Sai Ramudu Meka

Unusual Precipitation of Amorphous Silicon Nitride upon Nitriding Fe-2at.%Si Alloy

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.

Octapod-shaped, nanosized, amorphous precipitates in a crystalline ferrite matrix

The development of uniquely octapod-shaped nanosized amorphous silicon–nitride precipitates in a ferrite matrix was observed upon nitriding of Fe–4.5at.%Si alloy. The legs of the amorphous precipitate are oriented along ⟨1 1 1⟩-directions of the ferrite. The occurrence of such peculiarly shaped amorphous silicon–nitride precipitates, which experience a volume misfit of more than 100% with the surrounding ferrite, was attributed to precipitate growth influenced by long-range diffusion within the evolving highly anisotropic stress field around the developing precipitates after nucleation.

Diffraction-line shifts and broadenings in continuously and discontinuously coarsening precipitate-matrix systems: coarsening of initially coherent nitride precipitates in a ferrite matrix

The initial precipitation of misfitting particles can be accompanied by elastic accommodation of the precipitate/matrix misfit leading to considerable matrix lattice dilatation/contraction and variable lattice microstrain. In this stage, the entire assembly of matrix and precipitate particles, as a whole, can diffract coherently. Upon aging of the system, relaxation of the accommodated misfit can occur by continuous and/or discontinuous coarsening of the precipitates. These processes are associated with distinctly different, characteristic diffraction phenomena, also involving a transition from coherent to incoherent diffraction of precipitates and matrix. For the case of, initially fully coherent, alloying element nitrides in a homogeneously nitrided ferrite matrix, these effects have been identified and analyzed, thus allowing tracing of misfit-relaxation processes.

Interrelationships of defects, nitride modification and excess nitrogen in nitrided Fe-4.75 at.% Al alloy

Abstract In order to investigate the influence of crystal-lattice defects (dislocations) and of a relatively large Al content on the development of different modifications of aluminium nitride in ferrite and on the associated uptake of excess nitrogen, recrystallised and cold rolled specimens of Fe-4.75 at.% Al alloy were subjected to gaseous nitriding treatments at 550 °C employing a nitriding potential of 0.104 atm−1/2. In contrast with earlier results for an alloy of relatively low Al content, in both the nitrided cold rolled and the nitrided recrystallised specimens nanosized AlN precipitates developed, both of the metastable, cubic, NaCl type modification and of the stable, hexagonal, wurtzite type modification, which exhibit with the ferrite matrix a Baker–Nutting orientation relationship and a Pitsch–Schrader orientation relationship, respectively. A high-temperature hydrogen reduction treatment confirmed the stoichiometry of the precipitated nitrides as AlN. The fraction of cubic, NaCl type, AlN is significantly higher in the nitrided cold-rolled specimens because of its preferential development along dislocations. As compared to the nitrided recrystallised specimens, the higher dislocation density and the higher amount of cubic AlN in the nitrided cold-rolled specimens leads to a larger amount of excess nitrogen in the nitrided cold-rolled specimen.

Unusual nucleation and growth of γ′ iron nitride upon nitriding Fe–4.75 at.% Al alloy

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.

Coherency strain and precipitation kinetics: crystalline and amorphous nitride formation in ternary Fe–Ti/Cr/V–Si alloys

Specimens of iron-based binary Fe–Si alloy and ternary Fe–Me–Si alloys (with Me = Ti, Cr and V) were nitrided at 580 °C in a NH3/H2-gas mixture applying a nitriding potential of 0.1 atm−1/2 until nitrogen saturation in the specimens was attained. In contrast with recent observations in other Fe–Me1–Me2 alloys, no “mixed” (Me1, Me2) nitrides developed in Fe–Me–Si alloys upon nitriding: first, all Me precipitates as MeN; and thereafter, all Si precipitates as Si3N4. The MeN precipitates as crystalline, finely dispersed, nanosized platelets, obeying a Baker–Nutting orientation relationship (OR) with respect to the ferrite matrix. The Si3N4 precipitates as cubically, amorphous particles; the incoherent (part of the) MeN/α-Fe interface acts as heterogeneous nucleation site for Si3N4. The Si3N4-precipitation rate was found to be strongly dependent on the degree of coherency of the first precipitating MeN. The different, even opposite, kinetic effects observed for the various Fe–Me–Si alloys could be ascribed to the different time dependences of the coherent → incoherent transitions of the MeN particles in the different Fe–Me–Si alloys.

Continuous and discontinuous precipitation in Fe-1 at.%Cr-1 at.%Mo alloy upon nitriding; crystal structure and composition of ternary nitrides

Abstract The internal nitriding response of a ternary Fe–1 at.%Cr–1 at.%Mo alloy, which serves as a model alloy for many CrMo-based steels, was investigated. The nitrides developing upon nitriding were characterised by X-ray diffraction, scanning electron microscopy, electron probe microanalysis, transmission electron microscopy and atom probe tomography. The developed nitrides were shown to be (metastable) ternary mixed nitrides, which exhibit complex morphological, compositional and structural transformations as a function of nitriding time. Analogous to nitrided binary Fe–Cr and Fe–Mo alloys, in ternary Fe–Cr–Mo alloys initially continuous precipitation of fine, coherent, cubic, NaCl-type nitride platelets, here with the composition (Cr½,Mo½)N¾, occurs, with the broad faces of the platelets parallel to the {1 0 0}α-Fe lattice planes. These nitrides undergo a discontinuous precipitation reaction upon prolonged nitriding leading to the development of lamellae of a novel, hexagonal CrMoN2 nitride along {1 1 0}α-Fe lattice planes, and of spherical cubic, NaCl-type (Cr,Mo)Nx nitride particles within the ferrite lamellae. The observed structural and compositional changes of the ternary nitrides have been attributed to the thermodynamic and kinetic constraints for the internal precipitation of (misfitting) nitrides in the ferrite matrix.

Defect-dependent nitride surface layer development upon nitriding of Fe–1 at.% Mo alloy

Upon nitriding of binary Fe–1 at.% Mo alloy in a NH3/H2 gas mixture under conditions (thermodynamically) allowing γ′-Fe4N1– x compound layer growth (nitriding potential: 0.7 atm−1/2 at 753 K (480 °C) – 823 K (550 °C)), a strong dependency of the morphology of the formed compound layer on the defect density of the specimen was observed. Nitriding of cold-rolled Fe–1 at.% Mo specimens leads to the formation of a closed compound layer of approximately constant thickness, comparable to nitriding of pure iron. Within the compound layer, that is, in the near-surface region, Mo nitrides are present. The growth of the compound layer could be described by a modified parabolic growth law leading to an activation energy comparable to literature data for the activation energy of growth of a γ′-Fe4N1− x layer on pure iron. Upon low temperature nitriding (i.e. ⩽793 K (520 °C)) of recrystallized Fe–1 at.% Mo specimens, an irregular, ‘needle-like’ morphology of γ′-Fe4N1− x nucleated at the surface occurs. This γ′ iron nitride has an orientation relationship (OR) with the matrix close to the Nishiyama–Wassermann OR. The different morphologies of the formed compound layer can be interpreted as consequences of the ease or difficulty of precipitation of Mo as nitride as function of the defect density.

Microstructure and Kinetics of Nitride Precipitation in a Quaternary Iron-Based Model Fe-2.82 at. pct Cr-0.13 at. pct Mo-0.18 at. pct V Alloy

Internal nitride development in iron-based quaternary Fe-Cr-Mo-V alloy, as a model alloy for 31CrMoV9 steel, was investigated by performing controlled gaseous nitriding experiments. The nitride-precipitation process starts with the development of nanosized platelets of, coherent, cubic NaCl-type nitride, along {100} lattice planes of the ferrite matrix, in association with matrix-lattice dilation. The development of nitride platelets having a NaCl-type crystal structure, satisfying the Baker–Nutting orientation relationship with the ferrite matrix, and the nitrogen content of the nitrided zone suggest the development of a quaternary “mixed” (Crx,Vy, Mo1−x−y)N nitride, similar to the development of “mixed” ternary nitrides as reported for nitrided Fe-Cr-Al and Fe-Cr-Ti alloys. In a later stage, the nitride platelets undergo discontinuous coarsening resulting in the development of a lamellar microstructure consisting of nitride and ferrite lamellae. Kinetic analysis demonstrated that the thermally activated nature of growth of the diffusion zone is controlled with about equal weights, by the diffusion of nitrogen in the substrate matrix and the matrix lattice solubility of nitrogen.

Misfit-induced changes of lattice parameters in two-phase systems: coherent/incoherent precipitates in a matrix.

The article discusses misfit-induced lattice-parameter changes in two-phase systems as exposed by coherent diffraction of the assembly or incoherent diffraction by the matrix and the second-phase particles.