Calvin Lear

The mechanical behavior and microstructural evolution of a single body-centered-cubic (BCC) phase NbTaTiV refractory high-entropy alloy (RHEA) were studied over a wide range of strain-rates (from 1 × 10-4 to 2 × 103 s-1) and temperatures [from room temperature (RT) to 850 °C]. The evolution of yield strength as a function of strain rate and temperature showed that the present RHEA had less strain-rate dependence and a strong resistance to softening at high temperatures. The formation of thin type-I twins was observed during high-strain rate deformation. The present work also showed that the thin twins did not significantly affect the work-hardening/softening rate during high strain-rate testing at all temperatures. Contrary to the high strain-rate behavior, substantial strain-hardening was observed at quasi-static rates, when the materials were tested at cryogenic temperatures. This difference in behavior was attributed to the formation of “thick” twins at cryogenic temperatures, which not only act as stronger barriers to dislocation motion but also interact with each other leading to an increase in the work hardening rate. The temperature increase during high strain-rate testing was estimated by theoretical calculations. The relatively low stacking fault energies with high twinning stress for NbTaTiV were predicted by density functional theory calculations. Moreover, a modified Zerilli-Armstrong constitutive model was also fit to represent the strength of this material at varying strain rates and temperatures.

Joining nanostructured ferritic alloys (NFAs) has proved challenging, as the nano-oxides that provide superior strength, creep resistance, and radiation tolerance at high temperatures tend to agglomerate, redistribute, and coarsen during conventional fusion welding. In this study, capacitive discharge resistance welding (CDRW)—a solid-state variant of resistance welding—was used to join end caps and thin-walled cladding tubes of the NFA 14YWT. The resulting solid-state joints were found to be hermetically sealed and were characterized across the weld region using electron microscopy (macroscopic, microscopic, and nanometer scales) and nanoindentation. Microstructural evolution near the weld line was limited to narrow (~50–200 μm) thermo-mechanically affected zones (TMAZs) and to a reduction in pre-existing component textures. Dispersoid populations (i.e., nano-oxides and larger oxide particles) appeared unchanged by all but the highest energy and power CDRW condition, with this extreme producing only minor nano-oxide coarsening (~2 nm → ~5 nm Ø). Despite a minimal microstructural change, the TMAZs were found to be ~10% softer than the surrounding base material. These findings are considered in terms of past solid-state welding (SSW) efforts—cladding applications and NFA-like materials in particular—and in terms of strengthening mechanisms in NFAs and the potential impacts of localized temperature–strain conditions during SSW.

The effect of helium (He) concentration on ejecta production in OFHC-Copper was investigated using Richtmyer–Meshkov Instability (RMI) experiments. The experiments involved complex samples with periodic surface perturbations machined onto the surface. Each of the four target was implanted with a unique helium concentration that varied from 0 to 4000 appm. The perturbation’s wavelengths were λ ≈ 65 μ m, and their amplitudes h 0 were varied to determine the wavenumber ( 2 π / λ ) amplitude product k h 0 at which ejecta production beganfor Cu with and without He. The velocity and mass of the ejecta produced was quantified using Photon Doppler Velocimetry (PDV) and Lithium-Niobate (LN) pins, respectively. Our results show that there was an increase of 30% in the velocity at which the ejecta cloud was traveling in Copper with 4000 appm as compared to its unimplanted counterpart. Our work also shows that there was a finer cloud of ejecta particles that was not detected by the PDV probes but was detected by the early arrival of a “signal” at the LN pins. While the LN pins were not able to successfully quantify the mass produced due to it being in the solid state, they did provide information on timing. Our results show that ejecta was produced for a longer time in the 4000 appm copper.