Hume-Rothery Award Symposium: Alloy Phase Chemistry at the Atomic Level - Opportunities and Challenges: Session IV
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 2:00 PM
February 28, 2017
Room: 31C
Location: San Diego Convention Ctr

Session Chair: Hatem Zurob, McMaster University; Indrajit Charit, University of Idaho

2:00 PM  Invited
Interaction of Solutes with Interfaces in Iron: Matthias Militzer1; Hatem Zurob2; 1The University of British Columbia; 2McMaster University
    The interaction of alloying elements in solution with the bcc-fcc interface and grain boundaries can significantly affect microstructure evolution and resulting properties in low alloyed steels. To develop more fundamental microstructure evolution models it is critical to account for the atomistic mechanisms of the solute-interface interaction. Here, first principle density functional theory (DFT) simulations are presented for the binding energy of selected alloying elements X (X= Mn, Ni, Cr, Si, Mo) to bcc-bcc and bcc-fcc interfaces in Fe. The DFT simulation results are compared to 3D atom probe tomography studies of segregation of these elements to the bcc-fcc interface in ternary Fe-C-X alloys. The observed trends of segregation are consistent with those for the binding energies obtained with DFT simulations, i.e. Mo has the highest and Ni the lowest segregation tendency for the investigated solutes. An outlook will be provided for incorporating these atomistic data into overall microstructure evolution models.

2:30 PM  Invited
A New Look at Steel Martensite Tempering with Advanced Characterization Tools: Amy Clarke1; Michael Miller2; Daniel Coughlin3; Dean Pierce2; Jon Poplawsky2; Paul Gibbs4; Kester Clarke1; Virginia Judge1; Bjorn Clausen3; Jon Almer5; Robert Field1; Don Williamson1; David Alexander3; John Speer1; George Krauss1; 1Colorado School of Mines; 2Oak Ridge National Laboratory; 3Los Alamos National Laboratory; 4Sandia National Laboratories - Livermore; 5Argonne National Laboratory
    Steel is the world's most versatile structural material, thanks to the two temperature-dependent crystal structures of iron, the addition of interstitial carbon atoms, and alloying with selected substitutional alloying elements. Phases, microstructures and properties of steel depend upon where the atoms are and how they got there during processing. William Hume-Rothery spent his scientific career understanding chemical metallurgy and the link between atomic bonding and microstructure. G.D.W. Smith has dedicated his career to the study of fine-scale compositional variations in alloys, especially in steels. Advanced characterization tools such as atom probe tomography (APT) now afford unprecedented access to chemistry variations in three-dimensions at the atomic scale. Here we highlight atomic- and nano-scale microstructural evolution, with APT and complementary techniques, including transmission electron microscopy, Mössbauer effect spectroscopy, and synchrotron x-ray diffraction, in a medium carbon, low-alloyed steel after quenching and tempering over a wide range of tempering temperatures and times.

3:00 PM  
Atomistic Modelling of Carbon Redistribution in Martensite Phase: Helena Zapolsky1; Mykola Lavrskyi1; Armen Khachaturyan2; Frederic Danoix1; Renaud Patte2; Sophie Cazottes3; Mohamed Gouné4; Philippe Maugis5; 1University of Rouen; 2University of California and Rutgers University; 3INSA Lyon - MATEIS - SGM; 4University of Bordeaux; 5Aix-Marseille University Saint-Jerome
    Among various forms of steels, martensite obtained by quenching of the fcc austenite is the one with the highest strength. However, iron carbon martensite is not stable at room temperature and forms compositional modulations during aging. Although the morphology of this modulated microstructure was a subject of extensive experimental study the inter structure of the carbon-rich zones remains an open question. In this study we employ the Atomic Density Function theory (ADF) to model the low temperature kinetics of carbon redistribution in Fe-C system. In this model the elastic interaction between carbon atoms was taking into account. Our simulation results show that during growth the carbon-rich zones, after reaching the concentration around 11at% of carbon, undergo the inverse martensite transition from the bcc to the fcc phase. The employed atomistic approach makes possible the direct comparison of simulation results with experimental data obtained by Atom Probe Tomography.

3:20 PM Break

3:40 PM  Invited
Precipitation Kinetics: Quantitative In-situ Characterization Using Small-angle Scattering Helps Establish Models Validity: Alexis Deschamps1; Frederic De Geuser1; Mark Styles2; Christopher Hutchinson3; 1Grenoble Institute of Technology; 2CSIRO; 3Monash University
    Small-Angle Scattering, using X-rays (SAXS) or neutrons (SANS) enables a precise quantitative characterization of precipitates at the nanoscale. Being non-destructive and realized on samples of a relatively large volume, these techniques are well suited to time-resolved in-situ studies along any thermal treatment. As such, they are particularly suited in conjunction to precipitation models to establish their validity, their robustness in complex situations such as non-isothermal heat treatments, and obtain missing materials parameters such as the interfacial energy of forming metastable precipitates, that play a major role in the precipitation kinetics models. This talk will present several case studies on Aluminum alloys, Cu alloys and steels, where the interplay between quantitative characterization and modelling improves the understanding of the underling phenomena. In particular, the description of dynamic phenomena occurring during non-isothermal heat treatments will be addressed, as well as the influence of alloy supersaturation, using materials with a composition gradient.

4:10 PM  
Thermally Induced Phase Transformations in Beta-titanium Alloys and Corresponding Effects on Mechanical Properties: James Coakley1; Anna Radecka2; Paul Bagot3; David Dye4; Howard Stone1; Dieter Isheim5; David Seidman5; 1University of Cambridge; 2Rolls-Royce plc.; 3Oxford University; 4Imperial College London; 5Northwestern University
    With appropriate alloying and processing, the beta-titanium alloys possess remarkable mechanical properties that include very high strength-to-weight ratios arising from nanoscale alpha precipitate strengthening, and superelasticity due to a stress induced phase transformation in the elastic loading regime. It is necessary to study the thermal stability of this alloy system to broaden their applications in industry. This study correlates an increase in micro-hardness of Ti-5Al-5Mo-5V-3Cr wt.% during thermal exposure to nanoscale precipitation of isothermal omega phase and subsequent alpha phase, identified by TEM, atom-probe tomography (APT), and in-situ small angle neutron scattering (SANS). Furthermore, a loss of superelasticity and an increase in brittleness is identified during thermal exposure of Ti-24Nb-4Zr-8Sn wt.%, associated with a decrease in the number density of Nb-rich nanoscale embryos and precipitation of isothermal omega phase, respectively, identified by APT and TEM.

4:30 PM  
Method for Correcting Atom Probe Tomography Trajectory Aberration Artifacts in Multiphase Materials: Samuel Briggs1; Nathan Almirall2; Philip Edmondson3; Peter Wells2; G. Robert Odette2; Kumar Sridharan1; Kevin Field3; 1University of Wisconsin-Madison; 2University of California - Santa Barbara; 3Oak Ridge National Laboratory
    Atom probe tomography is a powerful tool for performing atomic-scale chemical analysis of materials and excels at characterization of precipitates and multiphase materials. However, the technique is subject to local magnification and trajectory aberration artifacts that can complicate quantification and lead to incorrect reporting of chemical and morphological data. A method of accounting for these trajectory aberration effects has been developed based on identification of unphysical atomic density variations between distinct phases in a reconstructed atom probe data set. Composition (and radius in the case of precipitates) can then be corrected based on the quantity of excess or deficient atoms in a volume as determined by the deviation of measured atomic density from the expected value. This specimen-wide aberration correction methodology has been applied to two-phase model systems, commercial alloys and low-alloy materials and has been shown to result in more representative precipitate and matrix compositions for these data sets.

4:50 PM  
Solute Distribution Analysis of Early Stages of Aging in Al-Mg-Si Alloys via Atom Probe Tomography: Phillip Dumitraschkewitz1; Gunther Rank2; Stephanie Sackl3; Stephan S.A. Gerstl4; Stefan Pogatscher1; 1Chair of Nonferrous Metallurgy, Montanuniversitaet Leoben; 2AMGA rolling GmbH; 3Chair of Physical Metallurgy and Metallic Materials, Montanuniversitaet Leoben; 4Scientific Center of Optical and Electron Microscopy, ETH Zurich
    Early stages of aging in Al-Mg-Si alloys are experimentally extremely difficult to observe. In this work a new strategy for the measurement of the as-quenched condition and various initial stages of aging via atom probe tomography (APT) has been applied. To hinder uncontrolled aging at room temperature after quenching, sample preparation and sample transfer into the atom probe analysis chamber was done under cryogenic conditions. However, the quantification of early stage clusters is still difficult due to the spatial noise of the APT data. Additionally surface migration of Si during the experiment is known to generate artifacts in the vicinity of poles. The data is corrected for artifacts and the early stages of aging of Al-Mg-Si alloys are discussed within the framework of a customized APT data analysis.