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

Abstract

AbstractIn 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.