Formation and Decomposition of Metastable α′′‐Fe16N2 from in situ Powder Neutron Diffraction and Thermal Analysis
Formation and Decomposition of Metastable α′′‐Fe16N2 from in situ Powder Neutron Diffraction and Thermal Analysis
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原位粉末中子衍射和热分析亚稳 α-Fe16N2 的形成和分解
DOI：
10.1002/zaac.201300379
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发表时间：
2013
期刊：
影响因子：
--
通讯作者：
R. Niewa
中科院分区：
文献类型：
--
作者：
M. Widenmeyer;T. Hansen;R. Niewa
为了进一步了解不同原料亚稳α‘’-Fe16N_2的形成和分解行为，进行了不同气氛、不同升温速率下的原位中子衍射和热分析。在惰性条件下，在较高的升温速率下，α‘-Fe16N_2在α-Fe和N_2中直接转化，而在较低的升温速率下，γ’-Fe_4N_2的分解发生。α‘-相的c/a比的变化与随后在分解过程中转变为马氏体相α’-Fe_8N有关。由于优化了实验装置，D20(Laue-Langevin研究所，ILL)的时间分辨率为2分钟，允许进行详细的Rietveld分析，因此收集了高质量的现场中子衍射数据。热分析支持所提出的α‘’-Fe16N_2的亚稳性质。对于所研究体系内的所有相变温度，存在与升温速率、样品的热历史、气体流动条件和颗粒大小的强烈依赖关系。特别是对于块体α‘’-Fe16N_2，改进了原位生成的铁粉与氨水反应的合成条件，得到了高纯度的样品[93(2)wt %]。
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.