High-throughput Characterization of Spall Strength of Niobium

With a high melting temperature (>2400 degree Celsius) and desirable ductility, niobium is a potential candidate for extreme environments that involve concurrent high operating temperatures and high strain rate deformations. Despite this potential, our understanding of the dynamic and shock-induced mechanical response of niobium with a simple body-centered cubic (bcc) structure remains limited both experimentally and theoretically. In this work, laser-driven micro-flyer impact experiments were performed on thin niobium foils (<300 micron) in a high-throughput manner to investigate the shock-induced spall failure at strain rates of 10^5 – 10^6 1/s. The particle velocity of the target back free surface was measured using photonic doppler velocimetry (PDV) with automated analysis of the obtained PDV signals. Spallation of niobium was observed above a critical impact velocity with the obtained spall strength values being higher than those previously reported from plate impact experiments, possibly because of the higher tensile strain rates achieved in our experiments. Increasing the shock stress from 6 to 11 GPa resulted in an increase of the spall strength, and efforts are currently underway to examine this trend at higher shock stress values. To understand the underlying mechanisms that govern spall failure, post-mortem characterization of the impact sites are currently being pursued using white-light interferometry, scanning electron microscopy, and X-ray micro computed tomography.