Refractory Metals 2023: Mechanical Behavior - Ultimate Plus
Sponsored by: TMS Structural Materials Division, TMS: Refractory Metals & Materials Committee
Program Organizers: Brady Butler, US Army Research Laboratory; Todd Leonhardt, Rhenium Alloys Inc.; Matthew Osborne, Global Advanced Metals; Zachary Levin, Los Alamos National Laboratory
Thursday 8:30 AM
March 23, 2023
Room: Aqua E
Location: Hilton
Session Chair: Todd Leonhardt, Rhenium Alloys Inc
8:30 AM
Mechanistic Models of the Inelastic Deformation of Refractory BCC Metals: Christopher Weinberger1; Anik Faisal1; Hunter Brumblay1; 1Colorado State University
In this talk, we will discuss our efforts in developing mechanistic models for the deformation of refractory body-centered-cubic metals integrating information from that atomistics to the continuum. Plasticity in BCC metals is known to exhibit strong temperature and moderate rate sensitivity in their flow stresses, and the dependence of flow stress on crystallographic orientation in single crystals deviates from the well-known Schmid’s law. In this presentation, we will describe atomistic simulations that predict the kink-pair activation enthalpy as a function of stress in that is able to accurately represent the strain rate and temperature dependent plasticity for single crystals. These results are integrated into a crystal plasticity finite element model that captures the deformation of polycrystals and ultimately are used to predict continuum yield surfaces. Similar strategies will be discussed regarding the role of deformation twinning and dislocation-crack interactions.
8:50 AM
Dislocation Emission from Crack Tips in Cr Studied by In-situ TEM: Daniel Kiener1; Michael Burtscher1; Inas Issa1; Klemens Schmuck1; Christoph Gammer2; Stefan Sandfeld3; 1University of Leoben; 2Erich Schmid Institute; 3Forschungszentrum Juelich
Despite their many favorable properties for high-performance applications in harsh environments, refractory metals commonly suffer from their inherent brittleness. Furthermore, when attempting to improve material strength by refining the microstructure into the ultra-fine grained regime, the failure mode commonly changes from transcrystalline to intercrystalline fracture. To circumvent this, a key factor is to reinforce weak grain boundaries by increasing grain boundary cohesion, e.g. using doping elements. In this work, crack tip processes in single crystal Cr and ultra-fine grained Cr, with and without C doping to strengthen grain boundaries, were studied using in-situ TEM nanomechanical testing. By deliberately placing the sharp crack tip in the grain interior or at the grain boundary, we examine differences with respect to local stress intensities required to nucleate dislocations from the crack tip in the grain compared to clean or doped grain boundaries and relate this to brittle cleavage of the interfaces.
9:10 AM
Insights on the Fatigue Crack Growth Behaviour of Rolled Tungsten for Its Application in Nuclear Fusion: Michael Pegritz1; Stefan Wurster1; Reinhard Pippan1; Anton Hohenwarter2; 1Erich Schid Institute of Materials Science of the Austrian Academy of Sciences; 2Montanuniversität Leoben
High heat flux materials in novel fusion reactors such as ITER or DEMO are exposed to remarkable cyclic thermal stresses. Therefore, knowledge of the fatigue properties in the fusion relevant temperature window is mandatory for the save operation and for lifetime calculations of structural components. Surprisingly, the fatigue crack growth behaviour of tungsten is barely investigated. This work provides first insights into the fatigue crack growth behaviour of rolled tungsten with ITER specification near the ductile to brittle transition temperature (DBTT) regime in high vacuum. A study on the temperature dependence of the crack growth behaviour above DBTT showed brittle fracture surface features to play a significant role up to 400°C. Therefore, different microstructures and respective crack orientations as well as different load ratios were investigated at 500°C. While a typical fatigue fracture surface for bcc-metals was observed at low load ratios, delamination-controlled fracture sets in for high load ratios.
9:30 AM
The Influence of Thermally Activated Dislocation Motion on the Brittle-ductile Transition of BCC Refractory Metals: Hunter Brumblay1; Tariqul Islam2; Gregory Thompson2; Christopher Weinberger1; 1Colorado State University; 2University of Alabama
Processing of refractory BCC metals like tungsten and molybdenum can be difficult due to their brittle nature at room temperature and thus require thermomechanical processing while others like tantalum are substantially more ductile. For example, tungsten has a brittle to ductile transition temperature (BDTT) near 500K while tantalum’s is below 70K. The origins of this discrepancy has not been fully elucidated, but could originate from either the intrinsic differences in dislocation mobility or impurities. In this talk, we examine the role thermally activated dislocations play in determining the BDTT in the BCC transition metals using a model of a crack coupled to discrete dislocation dynamics. These results provide insight into how dislocation mobility and intrinsic toughness influence the BDTT, shed light into whether impurities or intrinsic dislocation mobility control the BDTT, and may aid in refractory alloy design.
9:50 AM
Mechanical Properties of WMoFeNi Alloys during High Strain-rate Testing: Kerry Baker1; Riya Barua1; Zahidur Rahman1; T Balk1; 1University of Kentucky
Refractory high entropy alloys (RHEA) exhibit potential for various applications due to their high melting points, mechanical properties, and complex microstructure. Other studies have suggested that an alloy containing WMoFeNi undergoes adiabatic shear banding during high-speed testing. Further experiments are necessary to understand how the individual phases in WMoFeNi alloys deform at high strain rates. Nanoindentation was used to understand the small-scale mechanical behavior of these individual phases within a series of WMoFeNi alloys. In concert, Gleeble testing was used to measure the overall mechanical properties of the bulk alloy. Combined with detailed materials characterization, these results provide a deeper understanding of deformation in this complex alloy.
10:10 AM Break
10:25 AM
Stress Relaxation Behavior of Molybdenum Sheet: Alex Xie1; Gary Rozak2; Oliver Horst1; 1QSIL; 2H.C. Starck Solutions
Elevated temperature applications of pure Molybdenum (Mo) (BCC) sheet are limited by deformation that arise in long-term loading with high temperature cycles, for example, furnace applications. In this study, the stress relaxation mechanical deformation is characterized in pure Mo sheets for both powder metallurgy (PM) and arc casting consolidation followed by thermo-mechanical processing. A technique is developed for measuring deformation behavior via a “hot-grip” high temperature tensile test system at room up to 1350°C and stress levels up to 80% of the 0.2% off-set yield strengths at test temperature. The engineering strain rates are calculated and compared between the material types and testing conditions. Microstructures and surface conditions after testing are observed for guiding and understanding mechanism and directing future evaluation.
10:45 AM
Alloys of Chromium-Silicon Alloys with Iron and Nickel for Structural High Temperature Applications: Michael Kerbstadt1; Emma White1; Mathias Galetz1; 1DECHEMA-Forschungsinstitut
Cr-Si based-alloys are promising candidates due to their high melting point and good oxidation resistance. They are designed for working temperatures beyond Ni-base superalloys. The major drawbacks of Cr- based alloys are the embrittlement by Cr2N formation and high ductile to brittle transition temperatures (DBTT). This work investigates the effect of alloying Cr-Si-alloys with the elements Fe and Ni and targets the microstructure and precipitation of A15 phase. Varied compositions are manufactured by arc-melting and heat treatment at 1200°C, which enables controlled precipitation hardening by the Cr3Si-A15 phase.Ni can cause the effect of solution softening in Cr and increases the low-temperature ductility as a result. For Fe it is shown, that it stabilizes the two-phase structure consisting of nitration resistant A15 phase and Crss. Oxidation exposures at 1200°C in synthetic air indicate that Fe additions up to 5 at.% increase also the nitridation resistance of Crss.
11:05 AM
Strengthening in Mo-La Alloys at Temperatures from 1500 ⁰C to 1700 ⁰C: Monica Martinez Henriquez1; Gary Rozak2; Eric Taleff1; 1University of Texas at Austin; 2H.C. Starck Solutions
Lanthanated molybdenum (MoLa) alloys are commercially available and used at elevated temperatures, but limited data are available for the mechanical properties of MoLa alloys at high service temperatures. New data are presented for the strength and ductility of commercially-pure Mo and two MoLa alloys (1.0% and 0.6% La, by weight) at 1500 and 1700 ⁰C under constant true-strain rates of 10-4 and 3×10-5 s-1. The MoLa alloys retain greater strength at higher temperatures than pure Mo, although all the materials have a similar hardness at room temperature. Increasing La content decreases the propensity for recovery and recrystallization. Retained dislocation structures and fine grains in the MoLa alloys promote the formation of fine subgrains during elevated temperature deformation, which increase creep strength. The relationships between microstructure and strength at elevated temperatures are explored.