Abstract
The congruently melting, single phase, L12 intermetallic β-Ni3Ge 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.
Highlights
Intermetallic compounds are characterised by a high degree of chemical ordering and mixed covalent/ionic and metallic bonding, which gives rise to mechanical behaviour intermediate between ceramics and metals
Poor room temperature ductility limits formability, this can be increased by rapid solidification processing, wherein a reduction of the degree of chemical order and the formation of a fine pattern of antiphase domains (APDs) increases ductility [2,3,4]
In these the spherulites are the only morphology observed in range
Summary
Intermetallic compounds are characterised by a high degree of chemical ordering and mixed covalent/ionic and metallic bonding, which gives rise to mechanical behaviour intermediate between ceramics and metals. Due to these characteristics they can display desirable magnetic, superconducting and chemical properties [1]. Poor room temperature ductility limits formability, this can be increased by rapid solidification processing, wherein a reduction of the degree of chemical order and the formation of a fine pattern of antiphase domains (APDs) increases ductility [2,3,4]. We present here an analysis of the rapid solidification of β-Ni3 Ge, a congruently melting, single phase intermetallic having a melting point of 1405 K and a homogeneity range of 22.5 to 25 at %
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