A. Böttger

Lattice changes of iron-nitrogen martensite on aging at room temperature

The aging behavior of iron-nitrogen martensite (5.5 at. pct N ≙5.8N/100Fe) at about 297 K was investigated by employing X-ray diffractometry, thereby following, in particular, the changes in the {002} and {200} line profiles. Martensitic specimens were prepared by gaseous nitriding of pure iron in a mixture of NH3 and H2, followed by quenching in brine and subsequently in liquid nitrogen. The aging process can be divided into two stages. First, a redistribution of nitrogen atoms in the martensite matrix occurs (aging time < about 40 hours) in three ways: segregation of nitrogen to lattice defects (about 0.07N/100Fe), transfer of a small amount of nitrogen (about 0.06N/100Fe) fromalb- toc-type octahedral interstices, and local enrichment in an ordered way of the majority of the nitrogen atoms (coherent α′’-Fe16N2 precipitates). Second, formation of incoherent α″-Fe16N2 takes place (aging time > about 40 hours). Within the range of aging times employed (up to 670 hours), the diffraction by the residual austenite did not change.

Early stages of decomposition in iron-carbon and iron-nitrogen martensites: Diffraction analysis using synchrotron radiation

The early stages of decomposition of iron-carbon and iron-nitrogen martensites were studied by means of diffraction analysis. After aging at room temperature (RT) for 3. 5 years and after tempering at 405 K for 1 hour,ε/η transition carbide reflections but noα′-type (superstructure) reflections were detected for FeC martensites. However, for FeN martensites, theα′ superstructure reflections were observed on aging at RT for 3. 5 years and on tempering at 405 K for 1. 5 hour. For short aging times at RT (up to 60 hours),α′ reflections (not of the superstructure type) were observed for FeN martensites. Upon tempering, changes in theα′ crystal structure occur, which were discussed in terms of annihilation of structural vacancies on the nitrogen sublattice and stress relaxation. In the diffraction pattern recorded from FeC martensite (for short aging times at RT), weak reflections occurred which could not be identified conclusively yet and which disappeared after tempering for 1 hour at 405 K.

Amorphous precipitates in a crystalline matrix; precipitation of amorphous Si3N4 in α-Fe

Microstructural changes in solids due to phase transformations are induced in practice to bring about specific, desired properties. The present research project focused on the development of a kinetic model for the precipitation of silicon nitride in silicon containing {alpha}-Fe, more or less as performed earlier by the authors for the precipitation of CrN and AlN. In contrast with their earlier preliminary work from the past, a determined effort to reveal the structure of the nitrides was undertaken. It was found that the precipitates were amorphous, stoichiometric Si{sub 3}N{sub 4}. The evidence for this finding is presented here. Further, on the basis of recent work on the modeling of interface energies, it will be suggested here that the occurrence of initially amorphous precipitate particles may have a thermodynamic basis. The kinetic analysis will be presented elsewhere.

Smoothing of X-ray diffraction data and K (alpha)2 elimination using penalized likelihood and the composite link model

X-ray diffraction scans consist of series of counts; these numbers obey Poisson distributions with varying expected values. These scans are often smoothed and the K2 component is removed. This article proposes a framework in which both issues are treated. Penalized likelihood estimation is used to smooth the data. The penalty combines the Poisson log-likelihood and a measure for roughness based on ideas from generalized linear models. To remove the K doublet the model is extended using the composite link model. As a result the data are decomposed into two smooth components: a K1 and a K2 part. For both smoothing and K2 removal, the weight of the applied penalty is optimized automatically. The proposed methods are applied to experimental data and compared with the Savitzky–Golay algorithm for smoothing and the Rachinger method for K2 stripping. The new method shows better results with less local distortion. Freely available software in MATLAB and R has been developed.

The tempering of FeNiN martensite

The tempering behavior of ternary iron-nickel-nitrogen martensitic specimens (∼13.5 at. pct Ni, ∼4.7 at. pct N; ∼4.9 N atoms/100 metal atoms) in the temperature range of 270 to 670 K was investigated by analysis of the corresponding changes in the crystalline structure (X-ray diffraction), volume (dilatometry), and enthalpy (calorimetry). At least three stages of structural change can be distinguished: (1) redistribution of nitrogen atoms, involving segregation and formation of nitrogen enrichments, occurs at temperatures up to 370 K, (2) precipitation of γ′-(Fe, Ni)4N1-x nitride takes place in the temperature range of 370 to 430 K, and (3) coarsening of the γ′ precipitates and decomposition of a part of the retained austenite occur at temperatures above 430 K. From a comparison with the tempering behavior of a binary iron-nitrogen martensitic alloy containing a similar amount of interstitials, it was concluded that the presence of nickel suppresses the development of the intermediate α″-(Fe,Ni)16N2 nitride and advances the precipitation of γ′ nitride.

A diffracted-beam monochromator for long linear detectors in X-ray diffractometers with Bragg-Brentano parafocusing geometry

A new diffracted-beam monochromator has been developed for Bragg-Brentano X-ray diffractometers equipped with a linear detector. The monochromator consists of a cone-shaped graphite highly oriented pyrolytic graphite crystal oriented out of the equatorial plane such that the parafocusing geometry is preserved over the whole opening angle of the linear detector. In our standard setup a maximum wavelength discrimination of 3% is achieved with an overall efficiency of 20% and a small decrease in angular resolution of only 0.02 °2θ. In principle, an energy resolution as low as 1.5% can be achieved.

Diffraction Analysis of Ultra-Fine Structures;

Interpretation of variations in the position, shape and intensity of diffraction lines leads to detailed models for highly localized changes in the spatial periodicities of the density of solid matter, as the lattice spacings for crystalline material and the composition-modulation period of (even amorphous) multilayers. Ultrafine structures and changes thereof can be characterized. As examples analyses of (i) interdiffusion and the coupled stress and volume changes in amorphous multilayers and of (ii) preprecipitation phenomena in Fe-C and Fe-N martensites have been presented.