Phase Transformations and Microstructural Evolution: Solidification and Microstructural Evolution/General Topic
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Phase Transformations Committee
Program Organizers: Mohsen Asle Zaeem, Colorado School of Mines; Ramasis Goswami, Naval Research Laboratory; Saurabh Puri, VulcanForms Inc; Eric Payton, University of Cincinnati; Megumi Kawasaki, Oregon State University; Eric Lass, University of Tennessee-Knoxville

Thursday 8:30 AM
March 3, 2022
Room: 255B
Location: Anaheim Convention Center

8:30 AM
Growth Competition between Columnar Dendritic Grains - The Role of Microstructural Length Scales: Elaheh Dorari¹; Kaihua Ji¹; Gildas Guillemot²; Charles-Andre Gandin²; Alain Karma¹; ¹Northeastern University; ²MINES ParisTech
    During alloy solidification in material processing, such as casting and additive manufacturing, competitive growth occurs between columnar dendritic grains. As a result, grain boundaries (GBs) form with distinct microstructural features. We present the results of a comprehensive study based on quantitative phase-field (PF) simulations for bicrystal orientations. The transition from FOG to GL regimes is retrieved when decreasing the temperature gradient. We use the computed microstructural length scales at the GB to bridge Cellular Automaton (CA) and PF prediction of the GB orientation. Despite its inherent approximations, we demonstrate that the CA retrieves the PF results providing the choice of a cell size equal to the active secondary dendrite arm spacing. This result paves the path for future developments of the CA method.

8:50 AM
Growth, Coarsening, and Fragmentation of Dendritic Microstructures in Metallic Materials: Tiberiu Stan¹; Zachary Thompson¹; Peter Voorhees¹; ¹Northwestern University
    The dendritic microstructures that initially form and evolve during solidification of metals have a major impact on the final material properties. However, the details of dendritic growth, coarsening, and fragmentation are not well understood, making it difficult to accurately model these phenomena and design improved processing methods. We use in-situ x-ray computed tomography and serial sectioning to produce 4D and 3D datasets, respectively, and to gain new insights into the phase transformation and microstructural evolution phenomena. The dendritic morphologies, length scales, and crystallographic growth orientations are characterized for a variety of metallic alloys and processing conditions. Furthermore, insights into the causes of dendrite arm fragmentation events and their distribution throughout microstructures are discussed. These findings will be used to elucidate the microstructural evolution mechanisms and to produce benchmark data for modelling efforts.

9:10 AM
The Structural Evolution of the Metastable Ni₂₃B₆ Phase during Rapid Solidification of Undercooled Ni-B Melts: Lianjie Liu¹; ¹Shanghai Jiao Tong University
    Solidification of metastable Ni₂₃B₆ phase and subsequent transformation into the stable phase has been investigated in undercooled Ni_79.3B_20.7 and Ni₇₅B₂₅ melts by cooling curve and microstructure. The Ni₂₃B₆ metastable phase solidifies as a substitute for the stable Ni₃B phase above the critical undercooling ΔT_c1 = 170 K and ΔT_c2 = 240 K for Ni_79.3B_20.7 and Ni₇₅B₂₅ alloy, respectively. The structural evolution of metastable Ni₂₃B₆ phase in mushy zone is revealed during a two-step recalescence event. The final microstructure of the samples not only depends on undercooling, but also on the delay time of the second recalescence event. Decomposition of primary metastable phase accompanying with subsequent remelting and ripening plays an important role in the solidification microstructure formation. The delay time is a decisive parameter that does not affect the decomposition reaction, but it has a significant effect on the remelting and ripening.

9:30 AM
In Situ Interrogation of Dynamic Microstructural Evolution during Friction Stir Processing of Al – 4 at.% Si: Julian Escobar¹; Arun Bhattacharjee¹; Jorge dos Santos²; Jan Herrnring²; Luciano Bergmann²; Peter Staron²; Benjamin Klusemann¹; Bharat Gwalani²; Suveen Mathaudhu¹; Cynthia Powell¹; Arun Devaraj¹; ¹Pacific Northwest National Laboratory; ²Helmholtz-Zentrum Hereon
    Time-resolved analysis of microstructural evolution mechanisms during the Friction Stir Processing (FSP) can be achieved via synchrotron high energy X-ray diffraction (HEXRD). In this work, a customized portable FSP machine (Flexistir), was installed in the P07 Beamline at DESY Synchrotron, to study the microstructural evolution of an Al-4 at % Si alloy in situ during FSP. Tool rotational speeds of 350, 700 and 1400 revolutions per minute and welding speeds of 3, 6, 9, 12, 24 and 36 mm/s under a fixed axial force (2 kN) were studied. In situ results revealed process parameter dependent changes in lattice parameter and lattice strain of both Al and Si phases which was then correlated with detailed ex situ characterization results after FSP. The combination of in situ and ex situ study provided unprecedented insights to the microstructural evolution of this model alloy during FSP.

9:50 AM Break

10:10 AM
In-situ Observation of Coupled Growth Morphologies in Organic Peritectics under Pure Diffusion Conditions: Johann Mogeritsch¹; ¹Montanuniversitaet Leoben
    Isothermally coupled peritectic solidification for a hyper-peritectic alloy under pure diffusive conditions is presented. For this purpose, directional solidification experiments were performed aboard the International Space Station using a model system for peritectic layer solidification patterns. At constant temperature gradient of 3.0 K/mm and for pulling velocities ranging from 0.12 痠/s to 0.09 痠/s, coupled peritectic growth was observed. At lower pulling velocities, contrary to expectations, only a planar solidification front of the pro-peritectic phase was detected. Two effects were noticed, a significant effect of the pro-peritectic interface on the capability of the peritectic phase to nucleate and, in the further course of the experiments, a dynamic change of the coupled peritectic growth microstructure.

10:30 AM
Understanding Microstructural Evolution in Powder Bed Fusion Additive Manufacturing: Observations of Phase Transformation in Ti-6Al-4V Using In Situ TEM Heating Experiments: Sriram Vijayan¹; Carolin Fink¹; Joerg Jinschek¹; ¹The Ohio State University
    Powder bed fusion (PBF) additive manufacturing results in steep thermal gradients (~10^6 K/m) and rapid thermal cycles (~ 10^3 K/s), which significantly affects the microstructural evolution within the build. The non-equilibrium process conditions result in metastable microstructures and columnar grain morphologies, which deviate significantly from conventionally processed alloy microstructures. New in situ tools can observe thermally activated phase transformations (PT) under AM process conditions. Here, we use a modified MEMS-based heating stage to simulate ‘AM like’ processes inside a transmission electron microscope (TEM). This modified device enables us to study the microstructural response of a material under the combined influence of large thermal gradients (10^6 K/m) and rapid thermal cycling. The approach is used to observe the sequence of PT in Ti-6Al-4V to further our understanding on microstructural evolution under super-transus thermal cycling and sub-transus thermal cycling conditions as experienced during electron beam melting PBF processing.

10:50 AM
Experimental Investigation of Ni2Cr Long-range Ordering in Ni-Cr-Fe Based Model and Commercial Alloys: Nicholas Aerne¹; David Sprouster²; Julie Tucker¹; ¹Oregon State University; ²Stony Brook Univsersity
    Service temperatures in pressurized water reactors may promote nucleation and growth of the long-range order (LRO) Ni2Cr phase in Ni-based alloys. This phase transformation leads to drastic increases in model Ni-Cr alloy’s strength while decreasing the ductility. The formation of Ni2Cr with respect to Ni, Cr, and Fe compositions is disputed in literature. Further the rate of formation and nucleation factors is not well established in and between model and commercial alloys. In this research, isothermal aging of Ni-based commercial alloys: 625, 625 plus, 690, and Ni-Cr-Fe model alloys has been performed to quantify the impact of LRO over time at temperatures 330, 360, 418, and 475 蚓, aged up to ~21k, and ~10k hours for commercial and model alloys, respectively. The alloys were characterized by Vickers hardness testing and synchrotron x-ray diffraction. The hardness increased the most for model alloys with no Fe at 475蚓.