Formation and Decomposition of Metastable α′′-Fe16N2 from in situ Powder Neutron Diffraction and Thermal Analysis


  • Dedicated to Professor Wolfgang Bensch on the Occasion of His 60th Birthday

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In order to gain more information on the formation and decomposition behavior of metastable α′′-Fe16N2 from different starting materials in situ neutron diffraction and thermal analysis in different gas atmospheres and heating rates were carried out. Under inert conditions a direct conversion of α′′-Fe16N2 in α-Fe and N2 was observed using higher heating rates, while with lower heating rates the decomposition occurs via the formation of γ′-Fe4Ny. The changes in c/a ratio of the α′′-phase are related to a subsequent transformation into martensitic α′-Fe8N during the decomposition. In situ neutron diffraction data were collected in high quality, due to an optimized experimental setup with a time resolution of two minutes on D20 (Institut Laue-Langevin, ILL) allowing for detailed Rietveld analyses. Thermal analyses support the proposed metastable nature of α′′-Fe16N2. For all phase transition temperatures within the investigated system a strong dependency from heating rate, thermal history of the sample, gas flow conditions, and particle size exists. Particularly, for bulk α′′-Fe16N2 the synthesis conditions from reaction of in situ produced fine iron particles and ammonia were improved and a high purity sample [93(2) wt %] was achieved.