Christopher Wehrenberg

In situ X-ray diffraction measurement of shock-wave-driven twinning and lattice dynamics

Pressure-driven shock waves in solid materials can cause extreme damage and deformation. Understanding this deformation and the associated defects that are created in the material is crucial in the study of a wide range of phenomena, including planetary formation and asteroid impact sites, the formation of interstellar dust clouds, ballistic penetrators, spacecraft shielding and ductility in high-performance ceramics. At the lattice level, the basic mechanisms of plastic deformation are twinning (whereby crystallites with a mirror-image lattice form) and slip (whereby lattice dislocations are generated and move), but determining which of these mechanisms is active during deformation is challenging. Experiments that characterized lattice defects have typically examined the microstructure of samples after deformation, and so are complicated by post-shock annealing and reverberations. In addition, measurements have been limited to relatively modest pressures (less than 100 gigapascals). In situ X-ray diffraction experiments can provide insights into the dynamic behaviour of materials, but have only recently been applied to plasticity during shock compression and have yet to provide detailed insight into competing deformation mechanisms. Here we present X-ray diffraction experiments with femtosecond resolution that capture in situ, lattice-level information on the microstructural processes that drive shock-wave-driven deformation. To demonstrate this method we shock-compress the body-centred-cubic material tantalum—an important material for high-energy-density physics owing to its high shock impedance and high X-ray opacity. Tantalum is also a material for which previous shock compression simulations and experiments have provided conflicting information about the dominant deformation mechanism. Our experiments reveal twinning and related lattice rotation occurring on the timescale of tens of picoseconds. In addition, despite the common association between twinning and strong shocks, we find a transition from twinning to dislocation-slip-dominated plasticity at high pressure (more than 150 gigapascals), a regime that recovery experiments cannot accurately access. The techniques demonstrated here will be useful for studying shock waves and other high-strain-rate phenomena, as well as a broad range of processes induced by plasticity.

Nanoscale anisotropic Nd-Fe-B particles with high coercivity prepared by attrition milling

Nanoscale Nd-Fe-B particles were fabricated with room temperature coercivity of 4.9 kOe by attrition (stirred media) milling. For longer milling times, more than 80% of the particles were less than 500 nm diameter, and 50–200 nm particles can be selected by a decanting process. Anisotropy was demonstrated by field alignment, as shown by creation of strong texture in x-ray diffraction (XRD) patterns. Limited peak broadening in XRD patterns indicates that attrition milling produces less deformation and amorphous material than high-energy milling. Compared to other magnetic nanoparticles, the higher coercivity is attributed to limited plastic deformation and larger sized particles.

Modeling of grain size strengthening in tantalum at high pressures and strain rates

Laser-driven ramp wave compression experiments have been used to investigate the strength (flow stress) of tantalum and other metals at high pressures and high strain rates. Recently this kind of experiment has been used to assess the dependence of the strength on the average grain size of the material, finding no detectable variation with grain size. The insensitivity to grain size has been understood theoretically to result from the dominant effect of the high dislocation density generated at the extremely high strain rates of the experiment. Here we review the experiments and describe in detail the multiscale strength model used to simulate them. The multiscale strength model has been extended to include the effect of geometrically necessary dislocations generated at the grain boundaries during compatible plastic flow in the polycrystalline metal. We use the extended model to make predictions of the threshold strain rates and grain sizes below which grain size strengthening would be observed in the las...

Fabrication of ND-FE-B/ALPHA-FE nanocomposite magnets by shock compaction and heat treatment of mechanically milled powders

Bulk nanocomposite magnets based on the Nd-Fe-B/α-Fe system were fabricated using mechanical alloying and shock compaction. A high energy ball mill was used to combine Magnaquench MQA-T type Nd-Fe-B powder with varying amounts of pure Fe powder. The mechanically milled powders were shock compacted to near full density at a primary shock pressure of approximately 6.3 GPa. A range of heat treatments were applied to the recovered samples, and the crystallization behavior and magnetic properties were measured. The presence of additional iron increases magnetization saturation, but decreases coercivity. Heat treatment at 550°C increases the coercivity only marginally, which can be attributed to the amorphous material crystallizing to the α-Fe phase instead of the Nd2Fe14B phase. A subsequent shock experiment at 8.3 GPa produced a tetragonal to BCC phase transition in the Nd-Fe-B powder.