Imperial College London

DrBaptisteGault

Faculty of EngineeringDepartment of Materials

Reader in Materials
 
 
 
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b.gault

 
 
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Royal School of MinesSouth Kensington Campus

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Summary

 

Publications

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213 results found

He J, Makineni SK, Lu W, Shang Y, Lu Z, Li Z, Gault Bet al., 2020, On the formation of hierarchical microstructure in a Mo-doped NiCoCr medium-entropy alloy with enhanced strength-ductility synergy, Scripta Materialia, Vol: 175, Pages: 1-6, ISSN: 1359-6462

© 2019 Elsevier Ltd We demonstrate the formation of a hierarchical microstructure by optimized combination of Mo-doping and thermo-mechanical processing in a NiCoCr medium-entropy-alloy (MEA) that results in a significant improvement in yield-strength (~775 MPa) and ultimate-tensile-strength (~1200 MPa) with an elongation to fracture up to 35%. Addition of Mo slows down the recrystallization kinetics of the cold-rolled alloy at 700 °C. After 4 h annealing, the microstructure comprises soft recrystallized and hard non-recrystallized zones, dispersed σ precipitates, annealing twins and retained stacking faults/nanotwins that are shown to contribute to the overall mechanical behavior.

Journal article

Lopez-Galilea I, Ruttert B, He J, Hammerschmidt T, Drautz R, Gault B, Theisen Wet al., 2019, Additive manufacturing of CMSX-4 Ni-base superalloy by selective laser melting: Influence of processing parameters and heat treatment, Additive Manufacturing, Vol: 30

© 2019 Elsevier B.V. Selective laser melting (SLM) provides an economic approach to manufacturing Ni-base superalloy components for high-pressure gas turbines as well as repairing damaged blade sections during operation. In this study, two advanced processing routes are combined: SLM, to fabricate small specimens of the nonweldable CMSX-4, and hot isostatic pressing (HIP) with a rapid cooling rate as post-processing to heal defects while the target γ/γ´ microstructure is developed. An initial parametric study is carried out to investigate the influence of the SLM process parameters on the microstructure and defects occurring during SLM. Special emphasis is placed on understanding and characterizing the as-built SLM microstructures by means of high-resolution characterization techniques. The post-processing heat treatment is then optimized with respect to segregation and the γ/γ´ microstructure.

Journal article

Massey CP, Dryepondt SN, Edmondson PD, Frith MG, Littrell KC, Kini A, Gault B, Terrani KA, Zinkle SJet al., 2019, Corrigendum to Multiscale investigations of nanoprecipitate nucleation, growth, and coarsening in annealed low-Cr oxide dispersion strengthened FeCrAl powder (Acta Materialia (2019) 166 (1–17), (S135964541830939X), (10.1016/j.actamat.2018.11.062)), Acta Materialia, Vol: 180, ISSN: 1359-6454

© 2019 Acta Materialia Inc. The authors regret that the scale bars in Figure 4 for the visualized atom probe tomography control volumes are mislabelled and should be in units of nanometers, not micrometers. The authors would like to apologise for any inconvenience caused. Corrected Image:

Journal article

Harding I, Mouton I, Gault B, Raabe D, Kumar KSet al., 2019, Carbon partitioning and microstructure evolution during tempering of an Fe-Ni-C steel, SCRIPTA MATERIALIA, Vol: 172, Pages: 38-42, ISSN: 1359-6462

Journal article

Li L, Li Z, da Silva AK, Peng Z, Zhao H, Gault B, Raabe Det al., 2019, Segregation-driven grain boundary spinodal decomposition as a pathway for phase nucleation in a high-entropy alloy, ACTA MATERIALIA, Vol: 178, Pages: 1-9, ISSN: 1359-6454

Journal article

Kontis P, Chauvet E, Peng Z, He J, da Silva AK, Raabe D, Tassin C, Blandin J-J, Abed S, Dendievel R, Gault B, Martin Get al., 2019, Atomic-scale grain boundary engineering to overcome hot-cracking in additively-manufactured superalloys, ACTA MATERIALIA, Vol: 177, Pages: 209-221, ISSN: 1359-6454

Journal article

Dubosq R, Rogowitz A, Schweinar K, Gault B, Schneider DAet al., 2019, A 2D and 3D nanostructural study of naturally deformed pyrite: assessing the links between trace element mobility and defect structures, Contributions to Mineralogy and Petrology, Vol: 174, ISSN: 0010-7999

© 2019, Springer-Verlag GmbH Germany, part of Springer Nature. The links between deformation-induced micro- and nanostructures and trace element mobility in sulphide minerals have recently become a popular subject of research in the Earth sciences due to its connections with metallic ore paragenesis. It has been shown that plastic deformation in pyrite creates diffusion pathways in the form of low-angle grain boundaries that act as traps for base- and precious-metals. However, the plastic behavior of pyrite and the physiochemical processes that concentrate these trace elements in deformation-induced micro- and nanostructures remain poorly understood. In this study, we develop strategies for 2D and 3D analysis of naturally deformed sulphides by combining electron backscatter diffraction, electron channeling contrast imaging and atom probe tomography on pyrite in an attempt to better understand the underlying diffusion processes that mobilize trace elements. The combined results reveal structures associated with crystal-plastic deformation in the form of dislocations, stacking faults, and low-angle grain boundaries that are decorated by As and Co. Although our data support a dislocation-impurity pair diffusion model, we have evidence that multiple diffusion mechanisms may have acted simultaneously. In this study, we applied new data processing techniques that allow for orientation measurement of nanostructural crystal defects from atom probe tomography data. Dislocations within our studied sample occur along the (110) planes suggesting glide on {110}.

Journal article

Massey CP, Hoelzer DT, Edmondson PD, Kini A, Gault B, Terrani KA, Zinkle SJet al., 2019, Stability of a model Fe-14Cr nanostructured ferritic alloy after long-term thermal creep, Scripta Materialia, Vol: 170, Pages: 134-139, ISSN: 1359-6462

© 2019 Elsevier Ltd. The successful development of advanced materials such as nanostructured ferritic alloys (NFA) for next generation nuclear reactor concepts and ultra-supercritical steam power plants requires information on long term thermal creep. To support this initiative, this work examines the NFA MA957 (Fe-14Cr-1.0Ti-0.3Mo + 0.3Y2O3 in wt%) crept to 61,251 h at 825 °C and 70 MPa, the longest creep test available to date for this material. Using atom probe tomography and electron microscopy, it is shown that the grain size and nanoprecipitate size/composition are unaffected following this 7 yr creep test, although significant porosity is noted throughout the microstructure attributed to microvoid coalescence and growth.

Journal article

Mianroodi JR, Shanthraj P, Kontis P, Cormier J, Gault B, Svendsen B, Raabe Det al., 2019, Atomistic phase field chemomechanical modeling of dislocation-solute-precipitate interaction in Ni-Al-Co, ACTA MATERIALIA, Vol: 175, Pages: 250-261, ISSN: 1359-6454

Journal article

Massey CP, Hoelzer DT, Seibert RL, Edmondson PD, Kini A, Gault B, Terrani KA, Zinkle SJet al., 2019, Microstructural evaluation of a Fe-12Cr nanostructured ferritic alloy designed for impurity sequestration, Journal of Nuclear Materials, Vol: 522, Pages: 111-122, ISSN: 0022-3115

© 2019 Elsevier B.V. Fast reactor fuel cladding candidate materials require proficiency in extreme environments consisting of high temperatures and irradiation doses in excess of 150 displacements per atom (dpa). Nanostructured oxide dispersion strengthened (ODS) alloys have been developed extensively for this purpose due to their notable high temperature strength, creep resistance, and irradiation resistance. However, their properties can deteriorate if interstitial impurities such as C and N are not well controlled during the fabrication process. A new Fe-12Cr nanostructured ODS alloy OFRAC (Oak Ridge Fast Reactor Advanced Fuel Cladding) with solute additions of Mo, Ti, and Nb has been developed to provide the desired properties mentioned above while simultaneously sequestering impurities within the matrix. After extrusion at 850 °C, the as-extruded microstructure consists of an average 490 nm grain size and a high number density (6.8 × 1023 m-3) of 2.2 nm diameter (Y,Ti,O) nanoclusters distributed homogeneously in the matrix. Atom probe tomography investigations suggest non-stochiometric compositions for the smallest nanoclusters. In addition, a second population of nanometer scale (Nb,Ti) rich carbonitrides is also present in the microstructure that captures the potentially detrimental C and N impurity atoms present in the matrix. Atom probe tomography results indicate elemental segregation of Cr, Mo, and Nb to grain boundaries in the as-extruded material, consistent with previous investigations of solid solution strengthening by solute additions. The ability of OFRAC to sequester impurities introduced from the powder metallurgical approach to nanostructured ferritic alloy development, compounded with its beneficial mechanical properties, makes this alloy a competitive candidate for fast reactor applications. This manuscript has been authored by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United S

Journal article

Zhu M, Song W, Konze PM, Li T, Gault B, Chen X, Shen J, Lv S, Song Z, Wuttig M, Dronskowski Ret al., 2019, Direct atomic insight into the role of dopants in phase-change materials, NATURE COMMUNICATIONS, Vol: 10, ISSN: 2041-1723

Journal article

Springer H, Zhang JL, Szczepaniak A, Belde M, Gault B, Raabe Det al., 2019, Light, strong and cost effective: Martensitic steels based on the Fe – Al – C system, Materials Science and Engineering A, Vol: 762, ISSN: 0921-5093

© 2019 Elsevier B.V. We introduce a novel alloy design concept for density reduced ultra-high strength steels. It is based on the effects of C to increase martensite strength and the Al-solubility in austenite, in conjunction with Al to increase the martensite start temperature and to reduce density. This alloy combination results in inherently strong but light martensitic microstructures, whose mechanical properties (i.e. strength and ductility) can be readily adjusted over a wide range by applying straightforward and established heat treatments. The concept is validated on an Fe – 8 Al – 1.1C (wt.%) alloy subjected to quench and tempering treatments. The steel exhibits relatively low yield strength (~600 MPa) and reasonable ductility (~ 15% elongation) after hot rolling and soft annealing, but can be brought to a maximum hardness of 62 HRC after quenching and tempering at 250 °C. These attractive mechanical properties are coupled with a low density (6.95 g cm–3) and high elastic stiffness (Young's modulus 199 GPa). These lightweight martensitic steels are cost-effective structural materials for weight-critical applications, promising unprecedented specific strength. Advanced characterisation presented herein allows us to derive the fundamental underpinnings of carbide precipitation and phase transformation, and outline and discuss perspectives for refining the alloy composition and processing parameters.

Journal article

Palanisamy D, Raabe D, Gault B, 2019, On the compositional partitioning during phase transformation in a binary ferromagnetic MnAl alloy, ACTA MATERIALIA, Vol: 174, Pages: 227-236, ISSN: 1359-6454

Journal article

Rodenbough PP, Mao Z, Gault B, 2019, How to write a compelling (materials) science paper, Materialia, Vol: 6

Journal article

Chang YH, Mouton I, Stephenson L, Ashton M, Zhang GK, Szczpaniak A, Lu WJ, Ponge D, Raabe D, Gault Bet 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

Journal article

Lomuscio A, Rödel T, Schwarz T, Gault B, Melchiorre M, Raabe D, Siebentritt Set al., 2019, Quasi-Fermi-Level Splitting of Cu-Poor and Cu-Rich CuInS2 Absorber Layers, Physical Review Applied, Vol: 11

Journal article

Pandey P, Makineni SK, Gault B, Chattopadhyay Ket 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

Journal article

Pandey P, Sawant AK, Nithin B, Peng Z, Makineni SK, Gault B, Chattopadhyay Ket al., 2019, On the effect of Re addition on microstructural evolution of a CoNi-based superalloy

© 2019 Acta Materialia Inc. 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.

Working paper

Kwiatkowski da Silva A, Kamachali RD, Ponge D, Gault B, Neugebauer J, Raabe Det 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.

Journal article

Moutona I, Breen AJ, Wang S, Chang Y, Szczepaniak A, Kontis P, Stephenson LT, Raabe D, Herbig M, Britton TB, Gault Bet 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.

Journal article

Mompiou F, Tingaud D, Chang Y, Gault B, Dirras Get 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.

Journal article

Schwarz T, Redinger A, Siebentritt S, Peng Z, Gault B, Raabe D, Choi PPet 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.

Journal article

Yan F, Mouton I, Stephenson LT, Breen AJ, Chang Y, Ponge D, Raabe D, Gault Bet 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.

Journal article

Wu M, Li Z, Gault B, Munroe P, Baker Iet 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.

Journal article

Benzing JT, Kwiatkowski da Silva A, Morsdorf L, Bentley J, Ponge D, Dutta A, Han J, McBride JR, Van Leer B, Gault B, Raabe D, Wittig JEet 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

Journal article

Souza Filho IR, Kwiatkowski da Silva A, Sandim MJR, Ponge D, Gault B, Sandim HRZ, Raabe Det 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.

Journal article

Kontis P, Kostka A, Raabe D, Gault Bet 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.

Journal article

Massey CP, Dryepondt SN, Edmondson PD, Frith MG, Littrell KC, Kini A, Gault B, Terrani KA, Zinkle SJet 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.

Journal article

Chang Y, Lu W, Guenole J, Stephenson LT, Szczpaniak A, Kontis P, Ackerman AK, Dear FF, Mouton I, Zhong X, Zhang S, Dye D, Liebscher CH, Ponge D, Korte-Kerzel S, Raabe D, Gault Bet al., 2019, Ti and its alloys as examples of cryogenic focused ion beam milling of environmentally-sensitive materials, Nature Communications, Vol: 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.

Journal article

Kühbach M, Breen A, Herbig M, Gault Bet 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.

Journal article

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