Direct observation of dendrite fragmentation in the solidification of undercooled melts

Andrew M Mullis,Nafisul Haque

doi:10.1088/1757-899x/529/1/012020

Andrew M Mullis, Nafisul Haque

Open Access

https://doi.org/10.1088/1757-899x/529/1/012020

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Abstract

The fragmentation of dendrites immediately following the recalescence phase of growth during the solidification of undercooled melts has been invoked to explain various rapid solidification microstructures. Despite this, little direct evidence of such a fragmentation process usually survives in the as-solidified material. We report on the rapid solidification of the single phase, congruently melting intermetallic β-Ni3Ge. During equilibrium solidification this material solidifies to the chemically ordered L12 crystal structure. Conversely, during rapid solidification, disorder trapping results in solidification to a random fcc solid solution, thereby providing a means to distinguish the rapidly solidified structures. We present results which show a range of microstructures in which the dendrite fragmentation process has been captured in progress. Results from EBSD Euler mapping reveal that dendrite fragmentation is a potential, but not particularly efficient, route to grain refinement.

Highlights

Spontaneous grain refinement (SGR) during the solidification of deeply undercooled melts has been a subject of enduring interest within rapid solidification
Little direct evidence of such a fragmentation process usually survives in the as-solidified material
We report on the rapid solidification of the single phase, congruently melting intermetallic -Ni3Ge

Summary

Introduction

Spontaneous grain refinement (SGR) during the solidification of deeply undercooled melts has been a subject of enduring interest within rapid solidification. It is for instance difficult to see how remelting, which by definition must occur post-recalescence, can give rise to either a break in the velocity-undercooling curve or a change in the morphology of the solidification front That both occur concurrently with the onset of SGR must be ascribed to coincidence. Direct experimental evidence of dendrite fragmentation in rapidly solidified metals has been extremely difficult to obtain as any indication of such fragmentation is usually obscured in the final, as-solidified sample Such remelting has been predicted by [18] for 250 m droplets of Al-10 wt% Cu alloy, wherein it was found that the fraction of the recalescence solid subject to remelting was a strong function of the nucleation undercooling, with high undercooling leading to greater remelting. We report on drop-tube experiments upon the -Ni3Ge compound

Experimental Methods

Results and Discussion

Summary and Conclusions