Presidential History

News release announces the Presidential History of the University of Dayton

CRYSTAL ORIENTATION EFFECTS DURING FABRICATION OF SINGLE OR MULTI-CRYSTAL NB SRF CAVITIES*

Abstract Single and large-grain Nb SRF cavities are of interest due to possible reduction of cost and problems associated with inconsistent texture and surface finish among batches of rolled polycrystalline Nb sheet. The effect of crystal orientation on dislocation density, surface quality, and recrystallization after plastic deformation and e-beam welding was investigated, as understanding of their interrelations is needed. These were evaluated for three samples of different orientations at steps similar to those in typical cavity forming, with deformation modeled using a crystal plasticity approach. Initial dislocation density was higher than expected, increased with deformation, after welding was reduced in recovered areas, and was similar to initial density in recrystallized grains; there was also evidence that Nb has a higher tolerance for dislocations than other metals. Surface quality depends on a complex relation of crystal orientation, slip system activity, and prior surface treatment. Recrystallization nucleated outside the melt pool, and the new orientations grew both epitaxially into the weld as it solidified, and away until heat and time were insufficient to continue growth

Effect of strain rate on tensile mechanical properties of high-purity niobium single crystals for SRF applications

An investigation of the mechanical properties of high-purity niobium single crystals is presented. Specimens were cut with different crystallographic orientations from a large grain niobium disk and uniaxial tensile tests were conducted at strain rates between 10-4 and 103 s-1. The logarithmic strain rate sensitivity for crystals oriented close to the center of a tensile axis inverse pole figure (IPF) is ~0.14 for all strain rates. The strain at failure (ranging from 0.4 to 0.9) is very sensitive to crystal orientation and maximal at ~10-2 s-1 for crystals oriented close to the center of an IPF. The high anisotropy observed at quasi-static strain rates decreased with increasing strain rate. The activation of multiple slip systems in the dynamic tests could account for this reduction in anisotropy. A transition from strain hardening to softening in the plastic domain was observed at strain rates greater than approximately 6 × 10-2 s-1 for crystals oriented close to the center of a tensile axis IPF. Shear bands were observed in specimens with orientations having similarly high Schmid factors on both {110} and {112} slip families, and they are correlated with reduced ductility. Crystal rotations at fracture are compared for the different orientations using scanning electron microscopy images and EBSD orientation maps. A rotation toward the terminal stable [101] orientation was measured for the majority of specimens (with tensile axes more than ~17° from the [001] direction) at strain rates between 1.28 × 10-2 and 1000 s-1.The authors would like to acknowledge the work of CERN's Materials, Metrology and Non-Destructive Testing (EN-MME-MM) section for granting access to their equipment for specimen preparation and scanning electron microscope (SEM) analyses. The authors would also like to thank Mr. Larry Vladic of Elite Motion LLC for lending us the high-speed camera during the high strain rate tests performed ASU. This Marie Sklodowska-Curie Action (MSCA) Innovative Training Network (ITN) receives funding from the European Union's H2020 Framework Programme under grant agreement no. 764879. T.R. Bieler, D. Kang, E. Pai Kulyadi, P. Eisenlohr, C. Kale, and K.N. Solanki acknowledge support from DOE/OHEP grant DE-SC0009962

The Effect of Hot Deformation Parameters on Microstructure Evolution of the α-Phase in Ti-6Al-4V

The effect of high-temperature deformation and the influence of hot working parameters on microstructure evolution during isothermal hot forging of Ti-6Al-4V in the alpha phase field were investigated. A series of hot isothermal axis-symmetric compression tests were carried out at temperatures both low and high in the alpha stability field [(1153 K and 1223 K (880 °C and 950 °C), respectively], using three strain rates (0.01, 0.1 and 1.0/s) relevant to industrial press forging. The microstructures and orientation of the alpha laths were determined using optical microscopy and electron backscatter diffraction techniques. The experimental results show that there is a change in lath morphology of the secondary α phase under the influence of the deformation parameters, and that α lath thickness appears to have little influence on flow behavior