F. Danoix

Direct evidence of cementite dissolution in drawn pearlitic steels observed by tomographic atom probe

The dissolution of cementite during cold-drawing of pearlitic steels has been directly observed with the tomographic atom probe (TAP). Because of the nanometric interlamellar spacing and size of cementite lamellae, the TAP is shown to be extremely well suited for such a study. Analysis conditions and mass spectra peak deconvolution are shown to lead to quantitative analysis of both ferrite and cementite. It is also shown that specimen preparation always aligns the cementite lamellae habit plane with the analysis direction so that the analysed area is always perpendicular to their habit plane. Local magnification effects are also shown not to affect the carbon concentration measurement in the cementite. The first direct atomic scale quantitative concentration data across a few nanometer-thick cementite lamellae are given, and confirm the dissolution of cementite after cold-drawing. The derived compositions of ferrite, cementite and interfacial areas are obtained, giving information on the cementite dissolution mechanism as well as on its extent.

Composition of β″ precipitates in Al–Mg–Si alloys by atom probe tomography and first principles calculations

The composition of β″ precipitates in an Al–Mg–Si alloy has been investigated by atom probe tomography, ab initio density functional calculations, and quantitative electron diffraction. Atom probe analysis of an Al-0.72% Si-0.58% Mg (at. %) alloy heat treated at 175 °C for 36 h shows that the β″ phase contains ∼20 at. % Al and has a Mg/Si-ratio of 1.1, after correcting for a local magnification effect and for the influence of uneven evaporation rates. The composition difference is explained by an exchange of some Si with Al relative to the published β″-Mg5Si6 structure. Ab initio calculations show that replacing the Si3-site by aluminum leads to energetically favorable compositions consistent with the other phases in the precipitation sequence. Quantitative electron nanodiffraction is relatively insensitive to this substitution of Al by Si in the β″-phase.

An improved reconstruction procedure for the correction of local magnification effects in three‐dimensional atom‐probe

A new 3DAP reconstruction procedure is proposed that accounts for the evaporation field of a secondary phase. It applies the existing cluster selection software to identify the atoms of the second phase and, subsequently, an iterative algorithm to homogenise the volume laterally. This procedure, easily implementable on existing reconstruction software, has been applied successfully on simulated and real 3DAP analyses.

Atomic-scale observation and modelling of cementite dissolution in heavily deformed pearlitic steels

Abstract Heavily deformed pearlitic steel wires obtained by cold drawing have been investigated using a three-dimensional atom probe. Concentration profiles reveal the existence of pronounced gradients in the ferrite near ferrite-cementite interfaces. This indicates that cementite lamellae dissolve. Experiments are interpreted and dissolution kinetics are modelled semiquantitatively on the basis of thermodynamics and diffusion arguments. The dramatic increase in interface areas during drawing is considered as the driving force for dissolution through a Gibbs-Thomson effect. The relatively slow cooling of specimen from the temperature of drawing (100–300°C) to room temperature hence leads to the downhill diffusion of carbon from interfaces to the ferrite core. The kinetics equations are numerically solved in order to take into account the nonconstant mobility of carbon during cooling. The model highlights the key parameters which drive dissolution and their influence on kinetics. In comparison with experiments, predicted dissolution rates appear, however, to be underestimated.

Precipitation strengthening in high manganese austenitic TWIP steels

Abstract At low strains (∊ 0.3) the work hardening rate decreased slightly. Synchrotron X-ray diffraction analysis suggested that this was due to a reduction in the kinetics of twin formation. The highest strengthening coefficient in cold strips was obtained with Ti additions ≤0.1 wt.% (+1 380 MPa/wt.% Ti) but the effect quickly saturated after an increase of ∼+150 MPa. With Nb additions only modest hardening (+187 MPa/wt.%) could be achieved. The strengthening due to V was >530 MPa/wt.% for V additions ≤ 0.4 wt.% Saturation effects are less critical with V additions and yield stress increases of +375 MPa were demonstrated.

Investigation of wüstite (Fe1−xO) by femtosecond laser assisted atom probe tomography

In this paper, we report results obtained from laser assisted three-dimensional (3-D) atom probe tomography (APT) on wüstite (Fe(1-x)O). Oxides are generally insulating and hence hard to analyse in conventional electrical assisted APT. To overcome this problem, femtosecond laser pulses are used instead of voltage pulses. Here we discuss some aspects of pulsed laser field evaporation and optimization of parameters to achieve better chemical accuracy.

An atom-probe investigation of some correlated phase transformations in Cr, Ni, Mo containing supersaturated ferrites

Abstract At 300–400°C aged ferrite of duplex stainless steels may undergo complex demixing processes which depend on the alloying element nature and content. In addition to the spinodal decomposition of the δ ferrite solid-solution into α (Fe-rich) and α′ (Cr-rich) domains, which characterizes the unmixing process, variations in the alloying element content lead to a more or less important precipitation of an intermetallic G-phase. For a given Cr-content, at least four elements (C, Si, Ni and Mo) induce a particular sensitivity to G-phase precipitation. Two duplex stainless steels, the ferrite of which exhibits almost the same C, Si and Cr but different Ni and Mo contents have been carefully studied by atom probe. Because of its high spatial resolution, the atom probe is an attractive tool for the investigation of the fine-scale spinodal decomposition and precipitation process. In order to get both reliable chemical and spatial information, technical improvements as well as refinements in the statistical analysis of the experimental data were implemented. Characterization of both the spinodal decomposition and G-phase precipitation was carried out. Pertinent microstructural aging parameters were defined in such a way as to follow both kinetics. The investigations we conducted, demonstrate that G-phase precipitation is induced by the spinodal decomposition of the ferrite phase. The influence of Ni and Mo contents on G-phase precipitation is discussed.

Atom probe characterization of isotropic spinodal decompositions: spatial convolutions and related bias

Abstract The present study aims at characterizing the amplitude of chromium concentration fluctuations which occur in the spinodally decomposed ferrite phase of a duplex stainless steel. Measured amplitudes are shown to be dependent on both the value of the analysis diameter and the sampling depth as compared with the characteristic lengths of concentration fluctuations. A theoretical model has been designed in order to quantify this dependence. It takes into account axial smoothing, associated with depth sampling, and radial convolution, related to the analysed area which have been studied as a function of different analysis conditions. Optimized analysis parameters are deduced and the corresponding attenuation factor, relative to amplitude measurement, is calculated.

Investigation of precipitation in a new maraging stainless steel

Abstract A new type of maraging steel with the composition 12Cr9Ni4Mo2Cu (wt%) was developed by AB Sandvik Steel. According to the analytical electron microscopy (ATEM) investigations preceding this study, the high strengh (3000 MPa) of the material was assigned to a new quasicrystalline phase possessing icosahedral symmetry and of typical composition 35Mo40Fe16Cr2Ni7Si (at%). However, only precipitates after prolonged heat treatment could be successfully studied using ATEM technique. To better understand early stages of precipitation APFIM studies of the steels were therefore undertaken. Materials heat treated at 475°C for 5, 25 min, 4 and 400 h were analyzed. The Ni and Cu-rich precipitates were found to be the major precipitates for the short time heat treatments. In the materials after prolonged aging, Cu-rich areas in direct connection to Ni-rich precipitates were observed. Tomographic atom probe (TAP) investigation was performed to clarify the spatial distribution of elements in and close to the precipitates. A Mo-rich phase was detected in the steel after 4 h aging. The distribution of alloying elements in the observed precipitates is discussed.

The morphology of secondary-hardening carbides in a martensitic steel at the peak hardness by 3DFIM

The morphology and composition of secondary-hardening M(2)C carbides in a complex steel under non-isothermal tempering condition has been investigated with three-dimensional field ion microscopy and atom-probe tomography. The technical set-up and the condition of investigations have been developed. We will reveal for the first time, a virtually non-biased image of the so-called secondary-hardening microstructure, consisting in a very fine dispersion of nanometer-sized needles, idiomorphs and blocky carbides. Needles precipitate with a large number density at the maximum hardness peak. We have found out that this mixture of shape could be explained by the onset of coarsening, but the role of local factors have been evidenced: variation of composition among the carbides and even local strain effects due to the precipitation of a second phase can play a role in changing the growth conditions.