Rapid solidification morphologies in Ni 3 Ge: Spherulites, dendrites and dense-branched fractal structures

Abstract

Single-phase -Ni3Ge has been rapidly solidified via drop-tube processing. At low cooling rates (850 -300 m diameter particles, 700 -2800 K s -1 ) the dominant solidification morphology, revealed after etching, is that of isolated spherulites in an otherwise featureless matrix. At higher cooling rates (300 -75 m diameter particles, 2800 -25000 K s -1 ) the dominant solidification morphology is that of dendrites, again imbedded within a featureless matrix. As the cooling rate increases towards the higher end of this range the dendrites display non-orthogonal side-branching and tip splitting. At the highest cooling rates studied (< 75 m diameter particles, > 25000 K s -1 ), dense-branched fractal structures are observed. Selected area diffraction analysis in the TEM reveals the spherulites and dendrites are a disordered variant of -Ni3Ge, whilst the featureless matrix is the ordered variant of the same compound. We postulate that the spherulites and dendrites are the rapid solidification morphology and that the ordered, featureless matrix grew more slowly post-recalescence. Spherulites are most likely the result of kinetically limited growth, switching to thermal dendrites as the growth velocity increases. It is extremely uncommon to observe such a wide range of morphologies as a function of cooling rate in a single material.

The Role of Recrystallization in Spontaneous Grain Refinement of Rapidly Solidified Ni3Ge

“…[35] Nash and Nash [35] estimated that for drop-tube processed Ag 28.1 Cu 41.4 Ge 30.5 alloy, a maximum undercooling of DT = 168 K (168°C) (the undercooling at which Ahmad et al [25] observed a transition to fully disordered growth in b-Ni 3 Ge) would be achieved for droplets <280 lm in diameter. Interestingly, this is close to the droplet size (300 to 212 lm) at which Haque et al [26] first observed the formation of dendrites in their samples, an occurrence they associated with the transition to disordered growth. For droplets in the 53 to 75 lm diameter range undercoolings approaching 400 K (400°C) are estimated, which would exceed the maximum undercooling achieved by Nash and Nash.…”

“…Numerous crack-like features are apparent, although no such features may be observed prior to etching, so that despite their appearance the features are not therefore cracks. It has been shown previously [26] Fig. 1-Estimated cooling rate of droplets cooled in N 2 as a function of their diameter.…”

“…It has previously been established [26] that etching in this material distinguishes between the ordered and disordered material. A selected area diffraction pattern (Figure 7) confirms that the large blocky areas between the 'cracks' are indeed of the L1 2 ordered structure.…”

“…Rapid solidification of Ni 3 Ge has previously been studied by Ahmad et al [25] in deeply undercooled samples and by Haque et al [26] in rapidly cooled samples. Ahmad et al observed only single phase b-Ni 3 Ge at all undercoolings, up to the maximum undercooling achieved of 362 K (362°C), wherein the measured growth velocity was 3.55 m s À1 .…”

Mechanical Properties of Rapidly Solidified Ni5Ge3 Intermetallic

“…Samples were prepared for TEM analysis using a FEI Nova 200 Nanolab focused ion beam (FIB), with the sections cut being approximately 10 m  7 µm and between 55-70 nm in thickness. An example of a FIB milled section for the related -Ni 3 Ge compound is shown in [7]. For measurement of micro-hardness of the drop-tube samples a TUKONTM 1202 Wilson Hardness (micro-Vickers) test rig was used with 0.05 kg load.…”