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

TL;DR

This study shows that shock waves in space can create nanoscale iron-nickel alloys similar to those found in comets and meteorites.

Contribution

The paper demonstrates that low-velocity shocks can synthesize nanophase Fe-Ni alloys with meteoritic characteristics.

Findings

Shock-tube experiments produced nanophase Fe–Ni alloy under interstellar-like conditions.

The synthesized particles resemble those found in cometary and meteoritic samples.

Rapid quench crystallization under extreme conditions forms ordered bcc kamacite structures.

Abstract

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.

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