The effect of rapid solidification upon eutectic formation of Sn-Ag and Ag-Cu alloys

The rapid solidification behavior of hypereutectic Sn–Ag and eutectic Ag–Cu alloys was systematically investigated using a 6.5 m drop-tube technique. Powders with sizes ranging from over 850 μm to below 38 μm were produced, corresponding to equivalent cooling rates of Sn–Ag between approximately 250 and 14,800 K·s⁻¹ and of Ag–Cu between approximately 470 and 28,400 K·s⁻¹, with undercoolings from 20 to 210 K for the Ag–Cu system.

For the hypereutectic Sn–Ag system, dendritic β-Sn was identified as the primary solidification phase at all cooling rates, rather than proeutectic Ag₃Sn as predicted by the equilibrium phase diagram. The volume fraction of the interdendritic eutectic decreased with increasing cooling rate, while the Ag concentration in the residual liquid was estimated to be 12.5–15 wt.%—significantly exceeding the equilibrium eutectic composition of 3.5 wt.% Ag. The eutectic exhibited a blocky, divorced morphology, which is explained by the sluggish nucleation of the Ag₃Sn intermetallic coupled with a metastable phase diagram that allows substantial supersaturation of Ag under rapid solidification conditions.

For the eutectic Ag–Cu alloy, rapid solidification experiments revealed the formation of coarse anomalous eutectic structures at the eutectic cell margins and a distinct anomalous region at the centers of droplets smaller than 300 μm. The high-undercooling anomalous eutectic was attributed to the growth of a single-phase partitionless solid, followed by binodal and spinodal decomposition during subsequent solid-state cooling. This interpretation accounts for all observed features in morphology, chemistry, and crystallographic orientation, as well as the thermodynamic behavior of the anomalous eutectic and its partitionless precursor. No evidence of remelting or fragmentation during recalescence was required to explain the observed microstructures.

Together, these findings provide a comprehensive understanding of rapid solidification and anomalous eutectic formation mechanisms in Sn–Ag and Ag–Cu alloy systems. The results not only highlight the influence of intermetallic nucleation kinetics and metastable phase equilibria on eutectic evolution but also extend the concept of partitionless solidification as a key pathway for anomalous structure formation under extreme undercooling conditions.

In addition, the nucleation kinetics framework provides valuable insight into the behavior of non-faceted–faceted (nf–f) alloy systems under rapid solidification and offers theoretical guidance for the rational control of intermetallic composition and growth, as exemplified by systems such as Al–Fe or Fe–C. Meanwhile, the concept of partitionless solidification presents an alternative mechanism for the formation of anomalous eutectics, particularly when solidification proceeds beyond the T₀ temperature. In such cases, anomalous structures in other alloy systems may originate from solid-state decomposition following thermodynamically driven partitionless solidification. These findings may therefore offer a plausible explanation for the occurrence of anomalous eutectic morphologies in other systems such as Ni–Sn or Ni–Si.