Physics Based Crystal Plasticity Modeling Of Single Crystal Niobium PDF Download

One suggested method to manufacture superconducting radio frequency (SRF) cavities, which are used as driving units in particle accelerators, is the deep-drawing of Niobium (Nb) disks that were cut from large-grained ingots. This is a promising and cost-effective alternative to the current industry standard of deep-drawing rolled poly-crystalline sheets. It is essential to understand the sources of the observed variability in the performance of cavities fabricated by either process in order to define the most suitable fabrication route.Cavity performance is hampered by material defects such as dislocations or crystallite boundaries that can arise and evolve during deformation processes involved in the mechanical shaping of the cavity (mostly by deep-drawing). Such defects can trap magnetic flux causing significant radio frequency (RF) losses leading to reduced performance. Understanding dislocation mechanics in Nb based on deformation experiments allows predictive modeling to enable informed design strategies for optimal cavity fabrication and improved performance.Crystal plasticity (CP) modeling is a powerful and well-established computational materials science tool to investigate mechanical structure-property relations in crystalline materials. A dislocation mechanics-based constitutive description of plastic deformation in body-centered cubic (BCC) metals is used to formulate a CP model for Nb. Uniaxial tension experiments on several Nb single crystals cut from a large-grained ingot disk were conducted at several different strain rates. The specific selection of grains and the in-plane orientation of deformation samples cut from those grains were based on the active slip systems anticipated from SCHMID factor calculations. The results from these specifically designed deformation experiments are used to validate the model.Simulations and corresponding deformation experiments exhibit notable discrepancies in terms of the stress-strain response and lattice reorientation. These discrepancies are rationalized by considering the effect of a distribution in pre-existing dislocation densities across the possible slip systems, which entails a significant variability in the resulting slip system activity and associated crystal reorientation and strain hardening behavior. An exhaustive numerical study probing thousands of initial dislocation density distributions could be condensed into inverse pole figure (IPF) maps that chart the range of crystallographic tensile directions for which stable outcomes can be expected despite a given variability in the pre-existing dislocation density distribution. Consequently, it becomes clear that while single-crystal experiments can be a useful guide toward the development and testing of dislocation mechanics-based models, care must be exercised as to not expect a one-to-one matching between any specific experiment and its corresponding simulation, since the exact values and distribution of pre-existing dislocations in the sample is generally unknown (and likely unknowable).