Shock-induced nucleation of nanophase Fe-Ni alloy and its implications for interstellar iron reservoirs

Shock waves are ubiquitous in star-forming regions, protoplanetary disks, and cometary environments, yet their role in processing refractory metals remains poorly understood. Here, we show that laboratory shock-tube experiments produce nanophase Fe-Ni alloy from Fe and Ni powders under conditions resembling low-velocity (1-2 km/s) dust-heating shocks in the interstellar medium and cometary comae. The reflected-shock temperature exceeds 6000 K, and pressures reach around 14.5 bar, persisting for about 2-3 ms and completely vapourising the metal powders into an atomic vapour. Subsequent rarefaction drives a catastrophic thermal quench at ∼10⁶ K/s, inducing direct vapour-phase condensation of bcc kamacite (α-Fe-Ni) without an intervening taenite phase. X-ray diffraction and Rietveld refinement confirm a homogeneous kamacite solid solution, while FESEM reveals octagonal to sub-spherical particles consistent with condensation from transient vapour/melt droplets. HRTEM, SAED, and FFT analyses reveal well-ordered bcc lattices and high densities of dislocations and deformation twins, suggesting rapid quench crystallisation under extreme non-equilibrium conditions. HAADF-STEM and EDS mapping show atomic-scale compositional uniformity, with Fe:Ni ratios closely matching the initial composition. The microstructures, compositions, and sizes of these shock-synthesised nanophase Fe-Ni alloy particles closely resemble nanophase metals observed in GEMS-bearing IDPs and Wild 2 samples, aligning with Ni-enriched metal vapour inferred from Fe I and Ni I detections in cometary comae. Our results demonstrate that transient, low-velocity shocks can produce nanophase Fe-Ni metal with meteoritic and cometary characteristics, establishing a strong mechanistic link between metal vapour chemistry, dust reprocessing, and the formation of nanoscale kamacite in primitive solar system and interstellar materials.