Mechanisms of spall failure in niobium subjected to high-throughput laser-driven micro-flyer impact

Designing bcc alloys for shock resistance requires a fundamental understanding of the microstructural evolution and failure mechanisms under dynamic loading. However, two significant challenges arise. Firstly, conventional plate impact experiments are time-consuming and costly, limiting the acquisition of sufficient data to investigate these mechanisms. Secondly, the mechanistic understanding of deformation and failure in pure bcc metals remains limited, hindering purposeful alloy design strategies. To address these challenges, we employ a high-throughput laser-driven micro-flyer technique to establish the relationship between the spall failure of pure bcc niobium and its evolving microstructure at tensile strain rates of ∼ 10⁶ s⁻¹. By varying flyer thickness, peak shock stress ranging from 7.3 to 15.3 GPa were achieved. Post-mortem microstructural analysis of samples recovered at incipient and advanced spall failure states reveals that failure occurs in a ductile manner through mixed intergranular and intragranular fractures. For cavities nucleating within grains, dislocation emission from void surfaces is identified as the controlling void growth mechanism up to a void radius of ∼260 nm, after which cracks emanate from void surfaces extending outward along {101} planes. Local misorientation variations are observed around the cracks, with dislocation cells observed away from crack edges and high-angle grain boundaries at crack edges, revealing continuous dynamic recrystallization in regions of highly accumulated plastic strain. Our spall strength and microstructural evolution results are discussed in the context of analytical models for dynamic cavitation-driven failure.