Generating gradient germanium nanostructures by shock-induced amorphization and crystallization

Abstract

Gradient nanostructures are attracting considerable interest due to their potential to obtain superior structural and functional properties of materials. Applying powerful laser-driven shocks (stresses of up to one-third million atmospheres, or 33 gigapascals) to germanium, we report here a complex gradient nanostructure consisting of, near the surface, nanocrystals with high density of nanotwins. Beyond there, the structure exhibits arrays of amorphous bands which are preceded by planar defects such as stacking faults generated by partial dislocations. At a lower shock stress, the surface region of the recovered target is completely amorphous. We propose that germanium undergoes amorphization above a threshold stress and that the deformation-generated heat leads to nanocrystallization. These experiments are corroborated by molecular dynamics simulations which show that supersonic partial dislocation bursts play a role in triggering the crystalline-to-amorphous transition.amorphization | laser shock | nanocrystallization | germanium | gradient materials A morphous and gradient nanostructures are drawing intense attention due to their superior functional and mechanical properties (1, 2). Since they are thermodynamically metastable, amorphous materials can transform into nanocrystalline ones if appropriate treatments are applied (3). One of the most common methods to achieve amorphization is to quench a liquid at ultrafast cooling rates, which is extremely difficult for most pure elements (4). Alternatively, it has been shown that application of pressure leads to amorphization of materials whose melting point displays a negative Clapeyron slope (dT/dP < 0) (5-9); germanium (Ge) falls into this category (10). However, instead of pressure-induced amorphization, numerous studies, under both static (11, 12) and dynamic conditions (13-15), have shown that Ge undergoes a polymorphic transition at elevated pressures. Consequently, amorphization was not unambiguously identified in Ge until Clarke et al. (16) observed the indentation-induced crystalline-to-amorphous transition. More recently, a high-speed nanodroplet test also showed surface amorphization of Ge in an extremely localized manner (17).Despite being widely studied, the underlying microstructural mechanisms of pressure-induced amorphization remain vague. This is due to the notorious brittleness of germanium at room temperature which renders its recovery from pressurization extremely challenging. The deposition of high-power pulsed-laser energy onto a millimeter-scale target generates transient states of extreme stresses that promptly build up and decay rapidly as the pulse propagates. 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 ...

Structural transformations including melting and recrystallization during shock compression and release of germanium up to 45 GPa

“…The previous claim of shock-induced amorphization of Ge, based on an examination of recovered laser-shocked Ge samples [29], is contradicted by our results. The crystallization to the cd phase from the shocked state (lacking long range order) observed in our in situ measurements, upon uniaxial strain stress release, indicates that the shocked Ge was liquid rather than a metastable amorphous phase.…”

“…Recent preliminary XRD measurements in laser shocked Ge suggested a solid-solid transition precedes melting [28]; because the focus was to demonstrate simultaneous phase contrast imaging (PCI) and XRD measurements, neither the transition stresses nor the structure of high-pressure crystalline phase(s) were reported. In contrast, analysis of recovered Ge samples, laser shocked to ~33 GPa, and classical molecular dynamics (MD) simulations have reported that Ge amorphized under shock compression [29]. However, these results require further examination because time-scales of MD simulations and recovery experiments differ by many orders of magnitude.…”

Surface Nanocrystallization and Amorphization of Dual-Phase TC11 Titanium Alloys under Laser Induced Ultrahigh Strain-Rate Plastic Deformation

“…According to the results reported in References [ 34 , 35 , 36 , 37 ], the crystal-to-amorphous transition can be attributed to the local temperature rising to the melting point of the material due to plastic deformation and its subsequent fast cooling. Worswick and Yang [ 38 ] assumed that 5% of the plastic deformation work was stored in the grain defects and 95% was transformed into heat.…”

“…Similar results and formation mechanisms were reported by Wang et al [ 34 ], who found that the threshold peak pressure for amorphization of a NiTi alloy was about 3.3 GPa (the pressure is 4.4 GPa in this case). Meyers [ 35 , 36 ] found that shear bands were generated in AISI 304 stainless steel and germanium under shock loading conditions, and the formation of an amorphous phase was observed in these shear bands. The liquid-solid structure induced by the local temperature rising and subsequent fast cooling were the main reasons for the formation of an amorphous phase; however, the LSP-induced surface amorphization of titanium alloys has not been reported in the literature so far.…”

Shock-induced Amorphization in Covalently Bonded Solids

“…Owing to the development of high power pulsed lasers, high amplitude (10s to 100s of GPa) shock wave with ns pulse duration can be produced, which enables the exploration of materials science in this unknown regime. In this investigation, we have successfully shock recovered four different brittle materials, namely, Si [5][6], Ge [7], B 4 C [8], and SiC, all of which are covalently bonded.…”