Advances in Multi-Principal Element Alloys II: Thermal and Other Properties
Sponsored by: TMS Structural Materials Division, TMS Functional Materials Division, TMS: Mechanical Behavior of Materials Committee, TMS: Alloy Phases Committee
Program Organizers: Peter Liaw, University of Tennessee; Michael Gao, National Energy Technology Laboratory; E-Wen Huang, National Yang Ming Chiao Tung University; Jennifer Carter, Case Western Reserve University; Srivatsan Tirumalai; Xie Xie, FCA US LLC; James Brechtl, Oak Ridge National Laboratory; Gongyao Wang, Globus Medical

Thursday 2:00 PM
March 23, 2023
Room: Aqua D
Location: Hilton

Session Chair: Zhenzhen Yu, Colorado School of Mines; Elizabeth Opila, University of Virginia

2:00 PM  Invited
Phase Transformation Pathways in Compositionally-Complex BCC-B2 Alloys: Eric Lass¹; ¹University of Tennessee-Knoxville
    Refractory-based superalloys (RSAs) containing a BCC+B2 two-phase microstructure offer great potential for future high-temperature applications, but several issues currently hinder their further development. Among them, the higher-order BCC-B2 phase transition leads to undesirable microstructures. The present work details the underlying thermodynamics governing the BCC-B2 phase transition and the multitude of pathways available for the BCC to BCC+B2 transformation. Microstructural evolution in BCC-B2 RSAs parallels that in binary alloys such as Fe-Al, but the added compositional complexity opens new transformation pathways and may improve B2 phase stability. Previously reported RSAs are explained using a modeling framework constructed by combining modern computational thermodynamics with classic phase transformations theory, and new alloys are identified in previously uninvestigated systems. The ideas and discussion herein offer insight into the thermodynamics of microstructure development in RSAs and provide tools and guidance for future research in this promising class of materials.

2:20 PM  Invited
Multi-Principal Element Alloy Fillers to Mitigate Weldability and Joining Issues: Zhenzhen Yu¹; Benjamin Schneiderman¹; Abdelrahman Abdelmotagaly¹; ¹Colorado School of Mines
    Despite recent technological advances in manufacturing, a cross-cutting challenge remains in metallurgically joining of similar and dissimilar materials with poor weldability or incompatibility. Multi-principal element alloy (MPEA) fillers have demonstrated potential to meet these challenges, due to the stability of ductile solid-solution phases across a broad composition space. A rigorous down-selection strategy to identify MPEA compositions for particular applications has been developed, employing a suite of high-throughput computational techniques. A critical design consideration for the filler is tolerance to base material elements introduced during joining. In feasibility assessments, both initial MPEA filler compositions and prospective resultant compositions following joining must be evaluated. In this presentation, development of MPEAs as solutions for a variety of welding challenges will be discussed, such as liquid metal embrittlement in welding of Zn-coated automotive steels, high cracking susceptibility in repaired Ni-base superalloys, and joining of incompatible materials such titanium to stainless steel will be discussed.

2:40 PM  Invited
Impact of Ti on Phase Evolution and Oxidation Mechanisms within TiAlTa Alloys: Jaimie Tiley¹; Yanbo Wang²; Soumya Nag¹; Ercan Cakmak¹; Raymond Unocic¹; Pania Newell²; ¹Oak Ridge National Laboratory; ²University of Utah
    Meta-stable and heterogenous structures within refractory complex concentrated alloys (RCCAs) provide creative new high temperature materials for extreme environments. One approach involves the development of high temperature B2 phases in BCC RCCAs using Ti and Al additions. Although high temperature stability has been demonstrated in several RCCA material with these elements, there impact on oxidation mechanisms remained largely unknown. This work investigates the impact of Ti composition on phase evolution and oxidation mechanisms in TiAlTa alloys, aimed at understanding the potential for these elements in RCCAs. TiAlTa, 50Ti25Al25Ta, and 70Ti15Al15Ta (atomic %) samples were mechanically tested at room temperature, 250C, 500C, and 750C (in atmosphere) to study oxide film formation and related impacts on strength. TEM and XPS analysis was conducted at ORNL’s Center for Nanophase Materials Science user facility, while SEM, indentation and oxidation studies were conducted at the University of Utah.

3:00 PM  Invited
Processing of Refractory Multi-Principal Element Alloys for Ultrahigh Temperature Performance: Amy Clarke¹; Benjamin Ellyson¹; Adira Balzac¹; Nathan Peterson¹; William Waliser¹; Nelson Delfino de Campos Neto¹; Megan Le Corre¹; Abigail Miklas¹; Jonah Klemm-Toole¹; Francisco Coury²; Kester Clarke¹; Michael Kaufman¹; ¹Colorado School of Mines; ²Federal University of São Carlos
    Refractory multi-principal element alloys (RMPEAs) hold the promise to withstand continuous operation at ultrahigh temperatures (above 1200 °C). The majority of these alloys are single phase solid solutions with a body centered cubic structure, although multiphase microstructures may also hold some potential for ultrahigh temperature performance, especially for short-use and/or expendable environments (below 1200 °C). That said, processing of RMPEAs remains challenging, given their strong temperature dependence at low and high temperatures, potential sensitivity to impurities, and propensity to form heterogeneous microstructural characteristics. Here we highlight recent efforts to design and develop RMPEAs by thermodynamic, kinetic, and solid solution strengthening modeling and to tailor their microstructures by thermomechanical processing or additive manufacturing. These results will inform the development of processing maps and be used to understand and control RMPEA microstructural evolution and characteristics important to ultrahigh temperature performance.

3:20 PM  Invited
Oxidation of Group IV-V Refractory Multi-principal Element Alloys: Charlotte Brandenburg¹; David Beaudry²; Michael Waters³; Loïc Perrière⁴; Jean-Philippe Couzinie⁴; James Rondinelli³; Mitra Taheri²; Elizabeth Opila¹; ¹University of Virginia; ²Johns Hopkins University; ³Northwestern University; ⁴Univ Paris Est Creteil, CNRS, ICMPE
    Refractory multi-principal element alloys composed of Group IV (Ti,Zr,Hf) and Group V (Nb,Ta) metals are of interest for their potential high temperature capability, however, oxidation rates are rapid. Additions of common protective oxide formers Al, Cr, and Si lead to alloy embrittlement when added in sufficient amounts for oxidation resistance, thus other methods of increasing oxidation resistance are sought. In this work, the oxidation kinetics of equimolar NbTiZr and HfNbTaTiZr alloys are investigated using thermogravimetric analysis at temperatures between 900 and 1200°C in 1% oxygen (balance argon) for times up to 20 hours to understand fundamental oxidation mechanisms. Oxygen solubility varies in the alloy constituents resulting in phase separation into Group IV- and Group V-rich regions as interstitial oxygen content increases. X-ray diffraction analysis of the external oxide scales reveals complex oxides such as TiNb₂O₇ and Zr₆Nb₂O₁₇ form. Compositional and/or microstructural explanations for observed reductions in oxidation rates are described.

3:40 PM Break

4:00 PM  Invited
Attainments and Challenges of High Temperature Oxidation Resistance of Refractory High Entropy Alloys: Literature Review and Own Results: Bronislava Gorr¹; Steven Schellert²; Hans Juergen Christ²; Stephan Laube¹; Alexander Kauffmann¹; Martin Heilmaier¹; ¹Karlsruhe Institut für Technologie (KIT); ²Universität Siegen
    Refractory High Entropy Alloys (RHEA) are considered novel promising high temperature materials for structural applications at ultra-high temperatures primarily due to their attractive mechanical properties. While many RHEA suffer from poor oxidation resistance similar to that of pure refractory metals, some RHEA exhibit very good protectiveness which is attributed to the formation of either well-known protective scales such as α-Al2O3, or rarely encountered complex oxides such as CrTa-based oxides. In this contribution, the currently available literature on high temperature oxidation behavior of RHEA is reviewed with respect to the oxidation kinetics as well as oxide scale growth and constitution. In addition, own results on the formation and growth of complex CrTa-based oxides, which exhibit high thermodynamic stability and slow growth kinetics, are presented. Different types of oxidation mechanisms typical of RHEA are suggested.

4:20 PM  Invited
In-situ Investigation of the Initial Oxidation Steps in Refractory High-entropy Alloys by O₂ Gas Exposure: Heath Kersell¹; Xuesong Fan²; Alexander Herman¹; Zongyang Lyu²; Baldur Steingrimsson³; Peter Liaw²; Gregory Herman¹; ¹Oregon State University; ²The University of Tennessee; ³Imagars LLC
     Ti alloys find applications in biomedical implants and as high-temperature engine- and aerospace-components due to their low weight, corrosion resistance, low Young’s modulus, and tunable high-temperature strength and ductility. Recently, decreasing the Nb content in TiZrHfNb_x alloys was shown to produce enhanced wear-resistance. Using ambient pressure X-ray photoelectron spectroscopy (AP-XPS), we elucidate the initial oxidation stages in TiZrHfNb_0.3 refractory high-entropy alloys (RHEAs).Ar⁺ ion sputtering revealed metal-carbide species in the as-cast alloy subsurface. Subsequent vacuum heating enriched the near-surface carbide content while the relative Ti, Zr, Hf, and Nb concentrations remained stable. In contrast, stepwise O₂ exposures at 10^-5 mbar suppressed the carbide content and induced surface oxidation and enrichment of Hf and Zr. We characterize the initial oxidation kinetics in this regime and demonstrate the limiting near-surface composition as a function of depth while higher O₂ pressures induce the formation of a protective oxide layer on TiZrHfNb_0.3 surfaces.

4:40 PM  Invited
Synergistic Discontinuous Reactions Leading to Nano-lamellar Hierarchical Microstructures in High Entropy Alloys: Sriswaroop Dasari¹; Abhishek Sharma¹; Rajarshi Banerjee¹; ¹University of North Texas
    High entropy alloys (HEAs) often exhibit a wide range of interaction energies (ordering and clustering) due to the presence of multiple principal elements. The local interactions consequently lead to novel phase transformation pathways, typically not observed in conventional alloys. The current study reports a novel hierarchical nano-lamellar microstructure in Al0.3TixCoFeNi type HEAs. While the parent high-temperature phase is FCC in these alloys, isothermal annealing at 600°C leads to a complex microstructure, resulting from synergistic discontinuous reactions i.e., FCC to BCC+B2 (eutectoid) and FCC to FCC+L12 (discontinuous precipitation). These reactions aid each other in forming a hierarchical pearlite-like lamellar microstructure with 50-100 nm thick, alternating FCC+L12 / BCC+B2 lamellae. The relatively sluggish discontinuous reactions emanate from recrystallized FCC grain boundaries, eventually consuming the entire parent grain. This nano-lamellar microstructure in Al0.3CoFeNi exhibits tensile yield stress of 1074 MPa and elongation nearing 10%.

5:00 PM  Invited
Phase Formation in Compositionally Complex Alloy Thin Films: The Role of "Small" and "Large" Elements: Andrea Hodge¹; Daniel Goodelman¹; ¹University of Southern California
    Discrepancies in phase formation between bulk and thin film studies for high entropy systems are attributed to growth mechanisms, grain size, process temperature, and rapid diffusion during synthesis. For example, film synthesis features rapid cooling rates, which can restrict diffusion and minimize the nucleation and growth of intermetallic compounds in comparison to bulk. In this context, we aim to highlight the effect of changing elemental compositions on the crystallography of sputtered compositionally complex alloys films. This technique allows for easily tunable conditions such as operating power, working pressure, and working gas among others that influence the composition of each of the alloying elements present. Results indicate that there is a transition from BCC to FCC to amorphous crystallographic structures when altering the composition of each of the alloying elements.