Refractory Metals 2020: Nb Processing and Applications; Mechanical Behaviors of Refractory Metals
Sponsored by: TMS Structural Materials Division, TMS: Refractory Metals Committee
Program Organizers: Eric Taleff, University of Texas at Austin; Gary Rozak, H.C. Starck Solutions; Todd Leonhardt, Rhenium Alloys Inc.
Monday 8:00 AM
February 24, 2020
Room: Cardiff
Location: Marriott Marquis Hotel
Session Chair: Eric Taleff, University of Texas at Austin; Gary Rozak, H.C. Starck Inc.
8:00 AM
Effect of Dislocations and Grain Boundaries on Magnetic Flux Trapping in High-purity Niobium used for Superconducting Radio Frequency Cavities: Mingmin Wang1; Shreyas Balachandran2; Santosh Chetri2; Anatolii Polyanskii2; Peter Lee2; Thomas Bieler1; 1Michigan State University; 2National High Magnetic Field Laboratory
High-purity niobium is used for superconducting radio-frequency (SRF) cavities for particle accelerators with potential applications from water treatment to materials science. Trapped magnetic flux due to dislocations and grain boundaries in niobium can degrade the performance of SRF cavities, and hence is a scientific topic of growing interest. However, it is still unclear what defects actually trap magnetic flux, and which processing paths can minimize defects and ensure consistently high cavity performance. To investigate the mechanism of flux trapping by dislocations and grain boundaries, bi-crystal niobium samples with strategically chosen tensile axes were strained to about 5% to introduce dislocations. Electron channeling contrast imaging (ECCI) was used to characterize dislocation structures before and after the deformation, followed by magnetic flux observations at superconducting temperatures using magneto-optical (MO) imaging. The effect of dislocation density on flux trapping and conditions that increase the tendency to trap flux are discussed.
8:20 AM
The Influence of Dislocation Density on Thermal Conductivity in Pure Niobium from 2-9K: Peng Xu1; Thomas Bieler1; Neil Wright1; 1Michigan State University
To be announced
8:40 AM
Effect of Strain Rate on the Anisotropic Tensile Mechanical Properties of High Purity Niobium Single Crystals: Jean-Francois Croteau1; Eureka Pai Kulyadi2; Chaitanya Kale3; Di Kang2; Derek Siu4; Thomas Bieler2; Philip Eisenlohr2; Kiran Solanki3; Elisa Cantergiani1; Nicolas Jacques5; Daniel Balint4; Paul Hooper4; Said Atieh6; 1I-Cube Research; 2Michigan State University; 3Arizona State University; 4Imperial College; 5ENSTA Bretagne; 6CERN
An investigation of the mechanical properties of differently oriented high-purity niobium single crystals deformed in tension at strain rates between 10-4 to 103 s-1 is presented. Specimens were cut from a large grain niobium disk used for the manufacturing of SRF cavities. The strain rate sensitivity and the associated activation volume are presented for a specific orientation. Different crystallographic tension directions exhibit significantly different softening and hardening behaviors and elongation at fracture. Such anisotropy is reduced at high strain rates. The effect of early necking, adiabatic heating and the activation of multiple slip systems on the tensile split Hopkinson results is discussed. An attempt is made to explain the differences in flow stress based on crystal orientation evolution during deformation.
9:00 AM
Static and Dynamic Grain Growth in Niobium at 1200 to 1500℃: Emily Brady1; Eric Taleff1; 1University of Texas at Austin
Static and dynamic grain growth are investigated at 1200 to 1500°C in Type 1 (ASTM B393-18) niobium sheet material. Static grain growth occurs during annealing without concurrent plastic straining, while dynamic growth occurs during plastic straining at elevated temperature. Uniaxial tension tests at 10-4 s-1 and faster were used to measure flow and dynamic grain growth behaviors. Grain size and shape were characterized after static annealing (static case) and after tensile testing (dynamic case). Microstructures produced by different temperatures and strain rates are reported with the associated flow behaviors. Grain growth characteristics are contrasted between the static and dynamic cases.
9:20 AM Break
9:40 AM
Dynamic Grain Growth in Refractory Metals: Eric Taleff1; 1University of Texas at Austin
Dynamic grain growth (DGG) has been observed in body-centered-cubic metals, including refractory metals. Dynamic grain growth occurs during concurrent plastic deformation at elevated temperatures, a feature that distinguishes it from static grain growth, which occurs during static annealing. Both normal and abnormal forms of dynamic grain growth occur. Dynamic abnormal grain growth (DAGG) was observed to create large grains in Mo and Ta, with some instances in Mo producing single crystals several centimeters long. Recent microstructural studies of BCC Fe and Mo materials suggest a mechanism by which dynamic grain growth may occur, a mechanism involving subgrains formed during deformation. Microstructural data and a theoretical basis for this possible mechanism will be presented and discussed.
10:00 AM
Molybdenum Foil Tensile Testing: Brandon Kenny1; Gary Rozak2; 1Miami University; 2H.C. Starck Inc
The tensile properties of molybdenum (Mo) foil (<0.13 mm thick) are sparsely documented in open literature. Older tensile data may not reflect the current commercially pure Mo because refining methods are improving purity levels over time. Additionally, the strength of Mo is affected by the thermo-mechanical processing history. This presentation examines the tensile properties of molybdenum foil in various conditions with testing guided by ASTM protocol. Mo foil is prepared with different heat treat conditions, various sample preparation techniques, geometry, and test variables. The effects of processing and testing variables are evaluated via tensile properties, stress-strain curves, and fracture surfaces for a variety of test conditions. Molybdenum foil testing, results, and applications will be covered extensively throughout the presentation.
10:20 AM
Thermally Activated Deformation Processes in W-Re Alloys: Verena Maier-Kiener1; Johann Kappacher1; Daniel Kiener1; Helmut Clemens1; 1Montanuniversität Leoben
In this study, we investigate the influence of Re on mechanical properties in W-alloys by combining static heat treatments and high resolution imaging and in-situ high temperature nanoindentation experiments.to Three different alloy compositions were tested, commercially pure W as well as W5Re and W10Re. The alloys were examined in two different microstructural conditions, recrystallized coarse grained as well as ultra-fine grained. The latter one was achieved through high pressure torsion. Nanoindentation experiments were conducted from ambient temperatures up to 800°C in situ in a scannine electron microscope, with temperature increments of 100°C.In the coarse grained condition alloy softening was experimentally confirmed for the Re containing alloys, stemming from a reduced Peierl’s potential that was experimentally confirmed . The thermal stability of the ultra-fine grained microstructure could be increased through Re-alloying, which was evidenced from the nanoindentation experiments as well as additional static heat treatment experiments paired with EBSD measurements.
10:40 AM Cancelled
Mechanism of Hardening and Damage Initiation in Oxygen Embrittlement of Body-Centred-Cubic Niobium: Weizhong Han1; Ping-Jiong Yang1; 1Xi'an Jiaotong University
Body-centred-cubic metallic materials, such as niobium (Nb) and other refractory metals, are prone to embrittlement due to low levels of oxygen solutes. The mechanisms responsible for the oxygen-induced rampant hardening and damage are unclear. Here we illustrate that screw dislocations moving through a random repulsive force field imposed by impurity oxygen interstitials readily form cross-kinks and emit excess vacancies in Nb. The vacancies bind strongly with oxygen and screw dislocation in a three-body fashion, rendering dislocation motion difficult and hence pronounced dislocation storage and hardening. While self-interstitials anneal out fast during plastic flow, the vacancy-oxygen complexes are stable against passing dislocations. The debris in fact amplify the random force field, facilitating the generation of even more defects in a self-reinforcing loop. This leads to unusually high strain hardening rates and fast breeding of nano-cavities that underlie damage and failure. Ref. Acta Mater-2019-168-331.