Metastable Phases and Phase Equilibria: Towards Designing the Next Generation of Alloys: Session II
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Phase Transformations Committee
Program Organizers: Bij-Na Kim; Rajarshi Banerjee, University of North Texas; Gregory Thompson, University of Alabama; Eric Lass, University of Tennessee-Knoxville; Mohsen Asle Zaeem, Colorado School of Mines; Mark Aindow, University of Connecticut; Peeyush Nandwana, Oak Ridge National Laboratory; Dinc Erdeniz, University of Cincinnati; Andrew Bobel, General Motors Corporation
Wednesday 2:00 PM
February 26, 2020
Room: 31A
Location: San Diego Convention Ctr
Session Chair: Mohsen Zaeem, Colorado School of Mines; Andrew Bobel, General Motors; Dinc Erdeniz, Marquette University
2:00 PM Invited
Deformation Mode, Strain Path, and Strain Rate Effects on Austenite to Martensite Transformation in Gen 3 Steels: Louis Hector1; 1General Motors R&D Laboratories
Gen 3 steels, which are typically multiphase, often contain metastable retained austenite which dynamically regulates work hardening to delay necking instability during plastic straining. This occurs via the transformation induced plasticity (TRIP) effect involving the diffusionless shear transformation of austenite to martensite. Material constitutive model development for Gen 3 steel forming and component performance simulations requires measurement of the austenite volume fraction evolution with deformation mode, strain path, and strain rate. This presentation will review recently developed experimental methods for measuring austenite transformation to martensite for selected Gen 3 steels. Focus will be on homogeneous vs. inhomogeneous transformation, transformation occurring in a complex stamping with multiple deformation modes and strain paths, and the effect of strain rate on austenite stability. Model calibration and validation requirements in an Integrated Computational Materials Engineering (ICME) Framework aimed at predicting and then making new Gen 3 steels with improved properties will also be discussed.
2:30 PM
Resetting Mechanical Property of 9Cr Steel by Segregation Engineering: Minseok Kim1; Sang Jun Kim1; Ji Won Kim2; Eun Soo Park1; 1Seoul National University; 2Case Western Reserve University
There is a limit to reducing greenhouse gas emissions through reduction of materials, although only when materials are reused it can be effectively reduced. Therefore, this study focused on developing a novel reusable alloy by restting the mechanical properties. We selected the Mn element as an alloying element considering the phase stability and segregation tendency in the 9Cr steel, which is widely used as a structural material. After homogenization and air cooling, the steel is fully martensite structure. In the subsequent tempering process, partial austenitization occurs by the Mn element, which has been segregated to the grain boundary and lower the austenitization temperature. Finally, we find out that these meteastable austenites exhibit TRIP behavior. As a result, the strength of the 9Cr steel greatly increased with no significant reduction in elongation. Furthermore, we designed a heat treatment process for resetting mechanical properties by controling the reversible local phase transformation.
2:50 PM
Deformation Mechanisms in Metastable Fcc Alloys: Mulaine Shih1; Maryam Ghazisaeidi1; 1Ohio State University
We study the concept of average versus "local" stacking fault energy (SFE), and their fundamental role in dislocation behavior in fcc alloys. Recent studies in high entropy alloys (HEAs) have revealed the existence of composition-dependent metastable phases. Transitioning from fcc to hcp stable phases in fcc alloys correspond to a change from positive to negative average values for the SFE. The goal of this work is to understand the different deformation mechanisms at the atomic-scale as a function of composition, while the average SFE value changes sign. We perform atomistic simulations on random NiCo alloys as a model system to investigate the dislocation behavior in positive/negative SFE materials. Using molecular dynamics simulations, we show how deformation mechanisms vary as a function of concentration, temperature and applied stress. Our results provide a fundamental understanding of how changes in composition can tune deformation mechanisms, and consequently mechanical behavior in fcc solid solutions.
3:10 PM Invited
Modeling of Metastable Phase Formation for Sputtered Ti1-xAlxN Thin Films: Sida Liu1; Keke Chang2; Stanislav Mráz1; Xiang Chen1; Marcus Hans1; Denis Music1; Daniel Primetzhofer3; Jochen Schneider1; 1RWTH Aachen University; 2RWTH Aachen University; NIMTE, Chinese Academy of Sciences; 3Uppsala University
Metastable titanium aluminum nitride coatings are widely applied in cutting and forming applications. Although it is generally accepted that the phase formation of metastable TiAlN is governed by kinetic factors, modeling attempts today are based solely on energetics. In this work, the metastable phaseformation of TiAlN is predicted based on one combinatorial magnetron sputtering experiment, the activation energy for surface diffusion, the critical diffusion distance, as well as thermodynamic calculations. The phase formation data obtained from further combinatorial growth experiments varying chemical composition, deposition temperature, and deposition rate are in good agreement with the model. Furthermore, it is demonstrated that a significant extension of the predicted critical solubility range is enabled by taking kinetic factors into account. Explicit consideration of kinetics extends the Al solubility limit to lower values, previously unobtainable by energetics, but accessible experimentally.
3:40 PM Break
4:00 PM
Thermal Decomposition of Quasicrystals in Powder-processed Icosahedral-phase-strengthened Aluminum Alloys: Hannah Leonard1; Sarshad Rommel1; Mingxuan Li1; Thomas Watson2; Tod Policandriotes3; Mark Aindow1; 1University of Connecticut; 2Pratt & Whitney; 3Collins Aerospace
Recently, we have developed a series of Al-Cr-Mn-Co-Zr alloys that exhibit a nano-composite FCC Al plus I-phase microstructure in gas-atomized powders. The I-phase dispersoids exhibited a variation in microstructures and distribution that depended on the powder particle size (hence cooling rate) and the alloy composition. This microstructure is retained during consolidation of the powder to form bulk materials or cold-sprayed coatings, and the materials exhibit remarkable mechanical properties. Here we report a study on the thermal stability and decomposition of the I-phase in this Al alloy. A series of isothermal heat treatments were performed to determine the conditions under which the quasicrystalline phase decomposes. In-situ TEM heating experiments were performed with a MEMS-based heating holder on specimens prepared using a FIB technique from individual powder particles and from consolidated material. Experiments were conducted to investigate the transformation mechanisms for each of the different microstructures.
4:20 PM
Pseudo-in situ Characterization of Phase Transformation in an Al-Cu-Mn-Zr Alloy using Atom Probe Tomography: Bharat Gwalani1; Elizabeth Kautz1; Amit Shyam2; Jonathan Poplawsky2; Arun Devaraj1; 1Pacific Northwest National Laboratory; 2Oak Ridge National Laboratory
Aluminum (Al) alloys with improved high-temperature mechanical properties (up to 350oC) are needed to enable the next generation of higher efficiency, affordable vehicle engines. A significant challenge with traditional Al alloys is that at high temperatures the metastable, semi-coherent θ’(Al2Cu) transforms to θ(Al2Cu), the presence of which deteriorates mechanical properties. The compositionally optimized Al-Cu-Mn-Zr (ACMZ) alloys can be used at >325oC by utilizing an enveloping co-precipitation around θ’(Al2Cu) precipitates to extend the θ’ metastability. These co-precipitates could underpin the development of higher performance alloys (375-400oC). The current study focuses on characterization of coherent and incoherent interfacial changes, both compositional and structural, at fcc(matrix)- phase interfaces while aging. To have a detailed understanding of the coprecipitation mechanism, a novel in situ-atom probe tomography (APT) technique coupled with transmission electron microscopy (TEM) were used to probe early stage aging at 300 oC in an ultra-high vacuum environment.
4:40 PM
Harnessing the Stability of 𝜃ʹ-Al2Cu at Unprecedented High Temperatures: Dongwon Shin1; Amit Shyam1; Larry Allard1; Matthew Chisholm1; Jon Poplawsky1; J. Haynes1; 1Oak Ridge National Laboratory
Oak Ridge National Laboratory recently developed a family of cast Al-Cu alloys that can withstand up to 350°C (>200 hours) without appreciably losing mechanical strength. Extensive characterization has revealed that engineering the Al/𝜃′-Al2Cu interface via microalloying is the key to stabilizing this metastable strengthening phase at unprecedented high-temperatures. Inspired by this stimulating observation, we have performed massively parallelized first-principles density functional theory calculations (DFT) with modern supercomputing to further investigate the underlying mechanism. We present DFT databases of solute segregation energy at the interface between the aluminum matrix and 𝜃′-Al2Cu, and partitioning of solutes within 𝜃′. We also report Al/𝜃′ interfacial energies at under-investigated orientation obtained from DFT calculations of the experimentally observed structures. The research was sponsored by the LDRD Program of Oak Ridge National Laboratory and the Department of Energy, Vehicle Technologies Office, Propulsion Materials Program.
5:00 PM
Formation of Metastable Spiral Patterns during Directional Eutectic Solidification: Saman Moniri1; Ashwin Shahani1; 1University of Michigan
The recent discovery of spiral patterns in the Al-Ag-Cu ternary eutectic system has excited much interest in the solidification science community. Herein we report on a different class of eutectic spiral — one that is metastable, two-phase, and faceted — in the Zn-Mg alloy system. To better understand the emergence of such structures from a parent liquid phase, we have conducted a multimodal investigation using X-ray nanotomography, 3D electron backscatter diffraction, and further electron microscopy. Our correlative imaging workflow provides new insights into the complex morphology, crystallography, and underlying growth mechanism of the faceted eutectic spirals. It is anticipated that these results will provide the necessary benchmark data for simulations (e.g., phase field) of multiphase solidification patterns.