Continuous Phase Transformations: Session I
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Chemistry and Physics of Materials Committee, TMS: Phase Transformations Committee
Program Organizers: Jessica Krogstad, University of Illinois at Urbana-Champaign; Gregory Thompson, University of Alabama; Matthew Steiner, University of Cincinnati; Janelle Wharry, Purdue University

Monday 8:30 AM
March 15, 2021
Room: RM 55
Location: TMS2021 Virtual

Session Chair: Jessica Krogstad, University of Illinois at Urbana-Champaign; Gregory Thompson, University of Alabama


8:30 AM  Invited
Cluster Variation Model of Phase Behavior in Heusler-forming Alloy Systems: Michael Widom1; 1Carnegie Mellon University
    Energetics of chemical order in many alloys may be approximated by simple model Hamiltonians containing “clusters” of atoms. Each cluster contributes a certain energy that that can be fit to first principles calculations. The Cluster Variation Method represents the entropy in terms these clusters, and then varies cluster frequencies to minimize the free energy. This type of generalized mean field theory is convenient for exploring complex alloy phase diagrams. We apply the method to analyze phase transformation pathways in the family of Heusler compounds. The Heusler structure arises from a sequence of symmetry-breaking chemical ordering transitions on a body-centered cubic lattice, eventually leading to a set of four interpenetrating simple cubic sublattices; a total of eight distinct structures interpolate between the fully disordered BCC phase and the fully ordered quaternary Heusler. We discuss specific cluster-based models to represent specific phases and their transformations in several Heusler-forming compounds.

9:00 AM  Invited
Interfacial Spinodals: Reza Darvishi Kamachali1; 1Federal Institute for Materials Research and Testing (BAM)
    Despite their finite spatial extent, interfaces can have profound impacts on microstructure properties. This is because of their distinct phase-like properties distinguishing them from the adjacent homogeneous bulk structure. When noticed by solute atoms, interfaces can experience their own chemical phase changes. In this talk, we investigate the constrained states of chemically decomposed phases at interfaces. A density-based theory of interfaces is proposed to describe the confined chemical decomposition at general grain boundaries. Here the grain boundary is viewed as a lesser dense, defected structure with reference to the corresponding bulk structure. Using this picture, the thermodynamic origins of interfacial spinodal phenomena are revealed. We also show that transient interfacial spinodals can be activated over a large alloy composition range, enabling kinetic engineering of interfacial chemistry.

9:30 AM  
Competitive Partitioning and Decomposition Evolution in Nanocrystalline Alloys: Gregory Thompson1; Xuyang Zhou2; Reza Kamachali3; Jaber Mianroodi4; Alisson Kwiatkowski da Silva4; Pratheek Shanthraj5; Dirk Ponge4; Baptiste Gault4; Bob Svendsen6; Dierk Raabe4; Brad Boyce7; Blythe Clark; Blythe Clark; Blythe Clark7; 1University of Alabama; 2Max-Planck-Institut für Eisenforschung ; 3Federal Institute for Materials Research and Testing (BAM); 4Max-Planck-Institut für Eisenforschung; 5The University of Manchester; 6Aachen University; 7Sandia National Laboratories
    Spinodal decomposition is often described as a ‘barrier-free’ transformation. Unlike nucleation and growth, where defects play a dominate role in the nucleation of the secondary phase, such defects are largely considered inconsequential to this continuous phase transformation. At the nanoscale, where a high density of planar defects exist, their presence can have a profound influence on the temporal evolution of the transformation. In this presentation, the phase separation of a Pt(Au) nc alloy is characterized using a correlative method of electron diffraction and atom probe. A competition between interfacial segregation to the grain boundaries and spinodal decomposition is observed. Using a density-based phase field model for the grain boundary influence on the Gibbsian free energy for the transformation is used to elucidate how grain character influences the transformation. Furthermore, the presence of dislocations are also characterized revealing a distinct chemical structure in these defect structures.

9:50 AM  
Study of Precipitation Behavior of High-Cr Ni-based Filler Metals Using In-situ S/TEM: Cheng-Han Li1; Sriram Vijayan1; Carolin Fink1; Joerg Jinschek1; 1The Ohio State University
    High-Chromium Nickel-based filler metals are widely used for repair of structural components in nuclear power industry. During multi-pass weld repair applications these alloys are potentially susceptible to ductility-dip cracking (DDC), which is a solid-state grain boundary sliding phenomenon. Precipitation of carbides along grain boundaries upon cyclic heating and cooling is a factor for the occurrence of DDC. Additions of Molybdenum have been shown to enhance the cracking resistance in Ni-30Cr filler metals. However, there is a lack of insight on how Mo effects grain boundary carbide precipitation and results in increased resistance to DDC in multi-pass welds. In situ TEM heating experiments were performed to study preferred carbide nucleation sites and to understand the transformation mechanism of these carbides upon cyclic heat treatment. In situ observations were further validated with DSC and ex situ experiments. Elemental partitioning during precipitation were investigated using XEDS at high spatial-resolution in a probe-corrected S/TEM.

10:10 AM  Invited
Microstructural Engineering of Ni-based Superalloys Processed by Conventional and Additive Manufacturing: Felix Theska1; Nima Haghdadi1; Sophie Primig1; 1University of New South Wales
     The demand for Ni-based superalloys withstanding high mechanical workloads under aggressive high-temperature environments is growing continuously. Market forecasts predict an increase of ~30% in commercial aircraft and gas turbines. The ongoing technological success of superalloys is due to their multi-scale hierarchical microstructural design achieving contributions from different strengthening mechanisms. A typical microstructure of an advanced superalloy consists of various interfaces, micron-scale precipitates, complex nano-scale precipitates, solute clusters, and solute segregation at interfaces. Targeted engineering of these multi-scale hierarchical microstructures unlocks superior mechanical properties, and is driven by advanced microscopy. Current challenges are around hot-formability and weldability which can be overcome by advancements in conventional thermo-mechanical processing or additive manufacturing. This talk will summarize our recent progress in microstructural engineering and microscopy of superalloys. Advancements in processing of Alloy 718 and René 41 unlock enhanced strength and eliminate cracking. Innovative electron beam additive manufacturing of Alloy 738 promotes in-situ γ' precipitation.

10:40 AM  
Phase Competition in the Two Steps Continuous Phase Transformation during Solidification of Terbium: Huajing Song1; M.I. Mendelev2; 1Physics and Chemistry of Materials, Los Alamos National Lab; 2Ames Laboratory, US Department of Energy
    The competition among multiple phases ultimately determines the final microstructures of the materials. Such competition can originate at the very beginning of the solidification process. In the current study, a new developed Terbium (Tb) EAM potential was used in molecular dynamics simulation. A two steps continuous phase transformation is observed during the solidification of Tb. The undercooling liquid will always transform to a metastable body-centered cubic (bcc) structures first, then further transform to the equilibrium hexagonal close-packed (hcp) phase. The hcp phase can only form in the last liquid droplet or at the bcc grain boundary, and behaves sensitive to the system pressure and grain boundary orientations. The hcp phase grows by a solid-state massive transformation. In some high driving force conditions, a martensitic transformation can be activated. The result indicated that the kinetic factor can play a significant role to determine the transition pass for the solidification process.