Abstract
Al-Ni alloys (for Ni < 45 at.%) show a unique property in that, over at least part of the accessible undercooling range, the recalescence velocity measured in electromagnetically levitated samples is observed to decrease as the undercooling increases. This result has been subject to careful validation, including microgravity experiments utilising the TEMPUS levitation facility on-board the International Space Station (ISS). In these experiments, anomalous growth is observed to coincide with a recalescence morphology comprising multiple circular growth fronts [Herlach et al. Phys. Rev. Mat. 3, 073,402 (2019)], termed “scales”. In this paper we present an analysis of high speed video data from the ISS experiments in which we show that such scale-like growth is consistent with a recalescence front that is initially confined to a thin layer on the surface of the sample. This then nucleates a slower, radial inward growth, which is consistent with microstructures observed in Al-Ni droplets. We show that such surface recalescence would be favoured for samples which were surface enriched in Ni, wherein the recalescence velocity (at fixed nucleation temperature) increases rapidly with Ni-concentration. Moreover, it is shown that the anomalous velocity behaviour can be matched in all compositions studied if the surface enhancement in Ni is a linear function of the nucleation temperature with a gradient of 0.03 at.% K−1. Analysis of historical results from the literature indicates that such surface Ni-enhancement may have been present, but overlooked, in other experiments on Al-rich Al-Ni droplets.
Highlights
The solidification behaviour of deeply undercooled melts and the associated relationship between recalescence velocity and undercooling has been extensively studied for more than 70 years
For melts solidifying to a dendritic morphology, the velocityundercooling relationship is generally found to be of the form V ∝ ΔTβ (e.g. Wilnecker et al 1989; Battersby et al 2000; Dragnevski et al 2004), where V is growth velocity
Once initiated, the scales persist throughout the rest of the growth period, as shown in part (b) of the figure which shows the morphology of the recalescence front 36 frames ( = 0.55 r ) after nucleation of solidification
Summary
The solidification behaviour of deeply undercooled melts and the associated relationship between recalescence (or crystal growth) velocity and undercooling has been extensively studied for more than 70 years. Strong glass forming melts have been predicted to show such behaviour at very high undercooling (Orava and Greer 2014) In such melts the effect of decreasing atomic mobility as the nucleation temperature drops dominates the increase in thermodynamic driving force for crystal growth, wherein a maximum in the growth velocity is predicted. Further increases in undercooling lead to decreasing growth velocity Such behaviour has been observed in Cu-Zr (Wang et al 2014) and Ni-Zr (Kobold et al 2017) melts. A convincing explanation for the behaviour has been lacking
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