Wednesday  |  Conference Center A  |  09:40 AM–10:00 AM

#13272, Impact-induced Nanostructural Evolutions of Silver Single Crystals Lead to Improved Mechanical Properties

Processes that induce surface dynamic recrystallization, such as shot peening, are used to improve the mechanical properties of a material. However, the plasticity mechanisms governing the impact-induced grain refinement and subsequent improvement in mechanical properties under quasistatic loading remain elusive. With an initial complex grain morphology prior to impact and simultaneous multi-projectile bombardment, it is difficult to ascertain what plasticity mechanisms are at play during dynamic recrystallization of polycrystal samples subject to processes such as shot peening. To achieve a fundamental understanding, we study the impact-induced nanostructural evolutions in single-crystal silver (Ag) samples impacted along three primary crystallographic directions.

High-velocity microprojectile impacts are performed using a laser-induced projectile impact testing (LIPIT) apparatus, with the study material as the projectile or as the target. Seed-mediated grown Ag microcubes—with a near-defect free nanostructure and external cubic shape reflecting the internal crystallographic symmetry—are impacted onto a rigid target and selected for further study if the impact orientation is [100], [110], or [111]. Additionally, single crystal Ag substrates, with a surface normal of [100], [110], or [111] are impacted with rigid silica projectiles. Studying the impact of both particles and substrates of the same material and crystallographic orientations reveals the role of free surfaces in the nanostructural evolution. Nanostructural characterization through transmission Kikuchi diffraction (TKD) shows grain refinement near the impact site in both material forms. The extent of dynamic recrystallization depends on the impact orientation. Stress-strain curves obtained through in situ SEM microcompression testing of pillars FIB-milled from the impacted cubes show ultra-high hardening rates, high yield stresses, and large compressive toughness. The synergistic improvement in often conflicting properties is attributed to a combination of inter- and intragranular plasticity mechanisms elicited through the spatial gradient of nanograins to coarse grains representative of a gradient-nano-grained structure.

Claire Griesbach   University of Wisconsin-Madison
Jizhe Cai   University of Wisconsin-Madison
Ramathasan Thevamaran   University of Wisconsin-Madison