Title:
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Microstructural evolution and mechanical properties of deeply undercooled eutectic alloys
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An experimental investigation has been undertaken in an attempt to determine the
mechanism of formation of anomalous eutectics. A melt fluxing method and a drop
tube technique have been used to undercool eutectic Ag-Cu alloy. During the
fluxing experiments, the growth fronts of the undercooled alloy samples have been
monitored using high speed camera imaging. The evolved microstructures of the
fluxed samples undercooled ≤ 60 K exhibit a trizonal structure consisting of mixed
anomalous and lamellar eutectic. The high speed camera imaging reveals that the
growth front propagates in a spasmodic manner, where periods of rapid growth are
separated by significant intervals in which growth totally arrests. Depending upon
undercooling, growth is either continuous or spasmodic. Continuous growth is
characteristic of the advancement of a planar front, while during spasmodic growth
a double recalescence occurs, the first of which is characteristic of the propagation
of a dendritic front. The microstructure of drop tube processed Ag-Cu samples
comprises of a mixture of lamellar and anomalous eutectic structures and a silver-rich
phase, which appears as spherical inclusions at the eutectic cell boundaries. It
is concluded that, during spasmodic growth, the propagation of eutectic dendrites is
observed, which subsequently remelt to form the anomalous eutectic, while the
lamellar eutectic grows during post-recalescence cooling. It is also postulated that
this eutectic dendrite may be growing away from the eutectic point at high growth
rates, which could lead to silver building up ahead of the interface that, in the drop
tube samples, leads to the formation of the silver-rich phase observed at the cell
boundaries. In the fluxed samples, as the silver builds up, the dendrite is no longer
viable and growth arrests until sufficient silver atoms have diffused into the bulk
liquid for the transient growth cycle to restart.
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