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Thermal expansion and phase transformations of nitrogen‐expanded austenite studied with in situ synchrotron X‐ray diffraction
Author(s) -
Brink Bastian,
Ståhl Kenny,
Christiansen Thomas L.,
Somers Marcel A. J.
Publication year - 2014
Publication title -
journal of applied crystallography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.429
H-Index - 162
ISSN - 1600-5767
DOI - 10.1107/s1600576714005214
Subject(s) - austenite , materials science , thermal expansion , lattice constant , synchrotron , nitriding , nitrogen , rietveld refinement , analytical chemistry (journal) , hydrogen , diffraction , x ray crystallography , atmospheric temperature range , crystallography , metallurgy , thermodynamics , composite material , chemistry , crystal structure , microstructure , physics , organic chemistry , layer (electronics) , chromatography , nuclear physics , optics
Nitrogen‐expanded austenite, γ N , with high and low nitrogen contents was produced from AISI 316 grade stainless steel powder by gaseous nitriding in ammonia/hydrogen gas mixtures. In situ synchrotron 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 composition M 4 N ( 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.