Hume-Rothery Award Symposium: Alloy Phase Chemistry at the Atomic Level - Opportunities and Challenges: Session III
Sponsored by: TMS Functional Materials Division, TMS Structural Materials Division, TMS: Alloy Phases Committee, TMS: Nuclear Materials Committee, TMS: Phase Transformations Committee
Program Organizers: Wei Xiong, University of Pittsburgh; Shuanglin Chen, CompuTherm LLC; Frederic Danoix, Université de Rouen; Indrajit Charit, University of Idaho

Tuesday 8:30 AM
February 28, 2017
Room: 31C
Location: San Diego Convention Ctr

Session Chair: Amy Clarke, Colorado School of Mines; Peter Wells, University of California - Santa Barbara

8:30 AM  Invited
Advanced FIM and APT Techniques at the University of Oxford: Michael Moody1; Paul Bagot1; 1University of Oxford
    The FIM and Atom Probe Research Group, established by George D. W. Smith at the University of Oxford, has for over 40 years worked at the forefront of all aspects of the technique including the development of new instrumentation, advanced materials applications and data mining algorithms. The focus on this research continues to this day, with an overall goal to advance the applicability of the technique to address a wider range of materials problems and to maximise the insights it provides. In this talk advanced APT characterisation techniques and applications currently being developed at Oxford will be presented including: automated 3D FIM reconstruction, atomic scale hydrogen mapping in steels and a new machine learning cluster identification data mining algorithm.

9:00 AM  Invited
Advanced Microstructural and Chemical Characterization of Nano-scale NiMnSi Precipitates Formed in Irradiated Reactor Pressure Vessel Steels Using Atom Probe Tomography and Scanning Transmission Electron Microscopy: Philip Edmondson1; Chad Parish1; Randy Nanstad1; 1Oak Ridge National Laboratory
    Reactor pressure vessels (RPV) used in light-water nuclear reactors are typically constructed from low-alloy steels. Under high-fluence irradiation, second phases rich in Ni, Mn and Si are observed to form. These NiMnSi-rich precipitates are known to degrade the mechanical properties of the RPV steel. Atom probe tomography (APT) has typically been applied to the investigation of these precipitates, but accurate quantification of precipitate chemistry is difficult to obtain due to trajectory aberrations. APT along with Scanning Transmission Electron Microscopy with X-ray spectroscopy and advanced data mining techniques have been used to characterize a RPV steel irradiated during service at a nuclear power plant. The experimental results, combined with modeling of the X-ray data, provide unique insight into the chemistry of the NiMnSi precipitates. These results will be discussed in terms of the role of precipitate chemistry on second phase formation, and the use of correlative microscopy methods in nano-scale characterization.

9:30 AM  Invited
Design of Nd-Fe-B Permanent Magnets with Maximum Coercivity by Controlling Grain Boundary Chemistry at the Atomic Level: Kazuhiro Hono1; Taisuke Sasaki1; Hossein Sepehri-Amin1; Tadakatsu Ohkubo1; 1National Institute for Materials Science
    Coercivities of permanent magnets are extrinsic properties that are sensitive to microstructures, in particular the grain size and grain boundary chemistry in polycrystalline structures. Magnetic isolation of refined grains is considered to be most effective to maximize the coercivity; however, the commercial sintered magnets have the microstructure far from such an ideal case. In this talk, we will overview the microstructure features of various Nd-Fe-B based magnets based on recent correlative microscopy using SEM, TEM and APT, and point out why the coercivity of Nd-Fe-B magnets are substantially smaller compared to the anisotropy field. By micromagnetic modeling, we show what kind of microstructure and inter-grain exchange will give the maximum coercivity in the Nd-Fe-B system, and finally, present some experimental attempts to optimize the microstructure to maximize the coercivity.

10:00 AM Break

10:20 AM  Invited
Determination of Interfacial Free Energies in Two-phase Metallic Alloys: Atom-probe Tomographic Experiments and First-principles Calculations: David Seidman1; Zugang Mao Mao1; Chris Booth-Morrison1; 1Northwestern University
    A general approach is presented for determining interfacial free energies in two-phase metallic alloys for binary and ternary alloys, where the interfacial free energy is determined experimentally using atom-probe tomography (APT). The interfacial free energies are determined in the quasi-stationary coarsening regime using the mean field theory models developed by Calderon-Voorhees-Murray-Kostorz (binary alloys) and Kuehmann-Voorhees (ternary alloys), which include the thermodynamics of the matrix. Specific examples are presented for Ni-Al and Ni-Al-Cr alloys as studied by APT. The temperature dependencies of the gamma(fcc)-Ni/gamma-prime-Ni3Al(L12) interfacial free energies for the {100}, {110}, {111} heterophase interfaces are calculated using first-principles calculations, including both coherency strain energy and phonon vibrational entropy. The calculations, including ferromagnetic effects, predict that the {100}-type interface has the smallest free energy at elevated temperatures. The predicted morphology of the gamma-prime precipitates is in agreement with our APT observations of cuboidal precipitates, which have large {100}-type facets.

10:50 AM  Invited
First-principles Modeling of Anomalous Precipitation in W-Re Alloys under Neutron Irradiation: Duc Nguyen-Manh1; Jan Wrobel2; Michael Klimenkov3; Sergei Dudarev1; 1Culham Cenre for Fusion Energy; 2Warsaw University of Technology; 3Karlsruhe Institute of Technology
    Segregation occurring in under-saturated solid solutions under irradiation is a mysterious phenomenon that potentially has significant practical implications. In this study, a new multi-scale modelling approach, based on first-principles calculations, is developed, where solute atoms and vacancies (Vac) are treated as components of a ternary alloy. The free energy of ternary W-Re-Vac alloys is evaluated as functions of temperature and vacancy concentration using effective CE interactions and quasi-canonical Monte Carlo (MC) simulations. In the low solute concentration range (< 5 at%Re), our simulations predict the formation of precipitates with Re concentration that agrees with experimental observations on self-ion irradiated W-2at.% Re alloys. Our quenched MC predictions for 1.5% Re at T=1200K show that precipitates may adopt the form of (110) faceted voids enriched by Re atoms, with the average concentration of 14%. This agrees very well with recent Transmission Electron Microscopy investigations of neutron-irradiated W samples.

11:20 AM  
Design and Development of Novel High-temperature Creep Resistant 9% Cr Steels: Dieter Isheim1; Yao Du1; Cameron Gross1; Semyon Vaynman1; Yip-Wah Chung1; 1Northwestern University
    Structural steels soften remarkably when heated above several hundred °C due to the transition from athermal dislocation glide to thermally activated glide and diffusion processes accelerating changes of the microstruture critical to strength. We utilize a novel combination of several strategies for the design of 9% Cr steels with reduced carbon concentration to suppress Cr23C6, precipitating semi-coherent mono-carbonitride precipitates (MX, M = slow diffusing early transition metal, X = carbon or nitrogen), minimizing the driving force and nucleation of Laves and Z-phases, for enhanced microstructural stability, and addition of Mo and W for solid solution strengthening with the ultimate goal to obtain creep rupture times longer than 100,000 hours at 650°C and 35MPa, for applications in electric power generation. We present results from computational and experimental studies, including atom-probe tomography, phase stability and precipitation kinetics and estimations of the long-term microstructural stability and mechanical properties during exposure to above 700°C.

11:40 AM  
Diffusivity Determination of Slow Diffusion Systems using Diffusion Couples and Multiples: Zhangqi Chen1; Ji-Cheng Zhao1; 1The Ohio State University
    Diffusion coefficient is one of the fundamental properties in material science. Both diffusion couple and tracer experiments have been employed in the last several decades to obtain diffusion profiles for extraction of diffusion coefficients. For very slow diffusion systems, diffusion couple experiments are challenging since thousands of hours of annealing are required to obtain wide enough diffusion distance for reliable composition profiles for diffusivity extraction. A new analytical method is developed to extract diffusion coefficients from very steep composition profiles for slow diffusion systems. The Ti-Mo, Ti-Nb and Ti-Ta binary systems in Ti-Mo-Nb-Ta-Zr diffusion multiples are employed as examples to demonstrate the new analysis. This new analysis will accelerate the diffusion data gathering for the challenging slow diffusion systems.