Abstract
A comparative study of rapid solidification of Ag-Cu eutectic alloy processed via melt fluxing and drop-tube techniques is presented. A computational model is used to estimate the cooling rate and undercooling of the free fall droplets as this cannot be determined directly. SEM micrographs show that both materials consist of lamellar and anomalous eutectic structures. However, below the critical undercooling the morphologies of each are different in respect of the distribution and volume of anomalous eutectic. The anomalous eutectic in flux- undercooled samples preferentially forms at cell boundaries around the lamellar eutectic in the cell body. In drop-tube processed samples it tends to distribute randomly inside the droplets and at much smaller volume fractions. That the formation of the anomalous eutectic can, at least in part, be suppressed in the drop-tube is strongly suggestive that the formation of anomalous eutectic occurs via remelting process, which is suppressed by rapid cooling during solidification.
Highlights
Ag-Cu eutectic alloys have been widely studied for their remarkable strength and conductivity under both near-equilibrium and rapid solidification conditions
High undercoolings may be achieved after thermal cycling to remove possible impurities attached on the melt surface, but during post-recalescence solidification the highest cooling rate achieved is typically no more than 10 K s-1 [2], leading to significant post-recalescence modification of the original solidification structure [3]
In samples processed by melt fluxing there is extensive evidence for anomalous eutectic formation, as shown in figure 2(a), in which the microstructure is typically cellular, with cells consisting of lamellar eutectic in the cell body and anomalous eutectic occurring at the cell boundaries
Summary
Ag-Cu eutectic alloys have been widely studied for their remarkable strength and conductivity under both near-equilibrium and rapid solidification conditions. Various containerless processing techniques, such as splat quenching, drop tube processing, electromagnetic levitation and melt fluxing have been used to achieve high undercooling in Ag-Cu droplets [1]. High undercoolings may be achieved after thermal cycling to remove possible impurities attached on the melt surface, but during post-recalescence solidification the highest cooling rate achieved is typically no more than 10 K s-1 [2], leading to significant post-recalescence modification of the original solidification structure [3]. In drop-tube processing, a range of cooling rates from 103 to 106 K s-1 is achieved via the production of a fine spray of free falling droplets of varying sizes [4]. An indirect method of estimating these quantities from the size of the as-solidified droplet may be employed, details of which are given in [1, 5]
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