Publications
440 results found
Chang YH, Mouton I, Stephenson L, et al., 2019, Quantification of solute deuterium in titanium deuteride by atom probe tomography with both laser pulsing and high-voltage pulsing: influence of the surface electric field, NEW JOURNAL OF PHYSICS, Vol: 21, ISSN: 1367-2630
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- Citations: 24
Lomuscio A, Rödel T, Schwarz T, et al., 2019, Quasi-Fermi-Level Splitting of Cu-Poor and Cu-Rich CuInS2 Absorber Layers, Physical Review Applied, Vol: 11
Pandey P, Makineni SK, Gault B, et al., 2019, On the origin of a remarkable increase in the strength and stability of an Al rich Al-Ni eutectic alloy by Zr addition, ACTA MATERIALIA, Vol: 170, Pages: 205-217, ISSN: 1359-6454
Kwiatkowski da Silva A, Kamachali RD, Ponge D, et al., 2019, Thermodynamics of grain boundary segregation, interfacial spinodal and their relevance for nucleation during solid-solid phase transitions, Acta Materialia, Vol: 168, Pages: 109-120, ISSN: 1359-6454
© 2019 Acta Materialia Inc. Grain boundary segregation, embrittlement and phase nucleation are interconnected phenomena that are often treated separately, which is partly due to limitations of the current models to predict grain boundary segregation in non-ideal solid solutions. Here, a simple model is introduced to predict grain boundary segregation in solid solutions by coupling available bulk thermodynamic data with a mean-field description of the grain boundary character. The model is confronted with experimental results obtained in Fe-Mn alloys analysed by atom probe tomography. This model successfully predicts a first order transition or a discontinuous jump in the composition of the grain boundary which kinetically implies the formation of spinodal Mn fluctuations that tend to grow further with time within the segregated region. The increase in solute concentration at the grain boundary leads to an increase of the enthalpy of the boundary and to its embrittlement at lower temperatures. Once austenite is formed, the amount of segregated solute Mn on the grain boundaries is drastically reduced and the toughness of the grain boundary is increased.
Pandey P, Sawant AK, Nithin B, et al., 2019, On the effect of Re addition on microstructural evolution of a CoNi-based superalloy
In this study, the effect of rhenium (Re) addition on microstructural evolution of a new low-density Co-Ni-Al-Mo-Nb based superalloy is presented. Addition of Re significantly influences the γ′ precipitate morphology, the γ/γ′ lattice misfit and the γ/γ′ microstructural stability during long term aging. An addition of 2 at.% Re to a Co-30Ni-10Al-5Mo-2Nb (all in at.%) alloy, aged at 900 °C for 50 h, reduces the γ/γ′ lattice misfit by ∼ 40% (from +0.32% to +0.19%, measured at room temperature) and hence alters the γ′ morphology from cuboidal to round-cornered cuboidal precipitates. The composition profiles across the γ/γ′ interface by atom probe tomography (APT) reveal Re partitions to the γ phase (K Re =0.34) and also results in the partitioning reversal of Mo to the γ phase (K Mo =0.90) from the γ′ precipitate. An inhomogeneous distribution of Gibbsian interfacial excess for the solute Re (Γ Re , ranging from 0.8 to 9.6 atom.nm −2 ) has been observed at the γ/γ′ interface. A coarsening study at 900 °C (up to 1000 h) suggests that the coarsening of γ′ precipitates occurs solely by evaporation–condensation (EC) mechanism. This is contrary to that observed in the Co-30Ni-10Al-5Mo-2Nb alloy as well as in some of the Ni-Al based and high mass density Co-Al-W based superalloys, where γ′ precipitates coarsen by coagulation/coalescence mechanism with an extensive alignment of γ′ along <100> directions as a sign of microstructural instability. The γ′ coarsening rate constant (K r ) and γ/γ′ interfacial energy are estimated to be 1.13 × 10 −27 m 3 /s and 8.4 mJ/m 2 , which are comparable and lower than Co-Al-W based superalloys.
Moutona I, Breen AJ, Wang S, et al., 2019, Quantification challenges for atom probe tomography of hydrogen and deuterium in zircaloy-4, Microscopy and Microanalysis, Vol: 25, Pages: 481-488, ISSN: 1431-9276
Analysis and understanding of the role of hydrogen in metals is a significant challenge for the future of materials science, and this is a clear objective of recent work in the atom probe tomography (APT) community. Isotopic marking by deuteration has often been proposed as the preferred route to enable quantification of hydrogen by APT. Zircaloy-4 was charged electrochemically with hydrogen and deuterium under the same conditions to form large hydrides and deuterides. Our results from a Zr hydride and a Zr deuteride highlight the challenges associated with accurate quantification of hydrogen and deuterium, in particular associated with the overlap of peaks at a low mass-to-charge ratio and of hydrogen/deuterium containing molecular ions. We discuss possible ways to ensure that appropriate information is extracted from APT analysis of hydrogen in zirconium alloy systems that are important for nuclear power applications.
Mompiou F, Tingaud D, Chang Y, et al., 2019, Corrigendum to “Conventional vs harmonic-structured β-Ti-25Nb–25Zr alloys: A comparative study of deformation mechanisms” [Acta Mater. 161 (2018) 420–430](S1359645418307444)(10.1016/j.actamat.2018.09.032), Acta Materialia, Vol: 167, Pages: 287-288, ISSN: 1359-6454
© 2019 Acta Materialia Inc. The authors regret an error in the measure of pinning points distance and stress in the above-mentioned article. The correct Fig. 9 is drawn in Fig. 1. In p. 428, first column, the average value of τ l is now 104 MPa and the average distance between anchoring points is d = 63 nm, which yields to τ = μb/d = 128 MPa. The values extrapolated from dilute solid solutions are 154 MPa≤ τ T i ≤ 164 MPa. These values are still of the order of magnitude of the macroscopic yield shear stress of 220 MPa and 150 MPa. Taking into account an average Taylor factor of 1.5, the average critical resolved shear stress from mechanical tests should be of the order of 100–147 MPa which is close to the measured values. Fig. 1.[Figure presented] The authors would like to apologise for any inconvenience caused. The authors acknowledge W. A. Curtin for pointing out the error.
Schwarz T, Redinger A, Siebentritt S, et al., 2019, Variable chemical decoration of extended defects in Cu-poor C u2ZnSnS e4 thin films, Physical Review Materials, Vol: 3
© 2019 American Physical Society. We report on atom probe tomography studies of variable chemical decorations at extended defects in Cu-poor and Zn-rich Cu2ZnSnSe4 thin films. For a precursor film, which was co-evaporated at 320C, grain boundaries and dislocations are found enriched with Cu. Furthermore, Na out-diffusion from the soda-lime glass substrate occurs even at such a low temperature, resulting in Na segregation at defects. In contrast, stacking faults in the precursor film show clear Zn enrichment as well as Cu and Sn depletion. After an annealing step at 500C, we detect changes in the chemical composition of grain boundaries as compared to the precursor. Moreover, we measure an increase in the grain boundary excess of Na by one order of magnitude. We show that grain boundaries and dislocations in the annealed Cu2ZnSnSe4 film exhibit no or only slight variations in composition of the matrix elements. Thus, the effect of annealing is a homogenization of the chemical composition.
Yan F, Mouton I, Stephenson LT, et al., 2019, Atomic-scale investigation of hydrogen distribution in a Ti–Mo alloy, Scripta Materialia, Vol: 162, Pages: 321-325, ISSN: 1359-6462
Ingress of hydrogen is often linked to catastrophic failure of Ti-alloys. Here, we quantify the hydrogen distribution in fully β and α + β Ti–Mo alloys by using atom probe tomography. Hydrogen does not segregate at grain boundaries in the fully β sample but segregates at some α/β phase boundaries with a composition exceeding 20 at.% in the α + β sample. No stable hydrides were observed in either sample. The hydrogen concentration in β phases linearly decreases from ~13 at. % to ~4 at. % with increasing Mo-content, which is ascribed to the suppression of hydrogen uptake by Mo addition.
Wu M, Li Z, Gault B, et al., 2019, The effects of carbon on the phase stability and mechanical properties of heat-treated FeNiMnCrAl high entropy alloys, Materials Science and Engineering A, Vol: 748, Pages: 59-73, ISSN: 0921-5093
© 2019 This work systematically investigates the effect of carbon on the phase stability and room- temperature tensile performance of an annealed Fe 40.4 Ni 11.3 Mn 34.8 Al 7.5 Cr 6 (at%) high entropy alloy without (HEA) and with 1.1% carbon (CHEA). Four annealing conditions were investigated: 773 K for 13 d and 42 d, 973 K for 20 d, and 1073 K and 1423 K for 24 h. The resulting microstructures were analyzed using a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and atom probe tomography (APT). Vickers hardness testing and room- temperature tensile testing were used to determine the mechanical properties of the annealed HEA and the CHEA. APT composition profiles revealed fine Ni,Mn,Al-enriched matrix precipitates in the CHEA annealed at 773 K for 13 d. After ageing at 773 K for 42 d, colonies of (Ni,Fe) 2 MnAl- enriched Heusler phase lamellae were observed at the grain boundaries (GBs) and in the matrix for the HEA, while GB lamellar colonies of Mn,Cr-enriched M 23 C 6 carbides were observed for the CHEA. Due to the presence of the GB carbides, the resulting room-temperature elongation to fracture for the CHEA annealed at 773 K for 42 d was ~1% compared to ~11% for the HEA given the same anneal. At higher annealing temperatures, the microstructure contains Ni,Al-rich and Mn,Cr,C-rich precipitates that alternate along the GBs and appear to be associated with each other in the matrix. Electron diffraction analysis indicates the aforementioned Ni,Al-precipitates and Mn,Cr-carbides have b.c.c. and f.c.c. crystal structures, respectively. Once again, a low elongation to fracture (2%) was seen for the carbon-containing material compared to its un-doped counterpart (23%) which solely contained Ni,Al-rich b.c.c. precipitates.
Benzing JT, Kwiatkowski da Silva A, Morsdorf L, et al., 2019, Multi-scale characterization of austenite reversion and martensite recovery in a cold-rolled medium-Mn steel, Acta Materialia, Vol: 166, Pages: 512-530, ISSN: 1359-6454
© 2019 Acta Materialia Inc. A medium-Mn steel (Fe-12Mn-3Al-0.05C wt%) was designed using Thermo-Calc ® simulations to balance the fraction and stacking fault energy of reverted austenite. Intercritical annealing for 0.5, 8 and 48 h was carried out at 585 °C to investigate the microstructural evolution. X-ray diffraction (XRD), electron backscatter diffraction (EBSD), 3-dimensional EBSD, energy-dispersive spectroscopy via scanning-transmission electron microscopy (STEM-EDS) and atom probe tomography (APT) enable characterization of phase fraction, grain area, grain morphology and alloy partitioning. An increase in annealing time from 0.5 h to 48 h increases the amount of ultrafine-grained (UFG) reverted austenite from 3 to 40 vol %. EBSD and TEM reveal multiple morphologies of UFG austenite (equiaxed, rod-like and plate-like). In addition, most of the remaining microstructure consists of recovered α′-martensite that resembles the cold-rolled state, as well as a relatively small fraction of UFG ferrite (i.e., only a small amount of martensite recrystallization occurs). Multi-scale characterization results show that the location within the cold-rolled microstructure has a strong influence on boundary mobility and grain morphology during austenite reversion. Results from APT reveal Mn-decoration of dislocation networks and low-angle lath boundaries in the recovered α′-martensite, but an absence of Mn-decoration of defects in the vicinity of austenite grains, thereby promoting recovery. STEM-EDS and APT reveal Mn depletion zones in the ferrite/recovered α′-martensite near austenite boundaries, whereas gradients of C and Mn co-partitioning are visible within some of the austenite grains after annealing for 0.5 h. Relatively flat C-enriched austenite boundaries are present even after 8 h of annealing and indicate certain boundaries possess low mobility. At later stages the growth of austenite followed the local equilibrium
Souza Filho IR, Kwiatkowski da Silva A, Sandim MJR, et al., 2019, Martensite to austenite reversion in a high-Mn steel: Partitioning-dependent two-stage kinetics revealed by atom probe tomography, in-situ magnetic measurements and simulation, Acta Materialia, Vol: 166, Pages: 178-191, ISSN: 1359-6454
Austenite (γ) reversion in a cold-rolled 17.6 wt.% Mn steel was tracked by means of dilatometry and in-situ magnetic measurements during slow continuous annealing. A splitting of the γ-reversion into two stages was observed to be a result of strong elemental partitioning between γ and α′-martensite during the low temperature stage between 390 and 575 °C. Atom probe tomography (APT) results enable the characterization of the Mn-enriched reversed-γ and the Mn-depleted remaining α′-martensite. Because of its lower Mn content, the reversion of the remaining α′-martensite into austenite takes place at a higher temperature range between 600 and 685 °C. APT results agree with partitioning predictions made by thermo-kinetic simulations of the continuous annealing process. The critical composition for γ-nucleation was predicted by thermodynamic calculations (Thermo-Calc) and a good agreement was found with the APT data. Additional thermo-kinetic simulations were conducted to evaluate partitioning-governed γ-growth during isothermal annealing at 500 °C and 600 °C. Si partitioning to γ was predicted by DICTRA and confirmed by APT. Si accumulates near the moving interface during γ-growth and homogenizes over time. We used the chemical composition of the remaining α′-martensite from APT data to calculate its Curie temperature (TCurie) and found good agreement with magnetic measurements. These results indicate that elemental partitioning strongly influences not only γ-reversion but also the TCurie of this steel. The results are important to better understand the thermodynamics and kinetics of austenite reversion for a wide range of Mn containing steels and its effect on magnetic properties.
Kontis P, Kostka A, Raabe D, et al., 2019, Influence of composition and precipitation evolution on damage at grain boundaries in a crept polycrystalline Ni-based superalloy, Acta Materialia, Vol: 166, Pages: 158-167, ISSN: 1359-6454
The microstructural and compositional evolution of intergranular carbides and borides prior to and after creep deformation at 850 °C in a polycrystalline nickel-based superalloy was studied. Primary MC carbides, enveloped within intergranular γ′ layers, decomposed resulting in the formation of layers of the undesirable η phase. These layers have a composition corresponding to Ni3Ta as measured by atom probe tomography and their structure is consistent with the D024 hexagonal structure as revealed by transmission electron microscopy. Electron backscattered diffraction reveals that they assume various misorientations with regard to the adjacent grains. As a consequence, these layers act as brittle recrystallized zones and crack initiation sites. The composition of the MC carbides after creep was altered substantially, with the Ta content decreasing and the Hf and Zr contents increasing, suggesting a beneficial effect of Hf and Zr additions on the stability of MC carbides. By contrast, M5B3 borides were found to be microstructurally stable after creep and without substantial compositional changes. Borides at 850 °C were found to coarsen, resulting in some cases into γ′- depleted zones, where, however, no cracks were observed. The major consequences of secondary phases on the microstructural stability of superalloys during the design of new polycrystalline superalloys are discussed.
Massey CP, Dryepondt SN, Edmondson PD, et al., 2019, Multiscale investigations of nanoprecipitate nucleation, growth, and coarsening in annealed low-Cr oxide dispersion strengthened FeCrAl powder, Acta Materialia, Vol: 166, Pages: 1-17, ISSN: 1359-6454
A major challenge in the design of oxide dispersion strengthened (ODS) FeCrAl alloys is the optimization of the fine-scale particle size distribution that provides both beneficial mechanical properties and irradiation resistance. To address this obstacle, the nucleation, growth, and coarsening of the fine-scale (Y,Al,O) nanoprecipitates within an ODS FeCrAl powder was studied using atom probe tomography (APT) and small-angle neutron scattering (SANS). Mechanically alloyed Fe–10Cr-6.1Al-0.3Zr + Y2O3 wt.% (CrAZY) powders were heated in-situ from 20 to 1000 °C to capture the nucleation and growth of the nanoprecipitates using SANS. Furthermore, CrAZY powders were annealed at 1000 °C, 1050 °C, and 1100 °C for ageing times from 15 min to 500 h followed by either APT or magnetic SANS to study the structure, composition, and coarsening kinetics of the nanoprecipitates at high temperature. In-situ SANS results indicate nanoprecipitate nucleation and growth at low temperatures (200–600 °C). APT results revealed compositions corresponding to the cubic Y3Al5O12 garnet (YAG) stoichiometry with a possible transition towards the perovskite YAlO3 (YAP) phase for larger precipitates after sufficient thermal ageing. However, magnetic SANS results suggest a defective structure for the nanoprecipitates indicated by deviations of the calculated A-ratio from stoichiometric (Y,Al,O) phases. Particle coarsening kinetics follow n = 6 power law kinetics with respect to particle size, but the mechanism cannot be explained through the dislocation pipe diffusion mechanism. The potential effect of precipitate coarsening during pre- and post-consolidation heat treatments on the irradiation resistance of ODS FeCrAl alloys is discussed with respect to sink strength maximization.
Chang Y, Lu W, Guenole J, et al., 2019, Ti and its alloys as examples of cryogenic focused ion beam milling of environmentally-sensitive materials, Nature Communications, Vol: 10, Pages: 1-10, ISSN: 2041-1723
Hydrogen pick-up leading to hydride formation is often observed in commercially pure Ti (CP-Ti) and Ti-based alloys prepared for microscopic observation by conventional methods, such as electro-polishing and room temperature focused ion beam (FIB) milling. Here, we demonstrate that cryogenic FIB milling can effectively prevent undesired hydrogen pick-up. Specimens of CP-Ti and a Ti dual-phase alloy (Ti-6Al-2Sn-4Zr-6Mo, Ti6246, in wt.%) were prepared using a xenon-plasma FIB microscope equipped with a cryogenic stage reaching −135 °C. Transmission electron microscopy (TEM), selected area electron diffraction, and scanning TEM indicated no hydride formation in cryo-milled CP-Ti lamellae. Atom probe tomography further demonstrated that cryo-FIB significantly reduces hydrogen levels within the Ti6246 matrix compared with conventional methods. Supported by molecular dynamics simulations, we show that significantly lowering the thermal activation for H diffusion inhibits undesired environmental hydrogen pick-up during preparation and prevents pre-charged hydrogen from diffusing out of the sample, allowing for hydrogen embrittlement mechanisms of Ti-based alloys to be investigated at the nanoscale.
Kühbach M, Breen A, Herbig M, et al., 2019, Building a Library of Simulated Atom Probe Data for Different Crystal Structures and Tip Orientations Using TAPSim., Microsc Microanal, Pages: 1-11
The process of building an open source library of simulated field desorption maps for differently oriented synthetic tips of the face-centered cubic, body-centered cubic, and hexagonal-close-packed crystal structures using the open source software TAPSim is reported. Specifically, the field evaporation of a total set of 4 × 101 single-crystalline tips was simulated. Their lattices were oriented randomly to sample economically the fundamental zone of crystal orientations. Such data are intended to facilitate the interpretation of low-density zone lines and poles that are observed on detector hit maps during Atom Probe Tomography (APT) experiments. The datasets and corresponding tools have been made publicly available to the APT community in an effort to provide better access to simulated atom probe datasets. In addition, a computational performance analysis was conducted, from which recommendations are made as to which key tasks should be optimized in the future to improve the parallel efficiency of TAPSim.
Peng Z, Zanuttini D, Gervais B, et al., 2019, Unraveling the metastability of C n2+ (n = 2-4) clusters, Journal of Physical Chemistry Letters, Vol: 10, Pages: 581-588, ISSN: 1948-7185
Pure carbon clusters have received considerable attention for a long time. However, fundamental questions, such as what the smallest stable carbon cluster dication is, remain unclear. We investigated the stability and fragmentation behavior of C n2+ ( n = 2-4) dications using state-of-the-art atom probe tomography. These small doubly charged carbon cluster ions were produced by laser-pulsed field evaporation from a tungsten carbide field emitter. Correlation analysis of the fragments detected in coincidence reveals that they only decay to C n-1+ + C+. During C22+ → C+ + C+, significant kinetic energy release (∼5.75-7.8 eV) is evidenced. Through advanced experimental data processing combined with ab initio calculations and simulations, we show that the field-evaporated diatomic 12C22+ dications are either in weakly bound 3Πu and 3Σg- states, quickly dissociating under the intense electric field, or in a deeply bound electronic 5Σu- state with lifetimes >180 ps.
Peng Z, Lu Y, Hatzoglou C, et al., 2019, An Automated Computational Approach for Complete In-Plane Compositional Interface Analysis by Atom Probe Tomography., Microsc Microanal, Pages: 1-12
We introduce an efficient, automated computational approach for analyzing interfaces within atom probe tomography datasets, enabling quantitative mapping of their thickness, composition, as well as the Gibbsian interfacial excess of each solute. Detailed evaluation of an experimental dataset indicates that compared with the composition map, the interfacial excess map is more robust and exhibits a relatively higher resolution to reveal compositional variations. By field evaporation simulations with a predefined emitter mimicking the experimental dataset, the impact of trajectory aberrations on the measurement of the thickness, composition, and interfacial excess of the decorated interface are systematically analyzed and discussed.
Mouton I, Katnagallu S, Makineni SK, et al., 2019, Calibration of Atom Probe Tomography Reconstructions Through Correlation with Electron Micrographs., Microsc Microanal, Pages: 1-8
Although atom probe tomography (APT) reconstructions do not directly influence the local elemental analysis, any structural inferences from APT volumes demand a reliable reconstruction of the point cloud. Accurate estimation of the reconstruction parameters is crucial to obtain reliable spatial scaling. In the current work, a new automated approach of calibrating atom probe reconstructions is developed using only one correlative projection electron microscopy (EM) image. We employed an algorithm that implements a 2D cross-correlation of microstructural features observed in both the APT reconstructions and the corresponding EM image. We apply this protocol to calibrate reconstructions in a Cu(In,Ga)Se2-based semiconductor and in a Co-based superalloy. This work enables us to couple chemical precision to structural information with relative ease.
Sun B, Palanisamy D, Ponge D, et al., 2019, Revealing fracture mechanisms of medium manganese steels with and without delta-ferrite, Acta Materialia, Vol: 164, Pages: 683-696, ISSN: 1359-6454
© 2018 Medium Mn steels possess a composite like microstructure containing multiple phase constituents like metastable austenite, ferrite, δ-ferrite and α′-martensite with a wide range of fractions for each constituent. The high mechanical contrast among them and the deformation-driven evolution of the microstructure lead to complex fracture mechanisms. Here we investigate tensile fracture mechanisms of medium Mn steels with two typical types of microstructures. One group consists of ferrite (α) plus austenite (γ) and the other one of a layered structure with an austenite-ferrite constituent and δ-ferrite. Samples with the first type of microstructure show a dimple-type fracture due to void formation primarily at the ferrite/strain-induced α′-martensite (α′) interfaces. In contrast, the fracture surface of δ-ferrite containing steels shows a combination of cleavage in δ-ferrite and dimple/quasi-cleavage zones in the γ-α (or γ/α′-α) constituent. The embrittlement of δ-ferrite is due to the formation of B2 ordered phase. Failure of these samples is govern by crack initiation related to δ-ferrite and crack-arresting ability of the γ-α layers. Austenite stability is critical for the alloys' fracture resistance, in terms of influencing void growth and coalescence for the first type of microstructure and crack initiation and termination for the microstructure containing δ-ferrite. This effect is here utilized to increase ductility and toughness. By tailoring austenite stability towards higher fracture resistance, the total elongation of δ-ferrite containing steels increases from ∼13% to ∼33%. This approach opens a new pathway towards an austenite-stability-controlled microstructural design for substantially enhanced damage tolerance in steels containing metastable austenite and δ-ferrite.
Lei Z, Liu X, Wu Y, et al., 2019, Publisher Correction: Enhanced strength and ductility in a high-entropy alloy via ordered oxygen complexes., Nature, Vol: 565, Pages: E8-E8
Change history: In this Letter, owing to a production error, all the data points (except the two points for O-2 and N-2, respectively) were missing in Fig. 1b. The figure has been corrected online.
Pandey P, Makineni SK, Samanta A, et al., 2019, Elemental site occupancy in the L1<inf>2</inf> A<inf>3</inf>B ordered intermetallic phase in Co-based superalloys and its influence on the microstructure, Acta Materialia, Vol: 163, Pages: 140-153, ISSN: 1359-6454
© 2018 Acta Materialia Inc. We explore the effects of the elemental site occupancy in γ′-A3B (L12) intermetallic phases and their partitioning across the γ/γ′ interface in a class of multicomponent W-free Co-based superalloys. Atom probe tomography and first principles density functional theory calculations (DFT) were used to evaluate the Cr site occupancy behavior in the γ′ phase and its effect on the γ/γ′ partitioning behavior of other solutes in a series of Co-30Ni-10Al-5Mo-2Ta-2Ti-XCr alloys, where x is 0, 2, 5, and 8 at.% Cr, respectively. The increase in Cr content from 0 to 2 to 5 at.% leads to an inversion of the partitioning behavior of the solute Mo from the γ′ phase (KMo>1) into the γ matrix (KMo<1). At 5 at.% Cr, the Cr also has a preference to replace the excess anti-site Co atoms from the B-sites. At 8 at.% Cr, the Cr develops an additional preference to replace Co atoms from the A-sites. These compositional changes in the phases and the site partitioning behavior in the γ′ phase are accompanied by an overall decrease in the lattice misfit (δ) across the γ/γ′ interfaces as measured by high-resolution X-ray diffraction at room temperature. The reduction in misfit triggers a change in morphology of the γ′ phase from cuboidal (δ ∼ +0.48% at 0 at.% Cr) to round-cornered (δ ∼ +0.34% at 5 at.% Cr) to spheroidal shaped (δ ∼ +0.19% at 8 at.% Cr) precipitates. We also observed an increase in the solvus temperature from 1066 °C to 1105 °C when adding 5 at.% Cr to the alloy. These results on the effects of Cr in Co-base superalloys enable tuning the microstructure of these alloys and widening the alloy spectrum for designing improved high temperature alloys.
Langille M, Diak BJ, De Geuser F, et al., 2019, Understanding the role of cu and clustering on strain hardening and strain rate sensitivity of al-mg-si-cu alloys, Pages: 143-151, ISSN: 2367-1181
Increased demand for light-weighting in passenger vehicles has created a need for strong, light, ductile materials to be used in body-in-white applications. The AA6xxx-series of aluminum alloys are suitable candidates meeting most requirements but can fall short of the formability demands of designers, necessitating an understanding of what controls the formability in this alloy series. This work examines the effect of copper alloying in AA6xxx on the pre-ageing and natural ageing responses of the microstructure and mechanical properties. The changes in microstructure observed by differential scanning calorimetry and hardness testing are related to the work-hardening and strain-rate sensitivity parameters for these alloys measured by tensile testing. An observed asymmetry in the measured strain-rate sensitivity associated with increasing versus decreasing strain rate changes suggests that a different mechanism operates for the two conditions. It is postulated how this asymmetry in strain-rate sensitivity will impact the necking and ductility behaviour of these alloys.
Lilensten L, Antonov S, Raabe D, et al., 2019, Deformation of Borides in Nickel-based Superalloys: A study of segregation at dislocations, Microscopy and Microanalysis, Vol: 25, Pages: 2538-2539, ISSN: 1431-9276
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Sohn SS, Kwiatkowski da Silva A, Ikeda Y, et al., 2018, Ultrastrong Medium-Entropy Single-Phase Alloys Designed via Severe Lattice Distortion, Advanced Materials, ISSN: 0935-9648
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Severe lattice distortion is a core effect in the design of multiprincipal element alloys with the aim to enhance yield strength, a key indicator in structural engineering. Yet, the yield strength values of medium- and high-entropy alloys investigated so far do not substantially exceed those of conventional alloys owing to the insufficient utilization of lattice distortion. Here it is shown that a simple VCoNi equiatomic medium-entropy alloy exhibits a near 1 GPa yield strength and good ductility, outperforming conventional solid-solution alloys. It is demonstrated that a wide fluctuation of the atomic bond distances in such alloys, i.e., severe lattice distortion, improves both yield stress and its sensitivity to grain size. In addition, the dislocation-mediated plasticity effectively enhances the strength–ductility relationship by generating nanosized dislocation substructures due to massive pinning. The results demonstrate that severe lattice distortion is a key property for identifying extra-strong materials for structural engineering applications.
Stephenson LT, Szczepaniak A, Mouton I, et al., 2018, The Laplace Project: an integrated suite for preparing and transferring atom probe samples under cryogenic and UHV conditions, PLoS One, Vol: 13, ISSN: 1932-6203
We present sample transfer instrumentation and integrated protocols for the preparation and atom probe characterization of environmentally-sensitive materials. Ultra-high vacuum cryogenic suitcases allow specimen transfer between preparation, processing and several imaging platforms without exposure to atmospheric contamination. For expedient transfers, we installed a fast-docking station equipped with a cryogenic pump upon three systems; two atom probes, a scanning electron microscope / Xe-plasma focused ion beam and a N2-atmosphere glovebox. We also installed a plasma FIB with a solid-state cooling stage to reduce beam damage and contamination, through reducing chemical activity and with the cryogenic components as passive cryogenic traps. We demonstrate the efficacy of the new laboratory protocols by the successful preparation and transfer of two highly contamination- and temperature-sensitive samples—water and ice. Analysing pure magnesium atom probe data, we show that surface oxidation can be effectively suppressed using an entirely cryogenic protocol (during specimen preparation and during transfer). Starting with the cryogenically-cooled plasma FIB, we also prepared and transferred frozen ice samples while avoiding significant melting or sublimation, suggesting that we may be able to measure the nanostructure of other normally-liquid or soft materials. Isolated cryogenic protocols within the N2 glove box demonstrate the absence of ice condensation suggesting that environmental control can commence from fabrication until atom probe analysis.
Gault B, Breen AJ, Chang Y, et al., 2018, Interfaces and defect composition at the near-atomic scale through atom probe tomography investigations, Journal of Materials Research, Vol: 33, Pages: 4018-4030, ISSN: 0884-2914
Atom probe tomography (APT) is rising in influence across many parts of materials science and engineering thanks to its unique combination of highly sensitive composition measurement and three-dimensional microstructural characterization. In this invited article, we have selected a few recent applications that showcase the unique capacity of APT to measure the local composition at structural defects. Whether we consider dislocations, stacking faults, or grain boundary, the detailed compositional measurements tend to indicate specific partitioning behaviors for the different solutes in both complex engineering and model alloys we investigated.
Rusitzka KAK, Stephenson LT, Szczepaniak A, et al., 2018, A near atomic-scale view at the composition of amyloid-beta fibrils by atom probe tomography, Scientific Reports, Vol: 8, ISSN: 2045-2322
Amyloid-beta (Ab) proteins play an important role in a number of neurodegenerative diseases. Ab is found in senile plaques in brains of Alzeimer’s disease patients. The 42 residues of the monomer form dimers which stack to fibrils gaining several micrometers in length. Using Ab fibrils with 13C and 15N marker substitution, we developed an innovative approach to obtain insights to structural and chemical information of the protein. We deposited the modified protein fibrils to pre-sharped aluminium needles with >100-nm apex diameters and, using the position-sensitive mass-to-charge spectrometry technique of atom probe tomography, we acquired the chemically-resolved three dimensional information for every detected ion evaporated in small fragments from the protein. We also discuss the influence of experimental parameters such as pulse energy and pulse frequency of the used Laser beam which lead to differences in the size of the gained fragments, developing the capability of localising metal atom within Ab plaques.
Medrano S, Zhao H, De Geuser F, et al., 2018, Cluster hardening in Al-3Mg triggered by small Cu additions, Acta Materialia, Vol: 161, Pages: 12-20, ISSN: 1359-6454
© 2018 Acta Materialia Inc. The aging response of two Al-3Mg alloys with Cu addition <1 wt% has been tracked under simulated automotive paint bake conditions (∼ 20 min, 160 and 200 °C) to quantify the processes controlling hardening. The decomposition of the solid solution, observed by atom probe tomography, has been interpreted using a novel pair correlation function approach and incorporated into a model for prediction of precipitation hardening. It is shown that the hardening is controlled by clusters/Guinier-Preston-Bagaryatsky (GPB) zones similarly to what has been previously observed in much higher Cu containing 2XXX-series alloys. Interestingly, it is shown that very small additions of Cu (< 0.1 at%) can be used to catalyze a high number density of strengthening particles owing to the high enrichment in Mg compared to particles found in more conventional high Cu/low Mg alloys. This allows hardening during the first hour of aging that is as high as that obtainable in these high Cu alloys.
Mompiou F, Tingaud D, Chang Y, et al., 2018, Conventional vs harmonic-structured β-Ti-25Nb-25Zr alloys: A comparative study of deformation mechanisms, Acta Materialia, Vol: 161, Pages: 420-430, ISSN: 1359-6454
© 2018 Acta Materialia Inc. Harmonic alloys processed by powder metallurgy are constituted by a core of coarse grains embedded in an interconnected small grains shell. They have attracted attention due to their excellent strength combined with large ductility, the two properties being rather antagonist from the classical metallurgy point of view. In contrast, conventional β-Ti alloys are currently vastly studied owing their excellent properties especially for biomedical applications. In the present study, we explore at the micron scale the deformation mechanisms operating both in standard and harmonic-structured β-Ti-25Nb-25Zr alloys using transmission electron microscopy (TEM). Although we show some similarities, deformation mechanisms appear significantly different due to the activation of martensitic transformation in conventional samples. The combined use of automated crystal orientation in TEM and in-situ TEM straining reveals that deformation bands nucleate and grow according to a mechanism involving both martensitic transformation and twinning. The comparison between in-situ and post-mortem experiments shows globally a good agreement and highlights a strain relaxation mechanism between martensite and twin. More importantly, a cross-glide mechanism similar to what is observed in dilute solid solutions is proposed to explain the dynamics of dislocation motion. Stress estimations derived from the observations of dislocation curvature between pinning points, show a reasonable good agreement with macroscopic values. The observation of deformation mechanisms operating both in core and shell structures of the harmonic-structured alloy allows us to propose a scenario of plastic deformation in the early stages.
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