to investigate the thermal expansion and thermal stability of expanded austenite in the temperature range 385–920 K. Evaluation of the diffractograms of the sample with a high nitrogen content, corresponding to an occupancy of the interstitial lattice of 56%, with Rietveld refinement yielded a best convergence after including the stacking fault probability as a fitting parameter. The stacking fault density is constant for temperatures up to 680 K, whereafter it decreases to nil. Surprisingly, a transition phase with compositionM4N (M= Fe, Cr, Ni, Mo) appears for temperatures above 770 K. The linear coefficient of thermal expansion depends on the nitrogen content and is lowest for the sample with a high level of nitrogen.