Phase Transformations and Microstructural Evolution: Microstructure and Precipitation
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Phase Transformations Committee
Program Organizers: Rongpei Shi, Harbin Institute of Technology; Yipeng Gao, Jilin University; Fadi Abdeljawad, Lehigh University; Bharat Gwalani, North Carolina State Universtiy; Qi An, Iowa State University; Eric Lass, University of Tennessee-Knoxville; Huajing Song, Los Alamos National Laboratory
Tuesday 8:30 AM
March 16, 2021
Room: RM 57
Location: TMS2021 Virtual
Session Chair: Thomas Voisin, Lawrence Livermore National Laboratory; Kaila Bertsch, Lawrence Livermore National Laboratory
8:30 AM
Chemistry Effects on α' Precipitation in FeCrAl Alloys: Andrew Hoffman1; Soumya Nag1; Chen Shen1; Chao Jiang2; Yongfeng Zhang3; Raul Rebak1; 1GE Research; 2Idaho National Lab; 3University of Wisconsin-Madison
FeCrAl alloys have been shown to have excellent high temperature steam oxidation resistance due to Al forming a passive oxide layer making them an excellent candidate for accident tolerant nuclear fuel cladding. There are concerns, however, that at normal light water reactor operating temperatures (~300°C) precipitation of a Cr-enriched α’ phase can lead to issues with embrittlement and corrosion. While α’ has been well studied in Fe-Cr ferritic steels, little work has been conducted on the alloying effects of Al in the FeCrAl ternary system. Previous thermal aging studies at 475°C indicate that Al can enhance α’ precipitation at lower concentrations and suppress precipitations at high concentrations. In this aging study we show that while Al can enhance the precipitation kinetics of α’, it lowers the α-α’ miscibility gap. Additionally, the effects of Mo addition (common to commercial alloys) to this system will also be discussed.
8:50 AM
Effect of Slip and Twinning Microstructure on High Pressure Phase Transformation in Zirconium: Arul Kumar Mariyappan1; Yanbin Wang2; Rodney McCabe1; Laurent Capolungo1; Carlos Tome1; 1Los Alamos National Laboratory; 2Argonne National Laboratory
Group IV transition metal zirconium has a stable hexagonal-close-packed crystal structure (α) at ambient condition, and transforms to simple-hexagonal crystal structure (ω) under high pressure. The ω-phase is brittle compared to α-phase, and so the understanding of α-to-ω phase transformation is important to enhance the applicability of zirconium. The role of microstructure on this phase transformation is not understood yet. In this work, we study the effect of pre-existing slip and twinning microstructure on α-to-ω phase transformation and the stability of ω-phase using in-situ diffraction experiments at high pressure. The experiment reveals that pre-existing prismatic slip and tensile twinning microstructures favor the forward phase transformation when pressure is applied, and stabilizes the ω-phase when pressure is removed. On the other hand, pre-existing pyramidal slip and compression twinning microstructures delay the forward phase transformation under hydrostatic pressure and also mostly reverses back to α-phase upon pressure release.
9:10 AM
Probing the Plasticity and Microstructure Evolution of an Icosahedral Quasicrystal i-Al-Pd-Mn at Elevated Temperatures
: Yu Zou1; 1University of Toronto
Quasicrystalline materials possess a unique structure that is ordered yet not periodic. Despite their special configuration and manyuseful properties, they are typically very brittle at temperatures below ~75% of their melting points, rendering them difficult to process and often unsuitable for practical implementation. Here, we study the mechanical behavior of a typical icosahedral quasicrystal (i-Al-Pd-Mn) using micro-thermomechanical techniques over the temperature range of 25-500 °C, which has never been explored before. A few interesting phenomena have been observed, including micro-pillar shrinkage, phase transformations, grain refinement, and thermally induced transitions in deformation behavior (from brittle fracture to serrated plastic flows, and then to homogeneous flows). Furthermore, we discuss the multiple underlying mechanisms on the mechanical behavior of the quasicrystal in this temperature regime, exploring surface evaporation/diffusion, diffusion-enhanced plasticity, dislocation activities, and grain boundary rotation/sliding.
9:30 AM
Spinodal Decomposition in a Nanostructured Cu-Ti Alloy: Julian Rosalie1; Oliver Renk2; 1University of Leoben, Austria; 2Erich Schmid Institute, Austrian Academy of Sciences
One of the major limitations of nanostructured materials is their loss of work-hardening mechanisms and subsequent poor ductility. Conventional approaches to combat this by introducing second-phases are rarely successful due to grain-boundary solute segregation and strong competition for nucleation sites. We have addressed these problems by inducing spinodal decomposition in a nanostructured copper-titanium alloy produced by high-pressure torsion. Isothermal ageing increased the yield strength from 820 MPa to 900 MPa, while reducing the elongation to failure from 11% to 7%. Even in the peak-aged condition there was significant necking at the site of fracture. This work demonstrates that harnessing spinodal decomposition offers a means to control work-hardening and retain ductility in nanostructured materials.
9:50 AM
The Synergistic Role of Mn and Zr/Ti in Producing θ'/L12 Co-precipitates in Al-Cu Alloys: Jonathan Poplawsky1; Brian Milligan2; Patrick Shower3; Lawrence Allard1; Matthew Chisholm1; Dongwon Shin1; Amit Shyam1; 1Oak Ridge National Laboratory; 2Colorado School of Mines; 3GE Global Research
Typical Al-alloys used for automotive applications can only withstand 250C temperatures. A large metastable 𝜃' precipitate (Al2Cu) number density is critical for these alloys’ strength. ORNL recently developed an Al-Cu-Mn-Zr alloy that maintains strength after a >200 hrs exposure to 350C. Hardness results show that sole Zr additions don’t provide stability, sole Mn additions provide 300°C stability, while 350C stability is only achieved with combined Mn/Zr additions. Atom probe tomography (APT) and scanning transmission electron microscopy experiments coupled with computational simulations reveal that Mn/Zr stabilize 𝜃' through interfacial solute segregation and eventual L12/𝜃' co-precipitation. Mn segregation stabilizes 𝜃' through solute drag and interfacial energy reduction, which allows for slower diffusing Zr/Ti to form L12 at 𝜃' interfaces. L12 co-precipitation further improves 𝜃' stability with a nil interfacial energy, reduced strain, and ledge poisoning effect. APT was conducted at the CNMS, which is a U.S. DOE Office of Science user facility.
10:10 AM
Understanding the Influence of Thermal Gyrations on Solid-solid Interfaces in Ti-6Al-4V during EBM PBF Process Using In Situ TEM: Sriram Vijayan1; Meiyue Shao1; Joerg Jinschek1; 1The Ohio State University
In powder bed fusion (PBF) processes, additively manufactured (AM) Ti-6Al-4V (Ti64) builds experience extreme thermal gyrations (103-105 K/m & 103 K/s) during electron beam melting (EBM) of Ti64 powder. Using the default ‘raster scan’ EBM strategy, thermal gyrations experienced by the melt pool and previously deposited layers result in significant variations in microstructure across the build. Recently, researchers have revealed that modified EBM beam scanning strategies allow the control of thermal gradients and cooling rate, thereby controlling the local microstructure within an AM part. Here, we present an in situ TEM heating study, using a modified micro-heater to simulate AM like process conditions, to understand the influence of thermal gyrations across a solid-solid Ti64 interface at high spatial resolution. Subsequently, ex situ and calorimetric experiments were performed, to validate our in situ results and understand the mechanisms of solid-state processes and interface stability in builds during the non-equilibrium AM process.
10:30 AM
Variability of Grain Boundary Migration Behaviors among the Metastable Grain Boundary Structures: Eric Homer1; Darcey Britton1; Oliver Johnson1; Lydia Serafin1; Gus Hart1; 1Brigham Young University
Recent examinations of grain boundary mobility have demonstrated some very interesting phenomena, where grain boundaries can exhibit thermally activated and anti-thermal temperature dependence as well as a diversity of atomic migration mechanisms. Most of these studies focus on examining the migration behaviors associated with the minimum energy structure of a grain boundary. Since it is known that grain boundaries can take on a range of atomic structures, we examine the diversity of behaviors associated with these metastable grain boundary configurations. Initial examination indicates that the behaviors fall into groups of structures that exhibit similar behaviors. We will present on these observations as well as the application of machine learning techniques to find the correlations between the grain boundary structures and their observed behaviors.