Phase Transformations and Microstructural Evolution: Phase Transformation
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, Microstructure Engineering; Eric Payton, University of Cincinnati; Megumi Kawasaki, Oregon State University; Eric Lass, University of Tennessee-Knoxville

Monday 8:30 AM
February 28, 2022
Room: 255B
Location: Anaheim Convention Center

Session Chair: Eric Payton, Air Force Research Laboratory; Mohsen Asle Zaeem, Colorado School of Mines

8:30 AM  Invited
Tuning Nano-scale Phase Transitions to Expand Transformation-induced Plasticity: C. Tasan1; Shaolou Wei1; 1Massachusetts Institute of Technology
    To seek optimal strength-ductility balance in metallic alloys, athermic phase transformations upon plastic deformation are regarded as one of the most effective approaches. Decades of efforts in ferrous alloy design have documented the significant role of strain-induced martensitic transformation in mechanical performance improvement (namely, the transformation-induced plasticity effect, TRIP). Provided its mechanical benefit, the resultant transformation product of the TRIP-effect, martensite, is yet destructive. The extensive defect density within the martensitic phase and the hardenability discrepancy with its adjacency can lead to local embrittlement and hence fracture, inevitably rendering an extremum of property improvement. In light of this, the present talk will reveal two potential solutions: a sequential martensitic transformation mechanism and a mechanical faulting response. Further insights into mechanistically-guided alloy design will also be discussed.

9:00 AM  
Deformation-induced Phase Transformation and Microstructure Evolution in CrCoNi Medium Entropy Alloy: Hangman Chen1; Penghui Cao1; 1University of California, Irvine
    High entropy or medium entropy alloys have drawn enormous attention due to their excellent mechanical behaviors. The CrCoNi medium entropy alloy stands out among this class of materials because of its exceptional damage tolerance and high strength-ductility synergy. We perform multiscale atomistic simulations to elucidate the deformation mechanisms, phase transformation, and nano-/micro-structure evolution in CrCoNi, responsible for its extraordinary mechanical performance. By analyzing deformation trajectory, we reveal the fundamental dislocation processes underlying twinning, martensitic transformation (FCC to HCP), and sub-grain structure evolution. The role of local chemical short-range order on these deformation mechanisms will also be discussed.

9:20 AM  
NOW ON-DEMAND ONLY - Effect of Boron Segregation on Bainite Nucleation during Isothermal Transformation: Pierre Douguet1; Gregory Da Rosa2; Philippe Maugis1; Josée Drillet2; Khalid Hoummada1; 1Aix Marseille Univ, CNRS, IM2NP, Marseille, France; 2ArcelorMittal Maizières Research SA, Voie Romaine, BP30320, 57283 Maizières les Metz, France
    The effects of boron segregation and austenite grain size on the bainitic isothermal transformation of a high-strength steel were studied independently. Dilatometry, microstructural observations and previous atom probe tomography analyses showed that bainitic transformation rate decreases with increasing boron excess at austenite grain boundaries. We also found that boron segregation causes an unexpected grain size effect: large austenite grains transform faster than small ones. A kinetic model assuming slow nucleation at austenite triple junctions and rapid growth of bainitic nuclei successfully described our experimental data. These results confirm that bainite nucleation is inhibited by segregated boron at grain boundaries. On this basis, austenitic grain size could be optimized to make the most of boron addition in advanced high-strength steels.

9:40 AM  Invited
Localized Phase Transformation at Stacking Faults and Mechanism-based Alloy Design: Longsheng Feng1; Timothy Smith2; Ashton Egan1; Fan Zhang3; Michael Mills1; Yunzhi Wang1; 1The Ohio State University; 2NASA Glenn Research Center; 3CompuTherm LLC
    A new phenomenon was observed in some Ni-based superalloys where phase transformations occur at and are confined to stacking faults, which we refer to as localized phase transformations (LPTs). Alloys with different types of LPTs such as ordering and disordering are found to have drastically different creep properties. It is thus of great importance to understand the underlying physics behind this localized phase transformation. A thermodynamic model is proposed to understand the foundation of LPT and to address fundamental questions why LPT occurs, why LPT is confined at stacking faults, what types of LPT are expected at stacking faults and how LPT impacts mechanical properties. The proposed LPT mechanism is used to establish a new design strategy for the next-generation LPT-strengthened superalloys. This work is supported by NSF under DMREF program.

10:10 AM Break

10:25 AM  
Shear Deformation-induced Modification of Defect Structures and Hierarchical Microstructures in Metallic systems: Bharat Gwalani1; Matthew Olszta1; Miao Song1; Wenkai Fu1; Yulan Li1; Qin Pang1; Anqi Yu1; Mayur Pole1; Jia Liu1; Joshua Silverstein1; Xiaolong Ma1; Tanvi Anjantiwalay1; Aashish Rohatgi1; Mert Efe1; Peter Sushko1; Arun Devaraj1; Ayoub Soulami1; Suveen Mathaudhu1; Cynthia Powell1; Lei Li1; 1Pacific Northwest National Laboratory
    Deformation-induced microstructural modification is used in several advanced materials' processing methods such as friction-stir-based processing, welding, or additive manufacturing. In these processes, the mechanical-thermal coupling obscures a deep mechanistic understanding of microstructural evolution, and the knowledge of how these microstructures influence mechanical properties is in its nascency. We highlight the influence of high strain deformation on the microstructural hierarchy and mechanical properties of binary alloys such as Al-Si, Cu-X (X = Nb, Cr, or Ni). Our studies show that deformation-induced grain refinement, multiscale fragmentation, and metastable solute saturated phases with distinctive defect structures lead to a significant increase in the flow stresses measured via micropillar compression. Our results highlight that shear deformation during solid-phase processing can achieve persistent metastable microstructures with enhanced mechanical properties with deformation-dependent solute miscibility. The experimental insights obtained here are crucial for developing atomic to mesoscale models for microstructural evolution under high strain deformation.

10:45 AM  
Tricky Transformations in an Ion Irradiated Nickel-titanium Alloy: Alejandro Hinojos1; Daniel Hong1; Nan Li2; Khalid Hattar3; Peter Anderson1; Michael Mills1; 1The Ohio State University; 2Los Alamos National Laboratories; 3Sandia National Laboratories
    Controlling the martensitic transformation and subsequent mechanical properties in NiTi alloys through standard means of thermal and deformation processing has been probed for decades and has reached its realm of marginal gains. Prior work breaking with tradition utilizing ion irradiation showed little success in NiTi thin films. Using a combination of ion irradiation and plastic deformation crystallographic defects could produce unique heterogeneous microstructures to enhance psuedoelastic properties. Thermally cycled and uncycled NiTi were irradiated with two ion irradiation doses in order to understand the interaction between transformation and irradiation induced defects. In this work we have observed the transformation suppression of the thermal transformation in irradiated NiTi, but an enhancement in the reversible strain of the alloy. Here microstructural (XRD, EBSD, S/TEM) and mechanical (nanoindentation) characterization with in-situ cooling and micropillar testing will be used to assess the irradiation effects and transformation properties.

11:05 AM  
Unravelling the Mechanisms of Irradiation Induced Phase Transformation in Nanocrystalline Gold: James Nathaniel1; Douglas Medlin1; Khalid Hattar1; Mitra Taheri2; 1Sandia National Laboratories; 2Johns Hopkins University
    Gold is a noble metal typically stable as a solid in a face-centered cubic (FCC) structure; however, under particular circumstances (such as under high pressure and in nanoparticles) anomalous allotropes have been produced. Recently, stabilized hexagonal close packed (HCP) structure has been identified by diffraction in irradiated nanocrystalline gold thin-films. Though this phenomenon has been documented, the mechanism(s) of transformation have yet to be identified. Using in situ ion irradiation, atomic-resolution transmission electron microscopy (TEM), and automated crystal orientation mapping (ACOM) techniques, in this work, we investigate the specimen geometry, crystallography, defect morphology, and irradiation parameters which lead to the irradiation induced FCC to HCP phase transformation. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.