Eric N. Hahn

Phase Transformation in Tantalum under Extreme Laser Deformation

The structural and mechanical response of metals is intimately connected to phase transformations. For instance, the product of a phase transformation (martensite) is responsible for the extraordinary range of strength and toughness of steel, making it a versatile and important structural material. Although abundant in metals and alloys, the discovery of new phase transformations is not currently a common event and often requires a mix of experimentation, predictive computations, and luck. High-energy pulsed lasers enable the exploration of extreme pressures and temperatures, where such discoveries may lie. The formation of a hexagonal (omega) phase was observed in recovered monocrystalline body-centered cubic tantalum of four crystallographic orientations subjected to an extreme regime of pressure, temperature, and strain-rate. This was accomplished using high-energy pulsed lasers. The omega phase and twinning were identified by transmission electron microscopy at 70 GPa (determined by a corresponding VISAR experiment). It is proposed that the shear stresses generated by the uniaxial strain state of shock compression play an essential role in the transformation. Molecular dynamics simulations show the transformation of small nodules from body-centered cubic to a hexagonal close-packed structure under the same stress state (pressure and shear).

Non-equilibrium molecular dynamics simulations of spall in single crystal tantalum

Ductile tensile failure of tantalum is examined through large scale non-equilibrium molecular dynamics simulations. Several loading schemes including flyer plate impact, decaying shock loading via a frozen piston, and quasi-isentropic (constant strain-rate) expansion are employed to span tensile strain-rates of 108 to 1014 per second. Single crystals of 〈001〉 orientation are specifically evaluated to eliminate grain boundary effects. Heterogeneous void nucleation occurs principally at the intersection of deformation twins in single crystals. At high strain rates, multiple spall events occur throughout the material and voids continue to nucleate until relaxation waves arrive from adjacent events. At ultra-high strain rates, those approaching or exceeding the atomic vibrational frequency, spall strength saturates near the maximum theoretical spall strength.

Supersonic Dislocation Bursts in Silicon

Dislocations are the primary agents of permanent deformation in crystalline solids. Since the theoretical prediction of supersonic dislocations over half a century ago, there is a dearth of experimental evidence supporting their existence. Here we use non-equilibrium molecular dynamics simulations of shocked silicon to reveal transient supersonic partial dislocation motion at approximately 15 km/s, faster than any previous in-silico observation. Homogeneous dislocation nucleation occurs near the shock front and supersonic dislocation motion lasts just fractions of picoseconds before the dislocations catch the shock front and decelerate back to the elastic wave speed. Applying a modified analytical equation for dislocation evolution we successfully predict a dislocation density of 1.5 × 1012 cm−2 within the shocked volume, in agreement with the present simulations and realistic in regards to prior and on-going recovery experiments in silicon.

Generating gradient germanium nanostructures by shock-induced amorphization and crystallization

Significance Amorphization and nanocrystallization are two powerful methods to tailor material properties by altering their microstructure without changing the overall chemistry. Using powerful laser-driven shocks, we demonstrate that amorphization and nanocrystallization can be achieved within a time scale that is considerably shorter than other conventional techniques. Our results provide compelling insights into pressure/shear amorphization and propose a route to fabricate gradient semiconducting nanostructures using lasers. Additionally, shear-driven amorphization is demonstrated as the dominant deformation mechanism in this extreme regime. 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.

Nature's technical ceramic: the avian eggshell

Avian eggshells may break easily when impacted at a localized point; however, they exhibit impressive resistance when subjected to a well-distributed compressive load. For example, a common demonstration of material strength is firmly squeezing a chicken egg along its major axis between ones hands without breaking it. This research provides insight into the underlying mechanics by evaluating both macroscopic and microstructural features. Eggs of different size, varying from quail (30 mm) to ostrich (150 mm), are investigated. Compression experiments were conducted along the major axis of the egg using force-distributing rubber cushions between steel plates and the egg. The force at failure increases with egg size, reaching loads upwards of 5000 N for ostrich eggs. The corresponding strength, however, decreases with increasing shell thickness (intimately related to egg size); this is rationalized by a micro-defects model. Failure occurs by axial splitting parallel to the loading direction—the result of hoop tensile stresses due to the applied compressive load. Finite-element analysis is successfully employed to correlate the applied compressive force to tensile breaking strength for the eggs, and the influence of geometric ratio and microstructural heterogeneities on the shells strength and fracture toughness is established.

Towards predicting susceptibility of grain boundaries to failure in BCC materials

Several factors can affect the failure stress of a grain boundary, such as grain boundary structure, energy and excess volume, in addition to its interactions with dislocations. In this paper, we focus on the influence of grain boundary energy and excess volume at the boundary on the failure stress of a grain boundary in tantalum from molecular-dynamics simulations. Flyer plate simulations were carried out for a handful of boundary types with different energies and excess volumes. These boundaries were chosen as model systems to represent various boundaries observed in “real” materials. For a small, but representative, set of boundaries explored, no direct correlation was observed between the void nucleation stress of a boundary and either its energy and excess volume. This result suggests that average properties of grain boundaries alone are not sufficient indicators of the failure strength of a boundary.Several factors can affect the failure stress of a grain boundary, such as grain boundary structure, energy and excess volume, in addition to its interactions with dislocations. In this paper, we focus on the influence of grain boundary energy and excess volume at the boundary on the failure stress of a grain boundary in tantalum from molecular-dynamics simulations. Flyer plate simulations were carried out for a handful of boundary types with different energies and excess volumes. These boundaries were chosen as model systems to represent various boundaries observed in “real” materials. For a small, but representative, set of boundaries explored, no direct correlation was observed between the void nucleation stress of a boundary and either its energy and excess volume. This result suggests that average properties of grain boundaries alone are not sufficient indicators of the failure strength of a boundary.

The role of grain boundary orientation on void nucleation in tantalum

It is generally understood that microstructure plays a significant role in determining the deformation response of materials. During shock compression, grain boundaries serve as dislocation nucleation/pile-up/adsorption sites and grain size can alter the width of the shock front. During tensile release, grain boundaries are often “weak links” where spallation occurs. As such, a current deficit in predictive modeling capability is a quantitative description of these locations and their relative ability to serve as void nucleation sites - a challenging component of such a description is that spallation is inherently stochastic in nature. The inclination of the grain boundary plane with respect to the loading direction is thought to be a critical constituent in the resultant stress and failure at the boundary. Non-equilibrium molecular dynamics simulations are used to statistically quantify the influence of grain boundary inclination on the location of void nucleation and to highlight the emergence of stress hotspots at such boundaries. Boundaries oriented perpendicular to the loading direction are more likely to fail, but grain boundary inclination alone is not a complete predictor – i.e. not all perpendicular boundaries fail during spallation.It is generally understood that microstructure plays a significant role in determining the deformation response of materials. During shock compression, grain boundaries serve as dislocation nucleation/pile-up/adsorption sites and grain size can alter the width of the shock front. During tensile release, grain boundaries are often “weak links” where spallation occurs. As such, a current deficit in predictive modeling capability is a quantitative description of these locations and their relative ability to serve as void nucleation sites - a challenging component of such a description is that spallation is inherently stochastic in nature. The inclination of the grain boundary plane with respect to the loading direction is thought to be a critical constituent in the resultant stress and failure at the boundary. Non-equilibrium molecular dynamics simulations are used to statistically quantify the influence of grain boundary inclination on the location of void nucleation and to highlight the emergence of stress...