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Silicon-nitride precipitation in ferritic Fe–2at.%Si alloy was investigated upon nitriding in NH3/H2 gas mixtures, using light microscopy, hardness measurements, scanning and transmission electron microscopy, X-ray diffraction and electron-probe microanalysis for microstructural characterisation. Surprisingly, ideally weak nitriding behaviour occurred upon nitriding thick (1 mm) recrystallised Fe–2at.%Si alloy specimens. This phenomenon can be attributed to the onset of silicon-nitride precipitation only after a certain degree of nitrogen supersaturation has been established at all depths in the specimen. Silicon-nitride precipitates formed inside the ferrite grains and also along the ferrite grain boundaries. The precipitates were amorphous and had a stoichiometry of Si3N4. The amorphous nature of the tiny precipitates has a thermodynamic origin. Nitride precipitation occurred very slowly due to the very large volume misfit of the nitride with the matrix. An anomalous non-monotonous hardness change occurred with increasing nitriding time, which was ascribed to the initially fully elastic accommodation of precipitate/matrix misfit. The nitrogen uptake rate increased upon continued nitriding as a result of “self-catalysis”. The possible, favourable application of amorphous silicon-nitride precipitates as grain-growth inhibitors in the production of grain-oriented electrical steel is discussed. [ABSTRACT FROM PUBLISHER]

The influence of substitutionally dissolved Al in ferritic Fe–4.75 at.% Al alloy on the nucleation and growth of γ′ iron nitride (Fe4N1− x ) was investigated upon nitriding in NH3/H2 gas mixtures. The nitrided specimens were characterised employing optical microscopy, scanning electron microscopy, transmission electron microscopy, electron probe microanalysis and X-ray diffraction. As compared to the nitriding of pure ferrite (α-Fe), where a layer of γ′ develops at the surface, upon nitriding ferritic Fe–4.75 at.% Al an unusual morphology of γ′ plates develops at the surface, which plates deeply penetrate the substrate. In the diffusion zone, nano-sized precipitates of γ′ and of metastable, cubic (NaCl-type) AlN occur, having, with the ferrite matrix, a Nishiyama–Wassermann orientation relationship and a Bain orientation relationship, respectively. The γ′ plates contain a high density of stacking faults and fine ε iron nitride (Fe2N1− z ) precipitates, although the formation of ε iron nitride is not expected for the employed nitriding parameters. On the basis of dedicated nitriding experiments it is shown that the unusual microstructural development is a consequence of the negligible solubility of Al in γ′ and the obstructed precipitation of the thermodynamically stable, hexagonal (wurtzite-type) AlN in ferrite. [ABSTRACT FROM AUTHOR]

Abstract: A systematic investigation of the kinetics of the polytypic C36C14 equilibrium phase (shear) transformation occurring at about 1545 K was performed. A recently developed calibration and desmearing procedure for a differential thermal analysis (DTA) apparatus [Baumann et al. Thermochim. Acta 2008; 472:50] enabled the investigation of the progress of this transformation by isochronal DTA measurements, both upon heating and cooling. A kinetic model was developed on basis of the modular approach, which considers overlapping nucleation, growth and impingement. It was shown that athermal nucleation prevailed, i.e. pre-existing nuclei (here: arrays of dislocations) become activated as function of undercooling/overheating. The model fitting led to values for the equilibrium transformation temperature, the C36/C14 interface energy and the activation energy for growth. Moreover, a quantitative explanation was given of the composition-dependent thermal hysteresis of the transformation. [ABSTRACT FROM AUTHOR]

Differential dilatometry was applied to investigate the effect of an applied constant uniaxial compressive stress on the kinetics of the austenite (γ) → martensite (α′) transformation in the substitutional Fe–0.7 at.%Al alloy upon isochronal cooling/quenching with constant rate. All imposed stress levels are below the yield stresses of γ and α′ phases in the temperature range of the martensite formation. Albeit the start temperature of the γ → α′ transformation remains approximately constant in the range of stress explored, the overall transformation temperature range increases significantly with the increase of the uniaxial compressive stress. A modular phase transformation model, adopting a model for continuous nucleation and an anisotropic thermally-activated growth model, yielding a corresponding impingement correction, was employed to extract the nucleation rate and the γ/α′-interface velocity during the transformation. The kinetic analysis suggests that athermal nucleation and thermally activated growth govern the martensite transformation under the uniaxial compressive stress. More driving force is required when a larger uniaxial compressive stress is imposed, and the thus obtained velocity of the γ/α′-interface as function of temperature indicates a thermally activated growth governed by a relatively low activation energy. [ABSTRACT FROM AUTHOR]