Session: 04-12-02: Design of Materials and Discovery of Constitutive Models Linking Process-Structure-Property-Performance Relationships

Paper Number: 167067

An Integrated Microstructural High Strain-Rate Experimental and Computational Analysis of the Spall Behavior of Additively Manufactured Niobium C103 Alloys 

Niobium alloys, such as C103, have been used for high-temperature applications due to their oxidation resistance, high-temperature behavior, and ductility. These characteristics also render C103 as an attractive material for additive manufacturing (AM) processing. Refractory body centered cubic (BCC) metals and alloys, such as niobium, tantalum, molybdenum, and tungsten, have been a desirable material group due to their high-temperature behavior. Niobium, with its relatively low density and low-temperature fabricability, has been used for high-temperature applications, but oxidation can determinately affect behavior. Niobium alloys, such as C103, comprised of niobium, hafnium, and titanium, have also been shown to be oxidation-resistant. However, there is a lack of fundamental understanding of microstructural behavior resulting from high strain-rate and impact conditions for AM processed C103 alloys. To address these challenges, electron beam powder bed fusion (EB-PBF) was used to process and fabricate C103 samples with highly textured columnar grains. Test specimens were extracted from the AM fabricated samples with the grains either parallel or perpendicular to the build direction (BD) for experiments with impact velocities as high as 600 m/s. 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 C103. The experimental results provided a fundamental understanding of wave reflection, free surface velocities, and pullback velocities, and how it is related to microstructural behavior and spall. This integrated experimental, manufacturing, and computational predictions framework provided new 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. The computational predictions for the wave reflection, free surface velocities, and pullback were consistent and validated with the experimental results. This integrated experimental and computational approach provided a fundamental behavior of how high strain rate and spall behavior for the different microstructures that were normal and parallel BDs. The differences in spall behavior between the columnar grains parallel or perpendicular to the impact direction is due to the elongated and larger grain sizes in the parallel BD case, which results in higher dislocation-density velocities. These higher dislocation-density velocities resulted in the earlier onset of tensile waves and the subsequent formation of spall fracture surfaces in comparison with the perpendicular BD case. This interrelated approach provides an essential understanding of high strain-rate and dynamic fracture of textured AM microstructures that can be tailored to mitigate high-impact velocity and spall in niobium alloys.

 

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Presenting Author: Omar Eldaly North Carolina State University

Presenting Author Biography: Ph.D. Student at NC State University with a research focus on multiscale modeling

Authors:

Omar Eldaly North Carolina State University
Mohammed Zikry North Carolina State Univ