Abstract
An artificially prepared nanocrystalline iron sample with bimodal crystallite size distribution was nitrided and denitrided in the NH3/H2 atmosphere at 350 °C and 400 °C. The sample was a 1:1 mass ratio mixture of two iron samples with mean crystallite sizes of 48 nm and 21 nm. Phase transformations between α-Fe, γ’-Fe4N and ε-Fe3-2N were observed by the in situ X-ray powder diffraction method. At selected steps of nitriding or denitriding, phase transformations paused at 50% of mass conversion and resumed after prominent variation of the nitriding atmosphere. This effect was attributed to the separation of phase transformations occurring between sets of iron crystallites of 48 nm and 21 nm, respectively. This was due to the Gibbs–Thomson effect, which establishes the dependence of phase transformation conditions on crystallite sizes.
Highlights
Thermodynamic and kinetic relations concerning reactions of iron leading to the formation of iron nitrides have been developed theoretically and confirmed experimentally [1,2,3]
Happen, differed between processes of the iron nitriding and iron nitrides denitriding [10]. This shift between regions of phase occurrence during nitriding and denitriding processes was described as the hysteresis phenomenon in the nanocrystalline Fe–N system. It can be rephrased as a state where different ammonia concentrations are required to form materials with identical crystallographic structures, depending on whether they were obtained by the nitriding or denitriding process
Sample A is nanocrystalline iron obtained by laser evaporation with a mean crystallite sizes (MCS) of 48 nm, as assessed by the use of XRD
Summary
Thermodynamic and kinetic relations concerning reactions of iron leading to the formation of iron nitrides have been developed theoretically and confirmed experimentally [1,2,3]. It was pointed out that the nitriding potential values, at which phase transformations between α-Fe, γ’-Fe4 N and ε-Fe3-2 N happen, differed between processes of the iron nitriding and iron nitrides denitriding [10] This shift between regions of phase occurrence during nitriding and denitriding processes was described as the hysteresis phenomenon in the nanocrystalline Fe–N system. When determining the boundaries of parameters where given crystallographic phases occur, the influence of the crystallite size distribution should be taken into account [15] This relation has been studied in various ways in the Fe–N system for the nitriding and denitriding processes carried out in an atmosphere containing ammonia [16,17]. It is expected that for a given gas phase composition, a portion of the material (e.g., a subset of the smallest size crystallites) will occur as one crystallographic phase and the remaining portion of the material (e.g., a subset of the highest size crystallites) as another crystallographic phase
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
