Morphology of Spherulites in Rapidly Solidified Ni3Ge Droplets

Abstract

Abstract:The congruently melting, single phase, L1 2 intermetallic β-Ni 3 Ge has been subject to rapid solidification via drop-tube processing. Four different cooling rates are used in this process, at very low cooling rates (≥850 µm diameter particles, ≥700 K s −1 ) and slightly higher cooling rates (850-500 µm diameter particles, 700-1386 K s −1 ) the dominant solidification morphology, revealed after etching, is that of isolated spherulites in an otherwise featureless matrix. At higher cooling rates, (500-300 µm diameter particles, 1386-2790 K s −1 and (300-212 µm diameter particles, 2790-4600 K s −1 ) mixed spherulite and dendritic morphologies are observed. Indeed, at the higher cooling rate dendrites with side-branches composed of numerous small spherulites are observed. Selected area diffraction analysis in the TEM indicate that the formation of spherulites is due to an order-disorder transformation. Dark-field TEM imaging has confirmed that the spherulites appear to consist of lamellae of the ordered phase, with disordered material in the space between the lamellae. The lamellar width within a given spherulite is constant, but the width is a function of cooling rate, with higher cooling rates giving finer lamellae. As such, there are many parallels with spherulite growth in polymers.

Mechanical Properties of Rapidly Solidified Ni5Ge3 Intermetallic

“…We conclude that the contrast displayed by etching is because of incomplete chemical ordering, causing the etchant to attack the disordered material and leaving the ordered material unaffected. The lower chemical resistance of the disordered phase has resulted in this type of differential etching between ordered and disordered material to be observed previously in other intermetallic compounds [11,13]. The maximum hardness, 824 Hv0.05, was observed in smallest size drop-tube sample (212 -150 m diameter particles).…”

Evidence for dendritic fragmentation in as-solidified samples of deeply undercooled melts

“…EBSD does not appear to be able to distinguish between the ordered and disordered variants of the -Ni3Ge compound in these samples, this seemingly being a consequence of the additional lines in the resulting Kikuchi pattern being relatively weak, coupled with the small spatial extent of the ordered features in these samples. In fact, we have shown elsewhere [43] that the ordered regions are only consistently distinguished from the disordered regions once their projection area approaches 300 m 2 or greater. The previous XRD analysis, showing that all samples are single-phase -Ni3Ge is confirmed by the EBSD phase maps (not shown).…”

“…In fact, this is exactly what is observed. For sieve fraction between 850-300 m non-dendritic solidification morphologies are observed, with the dominant morphology being spherulites, as discussed in [43]. Dendritic fragmentation (pl > br) is predicted for droplets > 150 m in diameter, but for droplets > 300 m in diameter growth to the ordered L12 phase is predicted as insufficient undercooling is attained for disordered growth.…”

Order–Disorder Morphologies in Rapidly Solidified Ni3Ge Intermetallic

“…A reliable description of the solidification dynamics of Ni3Ge can be assembled from the SEM, TEM [13,17,18] For the spherulite morphology the TEM evidence is that the 'featureless' matrix is, as per the other morphologies, fully ordered, but the spherulites themselves are only partially disordered, consisting of alternating filaments of ordered and disordered material. The relationship between undercooling and chemical ordering was explored in detail for the Ni3Ge compound by Ahmad et al [19], with it being determined the fully disordered growth occurred for T > 168 K, this being evident by a rapid increase in growth velocity with undercooling above T = 168 K. Unfortunately, direct measurement of undercooling during drop-tube processing is not possible, although it would be expected that smaller droplets would experience higher undercooling, both because of their higher cooling rate and because of the melt sub-division effect.…”