Nina Gunkelmann

Interplay of plasticity and phase transformation in shock wave propagation in nanocrystalline iron

Strong shock waves create not only plasticity in Fe, but also phase transform the material from its bcc phase to the high-pressure hcp phase. We perform molecular-dynamics simulations of large, 8-million atom nanocrystalline Fe samples to study the interplay between these two mechanisms. We compare results for a potential that describes dislocation generation realistically but excludes phase change with another which in addition faithfully features the bcc → hcp transformation. With increasing shock strength, we find a transition from a two-wave structure (elastic and plastic wave) to a three-wave structure (an additional phase-transformation wave), in agreement with experiment. Our results demonstrate that the phase transformation is preceded by dislocation generation at the grain boundaries (GBs). Plasticity is mostly given by the formation of dislocation loops, which cross the grains and leave behind screw dislocations. We find that the phase transition occurs for a particle velocity between 0.6 and 0.7 km s−1. The phase transition takes only about 10 ps, and the transition time decreases with increasing shock pressure.

Morphological changes in polycrystalline Fe after compression and release

Despite a number of large-scale molecular dynamics simulations of shock compressed iron, the morphological properties of simulated recovered samples are still unexplored. Key questions remain open in this area, including the role of dislocation motion and deformation twinning in shear stress release. In this study, we present simulations of homogeneous uniaxial compression and recovery of large polycrystalline iron samples. Our results reveal significant recovery of the body-centered cubic grains with some deformation twinning driven by shear stress, in agreement with experimental results by Wang et al. [Sci. Rep. 3, 1086 (2013)]. The twin fraction agrees reasonably well with a semi-analytical model which assumes a critical shear stress for twinning. On reloading, twins disappear and the material reaches a very low strength value.

Influence of phase transition on shock-induced spallation in nanocrystalline iron

Intense shock waves may lead to spallation of the sample. Recent experiments show differences of shock spallation in iron depending on whether the samples underwent the pressure-induced bcc-hcp phase transformation or not. In this study, we perform molecular dynamics simulations of shock-induced spallation in polycrystalline iron. Our results show that the phase transformation decreases the probability of multiple spallation and crack formation. In agreement with experiments, the phase transformation changes the surface morphology showing smoother spallation surfaces.

Influence of porosity on collisions between dust aggregates

Context. Collisions between dust particles may lead to agglomerate growth or fragmentation, depending on the porosity of the dust and the collision velocity. Aims. We study the effect of agglomerate porosity and collision velocity on aggregate fragmentation and agglomeration. Methods. Granular-mechanics simulations are used to study the outcome of head-on dust aggregate collisions. The aggregates are composed of silica grains of 0.76 μ m radius and have filling factors of between 0.08 and 0.21. The simulations incorporate repulsive and viscoelastic, dissipative normal forces, and intergrain adhesion. The tangential forces are composed of gliding, rolling, and torsional friction. To study the effect of aggregate porosity, we prepared spherical aggregates with identical radius but differing particle numbers. Results. The threshold velocity for agglomerate fragmentation decreases with the porosity of the aggregates. Porous aggregates tend to fragment more easily, and the fragments are irregularly shaped. In the agglomeration regime, the merged aggregate is more compact than the initial collision partners. The collision velocity at which compaction is highest is independent of the initial porosity.

Low-velocity collisions of chondrules: How a thin dust cover helps enhance the sticking probability

Aims. The collision of two chondrules covered by a dust shell is investigated using a granular-mechanics algorithm. Methods. We focus on the specific case of chondrules of radius 25 μ m covered by dust of 0.76 μ m radius; the dust shells have thicknesses of up to 5 μ m and filling factors between 0.08 and 0.21. Results. We demonstrate that the bouncing velocity of the two chondrules increases by two orders of magnitude if a dust shell covers the chondrules. The shells become partly destroyed during the collision process, both by sputtering (monomer ejection) and by agglomeration to dust aggregates. Thicker and denser dust shells are more efficient in accommodating the collision energy than thin and porous shells.

Can we obtain the coefficient of restitution from the sound of a bouncing ball

The coefficient of restitution may be determined from the sound signal emitted by a sphere bouncing repeatedly off the ground. Although there is a large number of publications exploiting this method, so far, there is no quantitative discussion of the error related to this type of measurement. Analyzing the main error sources, we find that even tiny deviations of the shape from the perfect sphere may lead to substantial errors that dominate the overall error of the measurement. Therefore, we come to the conclusion that the well-established method to measure the coefficient of restitution through the emitted sound is applicable only for the case of nearly perfect spheres. For larger falling height, air drag may lead to considerable error, too.

Influence of C concentration on elastic moduli of α′-Fe1-xCxalloys

The elastic constants of tetragonally distorted - crystallites are calculated for several available interatomic interaction potentials. Besides embedded-atom-method-type potentials also a simple pair potential, modified embedded-atom-method and bond-order potentials are investigated. Care is taken to minimise the crystal structure properly in the presence of the C interstitials; we verify that the influence of statistics, i.e. the randomness of the C positions in the lattice, affects the elastic properties only little, as long as C is not allowed to cluster. We find that both sign and order of magnitude of the tetragonal elastic constants vary strongly between the predictions of the available potentials. Recent experimental data are available for the orientation-averaged elastic moduli; in contrast to the tetragonal constants, they feature only a mild dependence on C content. The experimental data are well reproduced by several of the potentials studied here. Existing deviations between experiment and predictions are discussed.