Anomalous eutectic formation in undercooled melts: Quantitative analysis

Quantitative electron microscopy has been used to map the distribution of anomalous eutectic within droplets of Ag-Cu eutectic alloy undercooled by 10-60 K in order to determine the volume fraction of the anomalous eutectic as a function of distance from the initial nucleation point and undercooling. The distribution of anomalous material is found to be very heterogeneous, with the maximum fraction of anomalous material being found near the nucleation point. More significantly, the results categorically disprove remelting as an origin for the anomalous eutectic, with the volume fraction of anomalous material significantly exceeding the maximum fraction solid that could have formed during the recalescence stage of solidification. To illustrate this, at T = 19 K we would expect the solid fraction formed during recalescence to be of the order of 4% whereas close to the nucleation site we find the volume fraction of anomalous eutectic to be 39.6%.