Phase transformations in epitaxial Ni/Fe bilayers upon low-temperature gaseous nitriding

Abstract

In this paper we describe studies on the low-temperature gaseous nitriding of epitaxial Ni-covered Fe layers. The epitaxial Ni/Fe bilayer system was grown by laser ablation deposition on an MgO͑001͒ substrate. Nitriding was done in an NH 3 ϩH 2 gas mixture in the 200°C-300°C temperature range. As a result of such nitriding, the main Fe-nitride phases could be produced as a pore-free material. The samples were analyzed by x-ray diffraction and conversion electron Mössbauer spectroscopy. It was observed that at the conditions applied the Fe-nitride phase formation occurs in the ␣→␣Ј, ␣Љ→␥Ј→→␥Љ,␥ٞ sequence, whereby each step is accompanied by grain refinement. The precise orientation relationships between the phases in the ␣→␣Ј, ␣Љ→␥Ј → sequence were determined. Based on the data obtained, a model is proposed that describes this transformation sequence on an atomic scale in terms of martensitic transition. Among other results, evidence is provided for two different types of orientation relationships that can occur between ␣-Fe and ␥Ј-Fe 4 N, depending on the prevailing conditions. The morphology of the Ni/Fe-N system is discussed.

On the formation of ultra-fine grained Fe-base alloys via phase transformations

“…From MS analysis the quantity of ␥ formed in both cases were found to be 95 and 50%, respectively. These values are comparable with the estimation of the phase ratio from XRD spectra, indicating that the offset between the ␣ and ␥ phases found for the epitaxial Fe films [7] does not play an important role in the rolled foils. Irrespective of the presence or absence of a pretreatment, the reduction of the foils containing ␥ -phase leads practically to the same result, i.e.…”

“…The ␣ → ε transformation of pure Fe occurs at 300 • C via the intermediate ␥ -phase [7]. For Ti and Cr containing alloys ε-nitrided at 400 • C, mixtures of ε-and ␥ -phases were obtained.…”

“…Due to the misfit between the ␣-, ␣ -and ␥ -phases, strain will build up in the material during phase transformation. If this strain does not affect the channel for the transformation and all the possible ORs occur with about equal probabilities, grain refinement and grain reorientation can be obtained by repeated cycling of bcc → fcc transitions [7]. When the nitrogen concentration in the film increases further, the hcp structured ε-Fe x N phase is formed.…”

“…In previous studies by the authors [7] it was shown that the transitions between bcc, fcc and hcp crystal structures occur at 300 • C during nitriding of a thin epitaxial Fe layer. As the nitrogen content in the film increases, the sequence of various phases is:…”

Magnetic properties and structural implications for nanocrystallineFe−Ti−Nthin films

“…The available literature indicates that as one adds nitrogen, there are progressive changes in structure. 18 One starts with body-centered-cubic (bcc) , with a nitrogen content N x up to 0.4 at. % or so, the nitrogen atoms can be dissolved in the bcc lattice with essentially no change in the structure.…”