Eric Aime Jägle

Kinetics of the allotropic hcp-fcc phase transformation in cobalt.

The allotropic, martensitic phase transformation (hcp → fcc) in cobalt was investigated by differential scanning calorimetry (DSC) upon isochronal annealing at heating rates in the range from 10 K min−1 to 40 K min−1. The microstructural evolution was traced by optical microscopy and X-ray diffractometry. The kinetics of the phase transformation from hcp to fcc Co upon isochronal annealing was described on the basis of a modular phase transformation model. Appropriate model descriptions for athermal nucleation and thermally activated, anisotropic interface controlled growth tailored to the martensitic phase transformation of Co were implemented into the modular model. Fitting of this model of phase transformation kinetics to simultaneously all isochronal DSC runs yielded values for the energy of the interface separating the hcp and fcc Co phase and the activation energy for growth.

Comparison of Maraging Steel Micro- and Nanostructure Produced Conventionally and by Laser Additive Manufacturing

Maraging steels are used to produce tools by Additive Manufacturing (AM) methods such as Laser Metal Deposition (LMD) and Selective Laser Melting (SLM). Although it is well established that dense parts can be produced by AM, the influence of the AM process on the microstructure—in particular the content of retained and reversed austenite as well as the nanostructure, especially the precipitate density and chemistry, are not yet explored. Here, we study these features using microhardness measurements, Optical Microscopy, Electron Backscatter Diffraction (EBSD), Energy Dispersive Spectroscopy (EDS), and Atom Probe Tomography (APT) in the as-produced state and during ageing heat treatment. We find that due to microsegregation, retained austenite exists in the as-LMD- and as-SLM-produced states but not in the conventionally-produced material. The hardness in the as-LMD-produced state is higher than in the conventionally and SLM-produced materials, however, not in the uppermost layers. By APT, it is confirmed that this is due to early stages of precipitation induced by the cyclic re-heating upon further deposition—i.e., the intrinsic heat treatment associated with LMD. In the peak-aged state, which is reached after a similar time in all materials, the hardness of SLM- and LMD-produced material is slightly lower than in conventionally-produced material due to the presence of retained austenite and reversed austenite formed during ageing.

The maximum separation cluster analysis algorithm for atom-probe tomography: parameter determination and accuracy.

Atom-probe tomography is a materials characterization method ideally suited for the investigation of clustering and precipitation phenomena. To distinguish the clusters from the surrounding matrix, the maximum separation algorithm is widely employed. However, the results of the cluster analysis strongly depend on the parameters used in the algorithm and hence, a wrong choice of parameters leads to erroneous results, e.g., for the cluster number density, concentration, and size. Here, a new method to determine the optimum value of the parameter dmax is proposed, which relies only on information contained in the measured atom-probe data set. Atom-probe simulations are employed to verify the method and to determine the sensitivity of the maximum separation algorithm to other input parameters. In addition, simulations are used to assess the accuracy of cluster analysis in the presence of trajectory aberrations caused by the local magnification effect. In the case of Cu-rich precipitates (Cu concentration 40-60 at% and radius 0.25-1.0 nm) in a bcc Fe-Si-Cu matrix, it is shown that the error in concentration is below 10 at% and the error in radius is <0.15 nm for all simulated conditions, provided that the correct value for dmax, as determined with the newly proposed method, is employed.

Characterizing solute hydrogen and hydrides in pure and alloyed titanium at the atomic scale

Abstract Ti and its alloys have a high affinity for hydrogen and are typical hydride formers. Ti-hydride are brittle phases which probably cause premature failure of Ti-alloys. Here, we used atom probe tomography and electron microscopy to investigate the hydrogen distribution in a set of specimens of commercially pure Ti, model and commercial Ti-alloys. Although likely partly introduced during specimen preparation with the focused-ion beam, we show formation of Ti-hydrides along α grain boundaries and α/β phase boundaries in commercial pure Ti and α+β binary model alloys. No hydrides are observed in the α phase in alloys with Al addition or quenched-in Mo supersaturation.

Simulation of the kinetics of grain-boundary nucleated phase transformations

The kinetics of phase transformations for which nucleation occurs on parent-micro-structure grain boundaries, and the resulting microstructures, were investigated by means ofgeometric simulations. The influences of parent microstructure grain-boundary area density,parent grain-size distribution and parent→product kinetics were analysed. Additionally, thesimulated kinetics were compared with predictions from two kinetic models, namely a modelproposed for spatially random nucleation and a model proposed for grain-boundary nucleation.It was found that the simulated transformed fraction as function of time lies in between the twomodel predictions for all investigated parent microstructures and parent→product kinetics.

Interplay of Kinetics and Microstructure in the Recrystallization of Pure Copper: Comparing Mesoscopic Simulations and Experiments

The kinetics and microstructure of the static recrystallization of strongly deformed, pure copper were investigated based on a comparison of mesoscopic, cellular automata simulations with experimental data. Physical models for the nucleation rate and the growth rate of recrystallized grains were employed, which required experimentally obtained informations about the deformed state as input. The orientation-distribution function and the misorientation-angle distribution for neighboring grains of deformed specimens were used to set up the microstructure serving as the start configuration for the simulations. The influences of a subgrain-size distribution, of an ongoing nucleation during recrystallization, of an inhomogeneously distributed stored energy in the deformed state, and of a misorientation-dependent interface mobility were investigated. The simulated microstructures after recrystallization exhibit a bimodal grain-size distribution instead of the broad, monomodal grain-size distribution found in the experiments. It is argued that spatially resolved determination of crystal (subgrain) orientation and (deformation) energy is necessary to arrive at realistic descriptions of both the recrystallization kinetics and the microstructure after completed recrystallization.

Kinetics of interface-controlled phase transformations: atomistic and mesoscopic simulations

Abstract This articles provides a summary of results of recent research on simulations of interface-controlled phase transformations with the aim of better understanding both the kinetics of these transformations and the resulting microstructure. Atomistic (kinetic Monte Carlo) simulations were performed to gain insight into the atomic (jump) processes occurring during interface movement in the massive austenite → ferrite phase transformation in pure iron. Mesoscopic (geometric) simulations were performed to reach an understanding of the formation of the microstructures resulting from phase transformations which obey specified kinetic models. As a special feature, the effect of non-random nucleation on the transformation kinetics was studied.

In-process Precipitation During Laser Additive Manufacturing Investigated by Atom Probe Tomography

Laser Metal Deposition (LMD) is a specific Laser Additive Manufacturing (LAM) process in which a focused laser beam creates a melt pool in the component’s surface. Metallic powder is then injected through a nozzle into the melt pool. As neighboring tracks and subsequent layers are deposited during the LMD process, already consolidated material experiences a cyclic re-heating. This intrinsic heat treatment (IHT) can be used to trigger the precipitation reaction in precipitation hardening alloys [1]. We used the rapid alloy prototyping capabilities of the LMD process to efficiently screen different alloy compositions by producing compositionally graded samples. In a simple model maraging steel containing 19at% Ni, we varied the Al concentration from 0 to 25at% to identify an alloy composition that responds well to the IHT of the LAM process to produce an in-process precipitation strengthened maraging steel. Due to its sub-nanometer resolution, Atom Probe Tomography (APT) provides compositional information at high-resolution and is therefore ideally suited for analyzing small clusters and precipitates. Additionally, we used High Energy Synchrotron X-Ray Diffraction (HEXRD) to provide crystallographic information with high sensitivity.

Co-deformation of crystalline-amorphous nanolaminates

Deformation of ductile crystalline-amorphous nanolaminates is not clearly understood due to the complex interplay of interface mechanics, shear banding and deformation-driven chemical mixing. In this work, we synthesized and indented model nanolaminates consisting of nanocrystalline Cu and amorphous CuZr (Fig1a-c). In order to study both, nanostructural and atomic-scale chemical deformation effects at the same specimen position we performed a joint analysis by transmission electron microscopy (TEM) and Atom Probe Tomography (APT).