HEA 2023: Characterization of HEAs II
Program Organizers: Andrew Detor, DARPA/DSO; Amy Clarke, Los Alamos National Laboratory
Monday 1:40 PM
November 13, 2023
Room: Riverboat
Location: Omni William Penn
Session Chair: Karin Ratschbacher, GfE Metals and Materials GmbH
1:40 PM Introductory Comments
1:45 PM Invited
Achieving Exceptional Mechanical Properties in High Entropy Alloys via Thermodynamically Guided Local Chemical Ordering: Sriswaroop Dasari1; Abhishek Sharma1; Chao Jiang2; Bharat Gwalani3; Stephane Gorsse4; An-Chou Yeh5; Srinivasan Srivilliputhur1; Rajarshi Banerjee1; 1University of North Texas; 2Idaho National Laboratory; 3North Carolina State University; 4University of Bordeaux; 5National Tsing Hua University
Understanding the local chemical ordering propensity in random solid solutions, and tailoring its strength, can guide the design and discovery of complex, paradigm-shifting multi-component alloys. A simple thermodynamic framework was employed, based solely on binary enthalpies of mixing, to select optimal alloying elements to control the nature and extent of chemical ordering in high entropy alloys (HEAs). Subsequently, high resolution electron microscopy, atom probe tomography, and computational modeling have been coupled to demonstrate how controlled additions of Al and Ti and subsequent annealing drive chemical ordering (from short-range ordered domains to long-range ordered precipitates) in nearly random equiatomic FCC CoFeNi solid solution, boosting its tensile yield strength by a factor of four while also substantially improving ductility, which breaks the so-called strength-ductility paradox. The generality of this approach was demonstrated via addition of Al for introducing chemical ordering and enhancing mechanical properties in another nearly random BCC refractory NbTaTi HEA.
2:15 PM
Development of Refractory Metal-based CCAs with Improved Mechanical Properties: Stephan Laube1; Steven Schellert2; Alexander Kauffmann1; Bronislava Gorr1; Yolita Eggeler1; Hans-Juergen Christ2; Martin Heilmaier1; 1KIT Karlsruhe; 2University of Siegen
Refractory compositionally complex alloys (RCCA) are promising candidates for high-temperature structural applications. To achieve good mechanical performance at elevated temperatures, the proper formation of a strengthening phase is crucial. In order to mimic the microstructures of precipitation-strengthened Ni-based superalloys, we developed an A2 matrix alloy, 82(TaMoTi)-8Cr-10Al (at.%) with B2 particles that are formed by a precipitation reaction, namely by diffusion-controlled nucleation and growth. The Al content is crucial to obtain the proper reaction sequence. By adjusting Al concentration, the ordering temperature can be tailored to be congruent with the maximum temperature of the two-phase field as a first prerequisite. In order to reveal the distinct nature of phase separation sequence, i.e. nucleation and growth, atom probe tomography and electron microscopy techniques were utilized on samples that were annealed over orders of magnitude in time at temperatures beyond 800°C and the impact of coherent particle strengthening on mechanical properties is discussed.
2:35 PM
Microstructures and Properties of AlCrFeMnV, AlCrFeTiV, and AlCrMnTiV High-entropy Alloys: Keith Knipling1; Patrick Callahan1; David Beaudry2; 1Naval Research Laboratory; 2Johns Hopkins University
A series of high-entropy alloys (HEAs) containing AlCrFeMnV, AlCrFeTiV, and AlCrMnTiV have been designed using a combination of thermodynamic prediction by Thermo-Calc and by experimental observation of the microstructure and phases present in arc-melted alloys. These alloys are predominantly BCC, with some alloys forming additional minor phases. A particularly intriguing microstructure is observed in the AlCrMnTiV alloy, which contains a high number density of ∼50 nm ordered B2 cuboids that are coherent with the BCC matrix, resembling the well-known γ-γ' microstructures in Ni-based superalloys but in a BCC system. We will correlate the alloy microstructures, observed using a combination of X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM/TEM), and atom-probe tomography (APT), to the alloys’ mechanical properties measured by Vickers microhardness and nanoindentation.
2:55 PM
Accelerating EXAFS Analysis of SRO in HEAs: Challenges and Opportunities: Howard Joress1; Elaf Anber2; Bruce Ravel1; Jonathan Hollenbach2; Mitra Taheri2; Brian DeCost1; 1NIST; 2Johns Hopkins
It has become clear that most HEAs are not completely disordered, with certain elements having a proclivity for bonding. Further, this short-range order can have a large effect on properties. Extended x-ray absorption fine structure (EXAFS) is a powerful tool for characterizing this disorder, however challenges exist in analysis of the data. In this talk we will describe in detail the difficulties inherent to applying EXAFS to HEAs, including low Z contrast, secondary phases, long-range order, and the large number of variables involved in the fits. We will also describe our efforts to accelerate EXAFS mapping and quantitative data analysis. To speed the acquisition of EXAFS data we demonstrate the use of on-the-fly unsupervised learning to select subsequent samples for measurement. We then demonstrate the use of modern statistical methods, including bayesian inference to quantify the short-range order.
3:15 PM Break
3:35 PM
Development of Coherent Ru-based BCC + B2 Alloys with High Thermal Stability: Carolina Frey1; Anthony Botros1; Sebastian Kube1; Haojun You1; Tresa Pollock1; 1University of California Santa Barbara
Refractory Multi-Principal Element Alloys (RMPEAs) are a new class of structural alloys for extreme environments with the potential to push operating temperatures above 1200 °C. However, the development of alloys that can operate at desired temperatures of 1300-1400 °C has been limited by a lack of coherent strengthening precipitates that can persist at the target temperatures. For this investigation, solution and ageable BCC + B2 alloys have been developed with Ru-based B2 phases that are coherent and stable to 1300-1900 °C. Relationships between the different elemental additions, B2 solvus temperatures, B2 volume fractions, and B2 compositions were determined via annealing studies and subsequent SEM investigations. The effect of aging on alloy hardness and precipitate morphology are also presented. Consistent with precipitate evolution in nickel superalloys, the morphologies of the coherent Ru-B2 precipitates evolved from spherical to cuboidal to cuboidal arrays with increasing aging time.
3:55 PM
Direct Determination of Short-range Order in Materials Using Spatial Statistics: Michael Xu1; Shaolou Wei1; Cemal Tasan1; James LeBeau1; 1Massachusetts Institute of Technology
While short-range order (SRO) in chemistry and structure can have significant impact on material properties, capturing their distribution at the nanoscale can be inherently difficult. Scanning transmission electron microscopy (STEM) offers a local probe of material chemistry and structure, yet projection of the three-dimensional structure being imaged presents its own challenges in distinguishing SRO from local random fluctuations. Using a combination of STEM and Geographic Information Systems (GIS), we demonstrate first how incorporating spatial statistics tools can enable quantification of SRO at the atomic scale. We will consider a case study oxide material to validate the approach and extend it to measure non-random SRO in the refractory high entropy alloy TiVNbHf. Comparing experiment with null-hypothesis random and ordered structures shows that chemical order exists at extremely local length scales (<1 nm). Finally, the addition of Al is demonstrated to enhance the degree of SRO as well.
4:15 PM
Phase Stability in Refractory High Entropy Alloys: Vishal Soni1; SriSwaroop Dasari1; Abhishek Sharma1; Advika Chesetti1; Oleg Senkov2; Daniel Miracle3; Rajarshi Banerjee1; 1University of North Texas; 2MRL Materials Resources LLC; 3Air Force Research Laboratory
Al-containing refractory high entropy alloys or complex concentrated alloys (RHEAs or RCCAs) often exhibit a patterned BCC+B2 microstructure, resembling the FCC+L12 microstructure typically observed in Ni/Co base superalloys. There is a rapidly growing interest in these BCC+B2 microstructures, especially due to their promising balance of elevated temperature mechanical properties. Unfortunately, such BCC+B2 microstructures are often reported to be unstable and upon long term annealing, results in transformation of the B2 phase to ordered omega phase which is detrimental to mechanical properties. The current study focuses on investigating the stability of BCC+B2 microstructure and the detrimental ordered omega phase in several Al-containing RHEAs at elevated temperatures (600C-1400C). The results are compared with CALPHAD based predictions to understand the stability of such phases and thus tune the alloy compositions to get elevated temperature stability of the BCC+B2 and at the same time, avoid the formation of undesired phases.
4:35 PM
Defect Mediated Microstructural Evolution and Phase Transformations in a BCC Based Al0.5NbTa0.8Ti1.5V0.2Zr Refractory High Entropy Alloy: Abhishek Sharma1; Advika Chesetti1; Tirthesh Ingale1; Vishal Soni1; Hamish Fraser2; Stéphane Gorsse3; Rajarshi Banerjee1; 1University of North Texas; 2The Ohio State University; 3Université de Bordeaux
This study focuses on how prior deformation influences the phase transformation pathway and the resultant microstructural evolution in the BCC-based Al0.5NbTa0.8Ti1.5V0.2Zr refractory high entropy alloy (RHEA). The solutionized alloy was cold-rolled prior to annealing, which resulted in an extremely refined HCP-based ordered omega phase dispersed within a disordered BCC matrix. This microstructure is in contrast to the B2+BCC microstructure that forms when this alloy is annealed at the same temperature, a transformation pathway which has been previously reported. Presence of deformation substructure consisting of dislocations, twins, etc. provides a high number density of heterogenous nucleation sites distributed within the matrix resulting in a lower nucleation barrier for crystallographically mismatched phases such as the hexagonal ordered omega phase. The compositional and structural changes associated with the ordered omega precipitation, were characterized in detail by TEM and APT, and provide insights for microstructural design of RHEAs.