The origin of anomalous eutectic structures in undercooled Ag–Cu alloy

Abstract

A melt encasement (fluxing) method has been used to undercool Ag-Cu alloy at its eutectic composition. The recalescence of the undercooled alloy has been filmed at high frame rate. For undercoolings < 60 K a microstructure consisting of mixed anomalous and lamellar eutectic is observed. Analysis of eutectic spacing in the lamellar eutectic reveals little dependence upon the undercooling of the bulk melt and is consistent with growth at an undercooling of 1.5 K.Depending upon undercooling, the progress of the recalescence front may be either continuous or spasmodic, wherein periods of rapid growth are separated by significant interludes in which growth totally arrests. Analysis of spot brightness profiles reveals that during continuous growth the recalescence is characteristic of the advancement of a planar, space-filling, front while during spasmodic growth a double recalescence occurs, the first of which is characteristic of the propagation of a dendritic, or non-space-filling, front. We conclude that during spasmodic growth we observe the propagation of two-phase, or eutectic, dendrites which subsequently remelt to form the anomalous eutectic, while the lamellar eutectic grows during post-recalescence cooling.

Phase Selection during Solidification of Ni-10.95 at. % and B-3.23 at. % Si Alloy

“…This result agrees with the findings in previous works, where it was observed that the volume fraction of the anomalous eutectic phase increases when undercooling of the eutectic reaction increases in the binary Ni-B system [23] and in other eutectic systems in binary alloys, such as Ag-Cu [29,30] and Ni-P [30]. An important feature of the R1 s -R2 s reaction sequence observed at high eutectic undercooling is that R2 s presents a high recalescence: 15 • C and 60 • C in the DSC and CCT experiments, respectively.…”

Microstructural Evolution and Phase Formation in Rapidly Solidified Ni-25.3 At. Pct Si Alloy

“…These have the same phase constitution, 1 -Ni 3 Si and Ni 25 Si 9 , but are morphologically distinct with (I) being a regular lamellar structure and (II) being a much more disordered eutectic structure. In fact it is clear that the structure II has the features of an anomalous eutectic, a structure that is widely observed [36,37] in the solidification of eutectic alloys from their undercooled parent melt, wherein the distinct lamellar morphology of a regular eutectic cannot be observed. Many studies [36,37] have indicated that the fraction of anomalous eutectic structure increases with increasing undercooling.…”

“…In fact it is clear that the structure II has the features of an anomalous eutectic, a structure that is widely observed [36,37] in the solidification of eutectic alloys from their undercooled parent melt, wherein the distinct lamellar morphology of a regular eutectic cannot be observed. Many studies [36,37] have indicated that the fraction of anomalous eutectic structure increases with increasing undercooling. The structure II can be observed in all ranges of type B droplet, while the fraction of the regular structure (I) presents a decreasing trend with decreasing particle size and has almost disappeared in all of the 75-106 m droplets.…”

“…Ag-Cu [37] and Ni-Sn [38], the coupled eutectic growth will break down in favour of single phase growth if the liquid experiences sufficiently high undercooling. According to the metastable phase diagram (Fig.…”

Effect of Sb Addition on the Solidification of Deeply Undercooled Ag-28.1 wt. % Cu Eutectic Alloy

“…With the increase of undercooling, the solidification rate increases and the solidification structures can be composed of a mixture of lamellar eutectics and anomalous eutectics or full anomalous eutectics [4][5][6][7][8]. Furthermore, the rapidly solidified alloys can have a series of good properties, such as high strength and toughness, high temperature creep resistance, electromagnetism and corrosion resistance, etc., because the properties of the as-solidified materials are determined by the conditions under which the liquid solidifies.…”