Directional amorphization of boron carbide subjected to laser shock compression

Abstract

Solid-state shock-wave propagation is strongly nonequilibrium in nature and hence rate dependent. Using high-power pulsed-laserdriven shock compression, unprecedented high strain rates can be achieved; here we report the directional amorphization in boron carbide polycrystals. At a shock pressure of 45∼50 GPa, multiple planar faults, slightly deviated from maximum shear direction, occur a few hundred nanometers below the shock surface. High-resolution transmission electron microscopy reveals that these planar faults are precursors of directional amorphization. It is proposed that the shear stresses cause the amorphization and that pressure assists the process by ensuring the integrity of the specimen. Thermal energy conversion calculations including heat transfer suggest that amorphization is a solid-state process. Such a phenomenon has significant effect on the ballistic performance of B 4 C.oron carbide is one of the hardest materials on earth while extremely lightweight, making it excellent for ballistic protection applications such as body armor (1-6). Thus, its dynamic behavior under impact/shock loading has been the subject of intensive studies for decades (1,(5)(6)(7)(8)(9)(10)(11)(12)(13). It is known that boron carbide undergoes an abrupt shear strength drop at a critical shock pressure around 20∼23 GPa, suggesting a deteriorated penetration resistance (8). Based on similar observations in geological materials (5), Grady (7) hypothesized that this was caused by localized softening mechanisms such as shear localization and/or melting. Examining fragments collected from a ballistic test using transmission electron microscopy (TEM), Chen et al. (14) were the first to identify localized amorphization in boron carbide, which appeared to be aligned to certain crystallographic planes. However, because the loading history of these fragments is unknown, its effect on the observed microstructure is not understood. Additionally, although the more well-defined loading conditions associated with quasi-static diamond-anvil cell (15) and nanoindentation (16, 17) experiments have provided greater insight into amorphization of boron carbide, they do not address the regime of high strain rate.The laser shock experimental technique offers promise in bridging the gaps of the previous experiments by enabling boron carbide to be shock compressed under controlled and prescribed uniaxial strain loading conditions and then recovered for postshock characterization by TEM. To ensure the integrity of the specimen, the duration of the stress pulse should be smaller than the characteristic time for crack propagation which is typically on the microsecond scale [limited by Rayleigh wave speed (18)]. Traditional dynamic loading methods such as plate impact and split Hopkinson bar cannot deliver the strain rates required because the stress pulses of both techniques occur on microsecond time scales. Therefore, brittle solids such as B 4 C will fail catastrophically by crack nucleation, propagation, and coalescence (19). To solve this ch...

Generating gradient germanium nanostructures by shock-induced amorphization and crystallization

“…The increase in temperature due to shock compression can be evaluated by considering both the homogeneous temperature rise (ΔT homo ) resulting from work done by hydrostatic pressure and the localized temperature rise (ΔT local ) resulting from work done by shear stress. The latter assumes a balance between relaxation of deviatoric strain energy and decrease in internal energy with heat loss to its surroundings, and thus gives a rough estimate of the temperature inside the amorphous band (19):…”

“…The short duration of the stress pulse and impedancematched encapsulation preserves the integrity of the target by suppressing the full development of cracks and enables postshock microstructure characterization. Using this methodology, we have previously reported shock-induced amorphization in silicon (18) and boron carbide (19). Before that, Jeanloz et al (20) discovered this phenomenon in olivine (iron/magnesium silicate) subjected to shock compression.…”

In search of amorphization-resistant boron carbide

“…Later, Chen et al [2] identified this deformation mechanism as localized amorphization (loss of crystalline order) by conducting transmission electron microscopy (TEM) of small fragments of B 4 C armor plates subjected to ballistic impact. Since then, this mechanism has been observed under indentation [3][4][5], ballistic impact [2,6], laser shock [7], diamond anvil cell [8], mechanical scratching [9], electric fields [10], and radiation [11]. Numerous characterization techniques, such as TEM [2,7], Raman spectroscopy [4,[12][13][14], photoluminiscence [14], FTIR [14], EELS [2], and neutron scattering [3], have been employed to observe the structure of the amorphized region.…”

“…Since then, this mechanism has been observed under indentation [3][4][5], ballistic impact [2,6], laser shock [7], diamond anvil cell [8], mechanical scratching [9], electric fields [10], and radiation [11]. Numerous characterization techniques, such as TEM [2,7], Raman spectroscopy [4,[12][13][14], photoluminiscence [14], FTIR [14], EELS [2], and neutron scattering [3], have been employed to observe the structure of the amorphized region. The consequences of amorphization include reduced ballistic performance [2], loss of hardness under dynamic loads [14][15][16], post-HEL softening [1], and change in electrical properties [10].…”

Amorphization‐induced volume change and residual stresses in boron carbide

“…These anomalies in the high-pressure performance of boron carbide spurred numerous investigations into the evolution of short-range disorder in the material, often referred to as "amorphization". Subsequent studies have also documented amorphization under Berkovich [8][9][10][11] and Vickers 3,12-16 indentations, laser shock impact, 17 mechanical scratching, 9,18 and nonhydrostatic loading in diamond anvil cell experiments. 19 Postmortem TEM of the deformed regions revealed inhomogeneous distributions of nanoscale amorphized zones within the crystalline matrix of boron carbide.…”