Solidification of Highly Undercooled Hypereutectic Ni-Ni 3 B Alloy Melt
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ation processing allows a melt to be undercooled below its equilibrium melting point. It is possible for a primarily metastable phase to solidify from a melt undercooled below the melting point of the metastable phase.[1] Such metastable phase ranges from metastable crystalline phases over supersaturated solid solutions and grain-refined alloys to disordered superlattice structures of intermetallic.[1–6] For eutectic alloys, some phenomenon such as a transition from regular lamellar eutectic colony to anomalous eutectic colony can take place with increasing undercooling.[7–11] Wei et al.[7] found that, for Co-Sb alloys, a transition from lamellar to anomalous eutectic takes place at 50 K (50 C) 340 K (340 C), the microstructure then transits from irregular eutectic to quasi-regular eutectic. Yang et al.[10] found that, for Fe83B17 alloy, if DT > 386 K (386 C), a metastable phase Fe3B remains in the as-solidified structure at room temperature. Leonhardt et al.[11] found that, for Ni-Nb alloy, if DT > 110 K (110 C), a metastable phase Ni3Nb occurs. As a classical eutectic alloy, Ni-Ni3B alloy acts generally as a fundamental material for studying the solidified mechanism of eutectic transformation, i.e., the transition from regular lamellar eutectic to anomalous eutectic with increasing undercooling.[12,13] In addition, phase selection, as one of the interesting solidification mechanisms, occurs upon solidification of Ni-Ni3B alloy.[12–17] On this basis, the solidification of undercooled hypoeutectic Ni-3.3 wt pct B alloy is revealed in Reference 13; i.e., the metastable eutectic reaction occurs if the undercooling of eutectic solidification is larger than a critical undercooling [~140 K (140 C)]. However, experimental data for rapid solidification of hypereutectic Ni-Ni3B alloys are still lacking, and effects of undercoolings on the solidification behavior are not yet clear. Since the eutectic composition is about Ni-3.7 wt pct B, in the present work, Ni-4.5 wt pct B alloy is chosen to study the solidification behaviors of hypereutectic Ni-Ni3B alloys. The characteristics of the cooling curves, microstructures, high-speed video (HSV) images, and growth velocities of solidification fronts are revealed. Especially, the microstructure at high undercooling is shown.
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EXPERIMENTS
To investigate the solidification of Ni-Ni3B alloy, 100 g hypereutectic alloy, mixed by high-purity Ni (99.98 wt pct) blocks and B (99.7 wt pct) powder, was arc melted in argon METALLURGICAL AND MATERIALS TRANSACTIONS A
atmosphere using a nonconsumable tungsten electrode. This operation was repeated several times to ensure the homogenization of the sample. Then the concentration of the sample was measured to be Ni-4.5 wt pct B alloy by the inductively coupled plasma–atomic emission spectroscopy (ICP-AES) technique, location in State Key Laboratory of Solidification Processing, Northwestern Polytechnic University, Xi’an, China. The experimental process is shown in Figure 1, where the sample (Ni-4.5 wt pct B), weighing about 4 g, together with some pi
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