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 $\lambda \approx 65\mathsf{\mu }$ m, and their amplitudes $h_0$ were varied to determine the wavenumber $(2\pi /\lambda )$ 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.

Kinetic Monte Carlo simulations have been performed to investigate the evolution of ordered domains in model alloys under irradiation. The alloys investigated were equiatomic binary alloys on a simple square lattice with first and second nearest-neighbor interactions, chosen so that a $2\times 2$ ordered structure is the equilibrium phase below a critical order-disorder transition temperature $T_{\mathrm{c}}$. The ratio of second to first nearest-neighbor interactions $R$ was varied from 0 to 0.45 to explore the effect of the thermodynamic frustrations induced by the proximity of the $2\times 1$ phase boundary, which occurs at $R=0.5$ for $T=0$. The atomic mixing produced by nuclear collisions was modeled by forcing the ballistic exchange of pairs of atoms at a controlled rate ${\mathrm{\Gamma }}_{\mathrm{b}}$. This disordering process competed with thermodynamic reordering, resulting in nonequilibrium steady states. Two trivial steady states were found, a disordered state at high ${\mathrm{\Gamma }}_{\mathrm{b}}$ and low $T$, and a long-range ordered state at low ${\mathrm{\Gamma }}_{\mathrm{b}}$ and low $T$. In the $R=0.45$ alloy, however, a third steady state was identified at intermediate ${\mathrm{\Gamma }}_{\mathrm{b}}$ and $T$ values, where multiple long-range ordered domains coexisted dynamically. It is shown that this state of patterning of order resulted from the coupling of the thermodynamic frustrations present in that alloy with the disorder introduced by irradiation. The practical relevance of this novel mechanism for patterning of order under irradiation is discussed in the context of recent observations of domain coexistence in irradiated ${\mathrm{Cu}}_3\mathrm{Au}$.