An integrated microstructural high strain-rate experimental and computational analysis of the spall behavior of additively manufactured niobium C-103 alloys

Abstract

Niobium alloys, such as C-103, have been used for high-temperature applications due to their oxidation resistance, high-temperature behavior, and ductility. These characteristics also render C-103 as an attractive material for additive manufacturing (AM) processing. However, there is a lack of fundamental understanding of how defects, such as dislocation density and dislocation density interactions, and texture affect high strain-rate and spall behavior in body-centered cubic (b.c.c.) AM processed C-103 alloys. To address these challenges, electron beam powder bed fusion (EB-PBF) was used to process and fabricate C-103 samples with highly textured columnar grains. Disc-shaped plate-impact test specimens were extracted from the AM-fabricated samples, with the grains oriented either parallel or perpendicular to the build direction, for experiments with loading velocities of up to 600 m/s. The tests were instrumented with a photonic Doppler velocimetry (PDV) system to obtain time-resolved free surface velocity data of the sample and compute the spall strength of C-103 across a wide range of loading rates. These experimental measurements were then integrated with computational predictions based on a dislocation-based crystalline plasticity (DCP) approach coupled with a fracture formulation to understand how defects, such as dislocation densities, affect the spall strength and the defect behavior of C-103. The predictive framework provided insights into how spall cracks nucleate due to a combination of tensile wave reflection and dislocation-density accumulation, and how immobile dislocation accumulation ahead of multiple crack fronts can blunt spall propagation. This interrelated approach provides an understanding of high strain-rate and dynamic fracture of textured AM b.c.c. microstructures that can be tailored to mitigate high-impact velocity and spall in niobium alloys.