On Strain Gradients and Size-Dependent Hardening Descriptions in Crystal Plasticity Frameworks

Han, Chung-Souk,Roters, Franz,Raabe, Dierk 대한금속ᆞ재료학회 2006 METALS AND MATERIALS International Vol.12 No.5

Oxidation behavior of AlN/CrN multilayered hard coatings

Tytko Darius,Choi Pyuck-Pa,Raabe Dierk 나노기술연구협의회 2017 Nano Convergence Vol.4 No.15

We report on the oxidation behavior of AlN/CrN multilayers at 900 °C, deposited by radio frequency magnetron sputtering. It is shown that oxidation in this system is controlled by diffusion of Cr towards the surface and formation of Cr2O3. Cr diffusion is found to mainly occur along grain boundaries. Thus, coherent cubic AlN/CrN multilayer regions with coarse columnar grain structures are found to be oxidation resistant, whereas regions decomposed into hexagonal AlN/cubic CrN are prone to oxidation.

Evaluation of Analysis Conditions for Laser-Pulsed Atom Probe Tomography: Example of Cemented Tungsten Carbide

Peng, Zirong,Choi, Pyuck-Pa,Gault, Baptiste,Raabe, Dierk Cambridge University Press 2017 Microscopy and Microanalysis Vol.23 No.2

<B>Abstract</B><P>Cemented tungsten carbide has been analyzed using laser-pulsed atom probe tomography (APT). The influence of experimental parameters, including laser pulse energy, pulse repetition rate, and specimen base temperature, on the acquired data were evaluated from different aspects, such as mass spectrum, chemical composition, noise-to-signal ratio, and multiple events. Within all the applied analysis conditions, only 1 MHz pulse repetition rate led to a strong detector saturation effect, resulting in a largely biased chemical composition. A comparative study of the laser energy settings showed that an ~12 times higher energy was required for the less focused green laser of the LEAP<SUP>TM</SUP> 3000X HR system to achieve a similar evaporation field as the finer spot ultraviolet laser of the LEAP<SUP>TM</SUP> 5000 XS system.</P>

Formation of nanometer-sized Cu-Sn-Se particles in Cu₂ZnSnSe₄ thin-films and their effect on solar cell efficiency

Schwarz, Torsten,Cojocaru-Miré,din, Oana,Mousel, Marina,Redinger, Alex,Raabe, Dierk,Choi, Pyuck-Pa Elsevier 2017 ACTA MATERIALIA Vol.132 No.-

<P>Atom probe tomography and transmission electron microscopy are used to study the formation of nano sized Cu-Sn-Se particles in Cu2ZnSnSe4 thin -films. For a Cu -rich precursor, which was deposited at 320 degrees C under Cu- and Zn-rich growth conditions, Cu2-xSe grains at the surface are detected. During annealing the precursor at 500 degrees C in a SnSe + Se atmosphere most of the Cu2-xSe is transformed to Cu2ZnSnSe4 via the consumption of excessive ZnSe and incorporation of Sn. However, atom probe tomography studies also reveal the formation of various nanometer-sized Cu-Sn-Se particles close to the CdS/Cu(2)ZnSnSe4 interface. One of those particles has a composition close to the Cu2SnSe3 compound. This phase has a smaller band gap than Cu2ZnSnSe4 and is proposed to lead to a significant drop in the open -circuit voltage and could be the main cause for a detrimental p-n junction and the zero efficiency of the final device. Possible effects of the other phases on solar cell performance and formation mechanisms are discussed as well. (C)2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.</P>

(Nb_<i>x</i>, Zr_1–<i>x</i>)₄AlC₃ MAX Phase Solid Solutions: Processing, Mechanical Properties, and Density Functional Theory Calculations

Lapauw, Thomas,Tytko, Darius,Vanmeensel, Kim,Huang, Shuigen,Choi, Pyuck-Pa,Raabe, Dierk,Caspi, El’ad N.,Ozeri, Offir,to Baben, Moritz,Schneider, Jochen M.,Lambrinou, Konstantina,Vleugels, Jozef American Chemical Society 2016 Inorganic Chemistry Vol.55 No.11

<P>The solubility of zirconium (Zr) in the Nb<SUB>4</SUB>AlC<SUB>3</SUB> host lattice was investigated by combining the experimental synthesis of (Nb<SUB><I>x</I></SUB>, Zr<SUB>1–<I>x</I></SUB>)<SUB>4</SUB>AlC<SUB>3</SUB> solid solutions with density functional theory calculations. High-purity solid solutions were prepared by reactive hot pressing of NbH<SUB>0.89</SUB>, ZrH<SUB>2</SUB>, Al, and C starting powder mixtures. The crystal structure of the produced solid solutions was determined using X-ray and neutron diffraction. The limited Zr solubility (maximum of 18.5% of the Nb content in the host lattice) in Nb<SUB>4</SUB>AlC<SUB>3</SUB> observed experimentally is consistent with the calculated minimum in the energy of mixing. The lattice parameters and microstructure were evaluated over the entire solubility range, while the chemical composition of (Nb<SUB>0.85</SUB>, Zr<SUB>0.15</SUB>)<SUB>4</SUB>AlC<SUB>3</SUB> was mapped using atom probe tomography. The hardness, Young’s modulus, and fracture toughness at room temperature as well as the high-temperature flexural strength and E-modulus of (Nb<SUB>0.85</SUB>, Zr<SUB>0.15</SUB>)<SUB>4</SUB>AlC<SUB>3</SUB> were investigated and compared to those of pure Nb<SUB>4</SUB>AlC<SUB>3</SUB>. Quite remarkably, an appreciable increase in fracture toughness was observed from 6.6 ± 0.1 MPa/m<SUP>1/2</SUP> for pure Nb<SUB>4</SUB>AlC<SUB>3</SUB> to 10.1 ± 0.3 MPa/m<SUP>1/2</SUP> for the (Nb<SUB>0.85</SUB>, Zr<SUB>0.15</SUB>)<SUB>4</SUB>AlC<SUB>3</SUB> solid solution.</P><P>The solubility of Zr in nanolaminated (Nb<SUB>1−<I>x</I></SUB>, Zr<SUB><I>x</I></SUB>)<SUB>4</SUB>AlC<SUB>3</SUB> is investigated. Experimentally, a maximum at <I>x</I> = 18.5% is found, which is consistent with a minimum in the calculated energy of mixing at low Zr contents. The mechanical behavior of this solid solution shows a significant improvement in fracture toughness and temperature stability compared to those of Nb<SUB>4</SUB>AlC<SUB>3</SUB>.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/inocaj/2016/inocaj.2016.55.issue-11/acs.inorgchem.6b00484/production/images/medium/ic-2016-00484u_0007.gif'></P>

Spatial Distributions of Alloying Elements Obtained from Atom Probe Tomography of the Amorphous Ribbon Fe₇₅C₁₁Si₂B₈Cr₄

Shin, Jinkyung,Yi, Seonghoon,Pradeep, Konda Gokuldoss,Choi, Pyuck-Pa,Raabe, Dierk Materials Research Society of Korea 2013 한국재료학회지 Vol.23 No.3

Spatial distributions of alloying elements of an Fe-based amorphous ribbon with a nominal composition of $Fe_{75}C_{11}Si_2B_8Cr_4$ were analyzed through the atom probe tomography method. The amorphous ribbon was prepared through the melt spinning method. The macroscopic amorphous natures were confirmed using an X-ray diffractometer (XRD) and a differential scanning calorimeter (DSC). Atom Probe (Cameca LEAP 3000X HR) analyses were carried out in pulsed voltage mode at a specimen base temperature of about 60 K, a pulse to base voltage ratio of 15 %, and a pulse frequency of 200 kHz. The target detection rate was set to 5 ions per 1000 pulses. Based on a statistical analyses of the data obtained from the volume of $59{\times}59{\times}33nm^3$, homogeneous distributions of alloying elements in nano-scales were concluded. Even with high carbon and strong carbide forming element contents, nano-scale segregation zones of alloying elements were not detected within the Fe-based amorphous ribbon. However, the existence of small sub-nanometer scale clusters due to short range ordering cannot be completely excluded.

Modulation of plastic flow in metallic glasses via nanoscale networks of chemical heterogeneities

Kim, Jinwoo,Oh, Hyun Seok,Kim, Wan,Choi, Pyuck-Pa,Raabe, Dierk,Park, Eun Soo Elsevier 2017 Acta materialia Vol.140 No.-

<P>We systematically investigate the microstructures of metallic glasses with nanoscale networks of chemical heterogeneities introduced by the presence of a metastable miscibility gap, and their effects on modulating plastic flow of the alloys. Microstructural analysis of as-quenched alloys and the associated thermodynamic assessment in Cu-Zr-Al-Y metallic glass-forming system suggest that the existence of a metastable miscibility gap can induce not only phase-separated microstructures with sharp phase interfaces but also compositional fluctuations without a clear interface ranging from atomic scale to a few-nanometer scale in the fully amorphous alloys. The statistical analysis of shear avalanches in such compositionally heterogeneous metallic glasses reveals that chemical heterogeneities extending over a few nanometers promote a relatively large population of shear deformation units jammed before the nucleation of mature shear bands. This leads to the multiple nucleation of shear bands and sluggish deformation behavior along them. However, phase interfaces formed by phase separation inside the miscibility gap promote rapid propagation of shear bands at low flow stress, while compositional fluctuations creating non-sharp interfaces emerging at the outside of miscibility gap have relatively high resistance against shear band propagation. We hence suggest that the optimization of nanoscale compositional fluctuations in metallic glasses in terms of topology, percolation and magnitude can be an effective route for improving the materials' damage tolerance upon plastic flow. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.</P>

Elemental partitioning and site-occupancy in γ/γ′ forming Co-Ti-Mo and Co-Ti-Cr alloys

Im, Hye Ji,Makineni, Surendra K.,Gault, Baptiste,Stein, Frank,Raabe, Dierk,Choi, Pyuck-Pa Elsevier 2018 Scripta materialia Vol.154 No.-

<P><B>Abstract</B></P> <P>We report on the sub-nanometer scale characterization of Co-12Ti-4Mo and Co-12Ti-4Cr (at.%) model alloys. Atom probe tomography reveals that Co and Cr partition to γ, whereas Ti and Mo to γ′. Additions of Mo and Cr to the reference Co-12Ti system lead to strong increases in γ′ volume fraction by about 25% and 12%, respectively. Element-specific spatial distribution maps along the [001] direction of the L1<SUB>2</SUB>-ordered γ′ phase reveal that both Mo and Cr preferentially replace Ti on its sublattice. The remaining excess Ti is available for formation of additional γ′, resulting in enhanced γ′ volume fractions.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

On the detection of multiple events in atom probe tomography

Peng, Zirong,Vurpillot, Francois,Choi, Pyuck-Pa,Li, Yujiao,Raabe, Dierk,Gault, Baptiste Elsevier 2018 Ultramicroscopy Vol.189 No.-

<P><B>Abstract</B></P> <P>In atom probe tomography (APT), multiple events can arise as a consequence of e.g. correlated field evaporation and molecular ion dissociation. They represent challenging cases for single-particle detectors and can cause compositional as well as spatial inaccuracies. Here, two state-of-the-art atom probe microscopes (Cameca LEAP 5000 XS and 5000 XR) were used to investigate cemented tungsten carbide, which exhibits high amounts of multiple events. By advanced data analysis methods, the natural character of the multiple events, as well as the performance of the APT detectors, are assessed. Accordingly, possible signal loss mechanisms are discussed.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Delay line detectors in atom probe tomography exhibit dead time and dead zone. </LI> <LI> For both Cameca LEAP 5000 XS and 5000 XR instruments, the dead time is about 3 ns. </LI> <LI> The dead zone evolves with the propagation of signals on the delay lines. </LI> <LI> The dead time and dead zone can cause signal loss during multiple events detection. </LI> <LI> The compositional and spatial accuracies can be strongly deteriorated. </LI> </UL> </P>

The effects of prior austenite grain boundaries and microstructural morphology on the impact toughness of intercritically annealed medium Mn steel

Han, Jeongho,da Silva, Alisson Kwiatkowski,Ponge, Dirk,Raabe, Dierk,Lee, Sang-Min,Lee, Young-Kook,Lee, Sang-In,Hwang, Byoungchul Elsevier 2017 Acta materialia Vol.122 No.-

<P><B>Abstract</B></P> <P>The effects of prior austenite (γ) grain boundaries and microstructural morphology on the impact toughness of an annealed Fe-7Mn-0.1C-0.5Si medium Mn steel were investigated for two different microstructure states, namely, hot-rolled and annealed (HRA) specimens and cold-rolled and annealed (CRA) specimens. Both types of specimens had a dual-phase microstructure consisting of retained austenite (γ<SUB>R</SUB>) and ferrite (α) after intercritical annealing at 640 °C for 30 min. The phase fractions and the chemical composition of γ<SUB>R</SUB> were almost identical in both types of specimens. However, their microstructural morphology was different. The HRA specimens had lath-shaped morphology and the CRA specimens had globular-shaped morphology. We find that both types of specimens showed transition in fracture mode from ductile and partly quasi-cleavage fracture to intergranular fracture with decreasing impact test temperature from room temperature to −196 °C. The HRA specimen had higher ductile to brittle transition temperature and lower low-temperature impact toughness compared to the CRA specimen. This was due to intergranular cracking in the HRA specimens along prior γ grain boundaries decorated by C, Mn and P. In the CRA specimen intergranular cracking occurred along the boundaries of the very fine α and α′ martensite grains. The results reveal that cold working prior to intercritical annealing promotes the elimination of the solute-decorated boundaries of coarse prior γ grains through the recrystallization of αʹ martensite prior to reverse transformation, hence improving the low-temperature impact toughness of medium Mn steel.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>