Solid State Precipitation: Session III
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Phase Transformations Committee
Program Organizers: Seth Imhoff, Los Alamos National Laboratory; Robert Hackenberg, Los Alamos National Laboratory; Gregory Thompson, University of Alabama

Thursday 2:00 PM
March 2, 2017
Room: 25A
Location: San Diego Convention Ctr

Session Chair: Seth Imhoff, Los Alamos National laboratory

2:00 PM  Invited
Atomic Theory of Spinodal Decomposition: Maylise Nastar1; 1CEA
    The phenomenological theories currently used to describe phase decomposition in solids fail to predict the early stage of a spinodal decomposition mainly because they rely on local equilibrium assumptions. Within the Self-Consistent Mean Field (SCMF) theory, it is shown that spinodal decomposition and more generally kinetics of nanoscale composition fluctuations are correctly described if deviations of short range order from local equilibrium are accounted for. The resulting structure function of a binary system is controlled by a sum of exponential laws, in agreement with atomic Monte Carlo simulations and small angle scattering observations of spinodal decomposition. Moreover, in the case of a point defect diffusion mechanism, diffusion of atoms is non random and the inter-diffusion coefficient controlling the concentration fluctuations is shown to be in discrepancy with a Cahn-Hilliard like formulation. These theoretical results are applied to the modeling of spinodal decomposition in Fe-Cr alloys.

2:30 PM  
Spinodal Decomposition and Ordering Transformation in U6Nb Alloy: Luke Hsiung1; 1Lawrence Livermore National Laboratory
    Microhardness measurement, tensile test, and transmission electron microscopy (TEM) analysis were conducted to investigate low-temperature aging behavior of a water-quenched U6Nb alloy. The alloy was subjected to the following heat-treatments: 1) thermal aging of an as-quenched alloy at 200°C, 2) natural aging of water-quenched alloy at ambient temperatures for 15 years (15-year-old alloy), and 3) thermal aging of 15-year-old alloy at 200°C and 212°C. During the early stages of thermal aging of as-quenched alloy at 200°C, microhardness increased as a result of spinodal decomposition; microhardness decreased after over-aging of the alloy for 16 hours. Atomic ordering was found to occur in the 15-year-old alloy according to TEM diffraction contrast analysis. Microhardness and tensile strength of the thermally aged 15-year-old alloy continuously increased even after prolonged aging for 240 hours at 200°C as a result of precipitation hardening. Transformation pathways and kinetics of ordering transformation will be proposed and discussed.

2:50 PM  
Atom Probe Characterization of Phase Separation during Age Hardening of a U-6wt.%Nb Alloy: Clarissa Yablinsky1; Seth Imhoff1; Yaqiao Wu2; Amy Clarke3; Robert Hackenberg1; 1Los Alamos National Laboratory; 2Center for Advanced Energy Studies / Boise State; 3Colorado School of Mines
    U-Nb alloys are of interest in defense applications and could potentially be used for metallic reactor fuels. However, long term aging and other thermal excursions have an impact on the corrosion resistance and mechanical properties of interest in these industries. The aging pathways are complex, and this research focuses on the non-lamellar (NL3) mechanism, which is kinetically competitive with a discontinuous precipitation (DP) reaction. The competition between the NL3 and DP reactions influences the subsequent decomposition steps. Samples aged at 500 and 600°C for 10, 100, and 10000 minutes that exhibit NL3 structures were investigated by local electrode atom probe. Results indicate that the precipitates have strong Nb partitioning (~0 at.% Nb within the precipitates, ~40 at.% Nb within matrix) and have coalesced at longer aging times. The Nb distribution inside of and between precipitates is used to gain insight into the details of the growth kinetics.

3:10 PM  Cancelled
Understanding the Decomposition Process of Immiscible Fe-Cu-Ag Alloy: B. Hornbuckle1; Anthony Roberts1; Tom Luckenbaugh1; Kris Darling1; 1U.S. Army Research Laboratory
    Solid state precipitation is essential in modern materials science as a strengthening mechanism for endless materials systems. An equally important phenomena is solid solution strengthening. This study looks to utilize the decomposition of a solid solution to engineer a new mechanism of strengthening. This will be accomplished by identifying the fundamental aspects of the decomposition of a Fe-Cu-Ag solid solution. The Fe-Cu-Ag system is an immiscible alloy system; consequently mechanical alloying was utilized to form the solid solutions because its a non-equilibrium process. Once a solid solution was formed after mechanical alloying, low temperature isothermal anneals were performed to initiate the decomposition process. Microstructural characterization will be used to identify both the structure of these distinct phase as well as the compositional evolution of them via transmission electron microscopy (TEM) and atom probe tomography (APT).

3:30 PM Break

3:50 PM  Invited
Hydride Precipitates in Zirconium Alloys: Evolution of Dissolution and Precipitation Temperatures during Thermal Cycling Correlated to Microstructure Features: Egle Conforto1; Stephane Cohendoz1; Patrick Girault1; Cyril Berziou1; Xavier Feaugas1; 1University of La Rochelle
    The fast and spontaneous hydrogen diffusion in zirconium alloys used in nuclear industry leads to the hydride precipitation which is often pointed as causing embrittlement and rupture. Our studies using XDR, TEM and SEM-EBSD have been demonstrating that the nature of the hydride phase precipitated depends on the hydrogen content and can show crystallographic orientation relationships (ORs) with the substrate. DSC has been used to identify the dissolution and precipitation energies at global scale. The difference of both can be associated to the misfit dislocation contribution to the precipitation. Local TEM “in-situ” dissolution observations confirm the dissolution temperature identified at global scale and demonstrate the fact that part of misfit dislocations can be depinning during dissolution process. The consequence of this mechanism is that dissolution and precipitation temperatures shift during thermal cyclic loading. This situation will be correlated to the nature of crystallographic hydride phases and their ORs.

4:20 PM  
Effect of Metalloid Addition on Anomalous Primary Crystallization of Al-RE Metallic Glasses: Mustafacan Kutsal1; Burcu Cam1; Eren Kalay1; 1METU
    Marginal glass forming alloys are known to show an anomalous primary crystallization reaction with unusually high nanocrystal number densities upon thermal devitrification. In order to get an insight of this puzzling behavior, we have studied the as-quenched states and devitrification of marginal glass forming Al90Y10 and Al87Y10Si3 metallic glasses with differential scanning calorimetry, X-ray diffraction, transmission electron microscopy and microhardness measurements. Our studies have shown that medium-range structural heterogeneity caused by chemistry alteration have direct impact on primary crystallization of Al-RE metallic glasses. Thermal analysis revealed that primary crystallization temperature is reduced from 480 K to 380 K with the addition of Si. Furthermore, microhardness measurements have showed 400 Hv hardness for Al87Y10Si3 metallic glasses, an ultrahigh value as compared to other Al-Rare-Earth based aluminum alloys. The underlying effects of Si addition will be discussed in detail with respect to short-to-medium-range correlations and corresponding structural models.

4:40 PM  
Formation of Complex Intermetallic Phases from Supersaturated Co Solid Solution in a Co-3.9Nb Alloy: Toshiaki Horiuchi1; Frank Stein2; Kohei Abe1; Shunsuke Taniguchi3; 1Hokkaido University of Science; 2Max-Planck-Institut für Eisenforschung GmbH; 3Nippon Steel & Sumitomo Metal Corporation
     The present study aimed at an investigation of the precipitation behaviour of complex monoclinic intermetallic phase Nb2Co7 from supersaturated Co solid solution in order to understand its formation process and morphology. The Co-3.9 at.%Nb alloy was solution heat treated at 1240oC resulting in a single-phase fcc-Co solid solution and was subsequently cooled to various temperatures for isothermal heat treatments.A distinct Widmannstätten-like discontinuous precipitation microstructure was observed after isothermal heat treatment at 900oC, and the underlying Co matrix was found to have an hcp structure in contrast to the expectations from the equilibrium phase diagram. On the other hand, the Co matrix after direct cooling from 1240oC to room temperature has an fcc structure and contains finely dispersed L12 ordered regions with a significantly increased Nb content, what indicates the occurrence of a metastable intermediate phase during this solid state reaction in the Co-Nb system.