Formation and Decomposition of Metastable α′′‐Fe 16 N 2 from in situ Powder Neutron Diffraction and Thermal Analysis

In order to gain more information on the formation and decomposition behavior of metastable α′′‐Fe 16 N 2 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′′‐Fe 16 N 2 in α‐Fe and N 2 was observed using higher heating rates, while with lower heating rates the decomposition occurs via the formation of γ′‐Fe 4 N y . The changes in c / a ratio of the α′′‐phase are related to a subsequent transformation into martensitic α′‐Fe 8 N 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′′‐Fe 16 N 2 . 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′′‐Fe 16 N 2 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.