Phase Transformations and Microstructural Evolution: Steels & General
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Phase Transformations Committee
Program Organizers: Gregory Thompson, University of Alabama; Rajarshi Banerjee, University of North Texas; Sudarsanam Babu, The University of Tennessee, Knoxville; Deep Choudhuri, University of North Texas; Raju Ramanujan, Nanyang Technological University; Monica Kapoor, National Energy Technology Lab

Monday 8:30 AM
February 27, 2017
Room: 16B
Location: San Diego Convention Ctr

Session Chair: Sudarsanam Babu, The University of Tennessee, Knoxville

8:30 AM  Invited
A Direct Evidence of Solute Interactions with a Moving Ferrite/Austenite Interface in a Model Fe-C-Mn Alloy: Goune Mohamed1; Fréderic Danoix2; Xavier Sauvage2; Didier Huin3; Lionel Germain4; 1ICMCB-Bordeaux1; 2GPM - Université de Rouen; 3ArcelorMittal; 4Université de Lorraine
    The coupled-solute drag during ferrite growth in steels is widely discussed in the literature and remains still controversial since little direct evidences were adduced to support this effect. In this paper, from a correlative microscopy approach, some selected migrating ferrite/austenite interfaces in a model Fe-Mn-C alloy are quantitatively analysed by atom probe tomography and Energy Dispersive X-Ray Spectroscopy at the nanoscale. They show unambiguously a strong co-segregation of both carbon and manganese at the interface during ferrite growth as a function of time at 680 °C. The kinetics of C and Mn accumulation at the α/γ interface is analysed and the binding energy for Mn and C to α/γ interface was thus estimated from the experimental data. The obtained results support the coupled-solute drag effect as an operating mechanism during ferrite growth.

9:00 AM  
An Experimental Assessment of the α + α' Miscibility Gap in Fe-Cr: Alexander Dahlstrom1; Frederic Danoix1; Peter Hedstrom2; Joakim Odqvist2; Helena Zapolsky1; 1Normandy University; 2KTH (Royal Institute of Technology)
    Phase separation in the Fe-Cr miscibility gap has been studied experimentally. A tendency to de-mix enable the formation of an α (Fe-rich) + α’ (Cr-rich) modulated nanostructure in Fe-Cr alloys. This phase separation triggers a change in mechanical properties known as the “475°C embrittlement”, causing a restriction in service life-time. The upper limit of the miscibility gap during the early stages of phase separation has been investigated and decided within ± 2°C. Vickers micro hardness has shown good potential effectively indicating the presence of phase separation. Utilizing atom probe tomography as a means when traditional metallographic characterization is challenged allows for analyzing the nanostructure on the atomic scale. To deepen the understanding of the limit of phase separation, the transition in the upper limit of the miscibility gap is further discussed based on atom probe analysis and the theory of spinodal decomposition by J.W. Cahn.

9:20 AM  
Diffusion Behavior of Alloy Elements in Martensite-austenite Constituent Formed in the Heat-affected Zone of a Low Alloy Carbon Steel: Masahiro Inomoto1; Hidenori Nako1; 1Kobe Steel, Ltd.
    Martensite-austenite constituent (MA) formed in the heat-affected zone (HAZ) has a detrimental effect on HAZ toughness due to its high hardness. In order to control MA, it is important to be aware of the diffusion and distribution behavior of alloy elements among the matrix phase and MA during thermal cycle. However, few studies have focused on these things during a short-time thermal cycle like HAZ. This study investigated the diffusion and distribution behavior during the thermal cycle. This was done by simulating intercritically reheated coarse-grained HAZ. Fe-C-Si-Mn-Al-Nb alloys were heated to austenitizing temperature, cooled to room temperature, subsequently reheated to intercritical temperatures and cooled at various rates. Atom probe analysis revealed that the carbon content in the martensite phase of MA formed in the thermal cycle with a cooling rate of 10 K/s was higher than in the water-quenched cycle and equilibrium concentration at intercritical temperature.

9:40 AM  
Direct Observation of the Movement of the Austenite-ferrite Interface in Fe-C-Mn Steels: William Rainforth1; John Nutter1; 1The University of Sheffield
    The transformation from austenite to ferrite is a key process in determining the final microstructure and properties of steels. The transformation is sensitive to both cooling rates and composition. Typically models of this transformation make assumptions about the material and the structure and motion of the interface. Using in situ hot stage transmission electron microscopy it is possible to directly observe the motion of the austenite-ferrite interface during transformations. Observed interfaces migrated with fluctuations in velocity but reasonable agreement with expectations. Interactions occurred between microstructural features including existing grain boundaries as well as other growing grains. By cycling the temperature within the two-phase region, the motion of the interface can be reversed. These interfaces do not show perfect reversibility, but can be used to examine the effect of changes in local chemistry caused by the partially complete transformation. How these observations can contribute towards refining existing models is discussed.

10:00 AM Break

10:20 AM  Invited
Synchrotron High-energy X-rays for In-situ Study of Phase Transformation of Advanced Materials: Yang Ren1; 1Argonne National Laboratory
    High-energy x-rays generated at synchrotron user facilities provide great research opportunities in materials science and engineering, especially for probing structural phase transformation of bulk materials in complex sample environments. The knowledge of the atomic-level structures and their evolution in realistic conditions provides fundamental basis for better understanding material properties and for further improvement of their performance and functionality. In this talk, we will present technical details and capacities of high-energy x-ray techniques available at the Advanced Photon Source (APS), Argonne National Laboratory, and their applications to in-situ structural investigations of phase transformation of advanced materials, including metals and alloys, magnetic materials, shape memory alloys and nanocomposites etc. under variable temperature, magnetic, electric and stress fields.

10:50 AM  
Harnessing the Kirkendall Effect for the Fabrication of Metallic Microtubes and Hollow Scaffolds: Ashley Paz y Puente1; Dinc Erdeniz1; David Dunand1; 1Northwestern University
    The Kirkendall effect, a consequence of the imbalance of diffusivities among atomic species, can result in the formation of so-called Kirkendall pores. Typically these Kirkendall pores are considered detrimental because they deteriorate the mechanical, thermal, and electrical properties of materials. However, this talk discusses harnessing the Kirkendall effect as a fabrication route for NiAl(Cr) and NiTi microtubes by taking advantage of the radial symmetry and spatial confinement. Both ex situ metallography techniques and in situ X-ray tomography were used to study the phase and Kirkendall pore formation and evolution in these systems. The extension of this Kirkendall based technique to 2D structures and 3D scaffolds of interest for a variety of applications from batteries to biomedical implants is also briefly discussed.

11:10 AM  
Interfacial Energy Evaluation in Binary Systems Using Diffusion-Multiples and Simulations: Qiaofu Zhang1; Surendra Makineni2; John Allison2; Ji-Cheng Zhao1; 1The Ohio State University; 2University of Michigan
    Diffusion multiples were first annealed at a high temperature to create solid solution compositions by interdiffusion and then dual annealed at a lower temperature to induce second phase precipitations as a function of composition/supersaturation for several binary systems in one sample. Precipitate morphology and distribution at different locations were characterized using high resolution SEM and TEM to obtain particle size distribution and mean particle radius. A MatLab program based upon the classical nucleation and growth theory (CNGT) and Kampmann-Wagner numerical (KWN) model was developed to simulate the precipitation process. By adjusting the pre-assumed interfacial energy value, simulated results can be matched with experimental results in order to obtain the interfacial energy between matrix and the second precipitate. The precipitation in both Ni-Al and Fe-Cu systems will be used to demonstrate this methodology.

11:30 AM  
Assessing Chemical and Microstructural Evolution at Interfaces of γ' - Strengthened Superalloys at High Temperatures by In Situ TEM Heating Experiments: Yolita Eggeler1; Erdmann Spiecker1; 1Friedrich Alexander Universität Erlangen-Nürnberg
    The scope of this work is to investigate the microstructural evolution of the γ and γꞌ phase towards a new thermodynamic equilibrium, corresponding to a selected temperature. In situ studies with chip-based heating systems (by DENSsolutions) are performed on a single crystal Ni based superalloy, ERBO1. The sample is heated to a fixed temperature for different time periods and is subsequently quenched. The element distribution was measured by EDXS at room temperature. It can be shown that the transition towards a new thermodynamic state can indeed be resolved, with progressing time. Such data contain valuable information on the kinetics of interdiffusion in a real superalloy microstructure. Quantitative evaluations of diffusion profiles will enable to simultaneously determine interdiffusion coefficients of various alloying elements. Finally the experimental results will be validated by complementary ex situ experiments on bulk samples employing quenching in a vertical oven.