Phase Transformations and Microstructural Evolution: Ti & Zr, and Lightweight Metals Al & Mg
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 2:00 PM
February 27, 2017
Room: 16B
Location: San Diego Convention Ctr

Session Chair: Rajarshi Banerjee, University of North Texas

2:00 PM  
Investigation of Nano-scale Instabilities in Titanium Alloys: Yufeng Zheng1; Robert Williams1; Rajarshi Banerjee2; Dipankar Banerjee3; Hamish Fraser1; 1The Ohio State University; 2University of North Texas; 3Indian Institute of Science
    Microstructural evolution in beta titanium alloys can be significantly influenced by the nano-scale instabilities in parent bcc structure beta phase matrix, such as the pre-formed nano-size hexagonal structure isothermal omega phase particles can assist the subsequent formation of super-refined hcp structure alpha phase precipitates. Recently, with the development of the atomic resolution z-contrast high angle annular dark field- scanning transmission electron microscopy (HAADF-STEM), two new types of nano-scale instabilities were characterized in beta titanium alloys for the first time, namely O’ phase and O” phase. The structure of two phases and their orientation relationship with parent beta phase revealed by the results from TEM and STEM will be introduced first. More importantly the transformation mechanisms involved, a novel {110}<1-10> type soft-phonon shuffle mechanism to produce nano-scale disordered orthorhombic structure O’ phase and ordering mechanism to produce nano-scale ordered orthorhombic structure O” phase will be discussed.

2:20 PM  
Deformation Modes in High-pressure ω-phase of Zr: A First-principles Study: Anil Kumar1; M. Arul Kumar1; Irene Beyerlein1; 1Los Alamos National Laboratory
    The phase diagram of Zr as function of temperature and pressure has been the subject of much research due to its excellent corrosion resistance and hence makes it attractive for aggressive environments such as nuclear reactors. Knowledge of the dominant plastic deformation modes in the high-pressure phase of Zr (ω-phase) is important for understanding the plasticity of this system; however, they have yet to be clarified. In this work, we use first-principles DFT calculations to determine the generalized stacking fault energy surfaces for the possible slip systems in the ω-phase of Zr to identify the dominant slip modes. We employ an effective medium visco-plastic self-consistent model to simulate the deformation texture using these slip modes. We show that they compare well with experimental deformation textures of ω-Zr. Our results are relevant to the (a) study plasticity of ω-Zr, and (b) for understanding the mechanisms underlying the α-ω phase transformation.

2:40 PM  
Crystallization Pathway in Al-Sm Alloys Prepared by Melt Spinning and Magnetron Sputtering: Fanqiang Meng1; Wenjie Wang1; Shihuai Zhou1; Matthew Besser1; Matthew Kramer1; Ryan Ott1; 1Ames Laboratory
    Clarification of crystallization pathway is not only helpful for the structural design, but also can provide important insight into the starting amorphous structure. Two Al-Sm amorphous structures realized in melt-spun ribbons (MSR) and magnetron sputtered thin films (STF) exhibit very similar structural order, as measured by HRTEM and HEXRD; yet these two structures devitrify following vastly different pathways. The MSR alloy exhibits a polymorphic transformation to the “big cubic phase” (BCP), while the STF develops pronounced compositional inhomogeneities before the formation of fcc-Al and “big hexagonal phase” (BHCP). In both cases, the initial devitrifying transition consumes the entire sample, after which the resulting crystalline phases follows different pathways. Both pathways eventually converge toward the same phase composition. The difference in the structural motifs between BCP and BHCP and its relationship to an atomic “Gene” in the starting amorphous structures of the MSR and STF is discussed.

3:00 PM  
Microstructural and Texture Transitions Observed Using Shear Assisted Processing and Extrusion (ShAPE) of Melt Spun AZ91E Precursors: Nicole Overman1; Scott Whalen1; Matt Olszta1; Karen Kruska1; Jens Darsell1; Vineet Joshi1; Hellen Jiang1; Suveen Mathaudhu1; 1Pacific Northwest National Laboratory
    Processing improvements for lightweight structural materials are at the forefront of vehicle technologies development. For these reasons, a unique processing methodology has been explored. Consolidation of melt spun AZ91E flake precursor material was accomplished using Shear Assisted Processing and Extrusion (ShAPE). Microstructural evolution was evaluated at various stages of processing. Compositional segregation and grain growth were seen during consolidation, followed by grain refinement, homogenization of second phases and basal texture development during extrusion. Scanning and transmission electron microscopy were used to evaluate the formation and distribution of enriched Al-Mn and Al-Zn second phases. Interestingly, the equilibrium Mg17Al12 phase was not seen despite the higher temperatures (~450C) involved during processing. The unique phases identified are likely a result of far-from-equilibrium precursors. Hardness testing of the extrudate indicated structural uniformity. As a result, ShAPE processing coupled with rapid solidification was shown to produce materials with novel second phase compositions and crystallographic texture.

3:20 PM Break

3:40 PM  
Neutron Diffraction Study on Atomic Structures and Phase Transition of Magnesium-lithium Alloy: Ye Cui1; Zhongwu Zhang1; 1Harbin Engineering University
    Magnesium-lithium alloy has become the most important lightweight alloy material in the world for its outstanding strength, stiffness and excellent processing properties. The study on phase transition mechanism of magnesium-lithium alloy can be used to guide the composition design and preparation of magnesium-lithium alloy. But lithium atoms are difficult to observe by most methods, which hinders the study of distribution of lithium atom in magnesium-lithium alloy and its influence on phase transition of magnesium-lithium alloy. Neutron diffraction is a useful method to analyze lithium atoms and provides a useful way to study the atomic structure and phase transformation mechanism of magnesium-lithium alloy. In this work, neutron diffraction is used to analyze magnesium-lithium alloy, the atomic structures of magnesium-lithium alloy with different phases are obtained by pair distribution function method(PDF), and the atomic diffusion behavior and phase transition mechanism of magnesium-lithium alloy are studied based on the atomic structures.

4:00 PM  
Solute Segregation in Aluminum Alloys: Dongwon Shin1; Shibayan Roy1; Baishakhi Mazumder1; Larry Allard1; James Haynes1; Amit Shyam1; 1Oak Ridge National Laboratory
    Recently, it has been demonstrated that the operation temperature of cast Al-Cu alloys for automotive applications can be extended up to 300C. The mechanism behind this breakthrough observation is the stabilization of strength-contributing 𝜃′-Al2Cu at higher temperatures than previously reported. An extensive experimental investigation using scanning transmission electron microscopy and atom probe tomography revealed that segregated solute atoms at the semi-coherent interface between the aluminum matrix and 𝜃′-Al2Cu significantly contribute to the stabilization of this highly mobile interface. Here, we present a large database of solute segregation energies at the interface between aluminum and 𝜃′ from first-principles calculations. The interfacial energy change due to solute segregation agree well with experiments, wherever available. We have identified microalloying elements which may offer opportunities to further increase the temperature limit of Al-Cu alloys. This project was supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy Vehicle Technologies Program.