Liquid Ni-31.7%Sn-2.5%Ge alloy was highly undercooled by up to 238 K (0.17T L ) with glass fluxing and drop tube techniques. The dendritic growth velocity of primary Ni 3 Sn compound shows a power-law relation to undercooling and achieves a maximum velocity of 380 mm/s. The addition of Ge reduces its growth velocity as compared with the binary Ni 75 Sn 25 alloy. A structural transition from coarse dendrites into equiaxed grains occurs once undercooling exceeds a critical value of about 125 K, which is accompanied by both grain refinement and solute trapping. The Ni 3 Sn intermetallic compound behaves like a normal solid solution phase showing nonfaceted growth during rapid solidification. Intermetallic compounds have developed into an important category of structural materials with potential high temperature applications. Since their microstructural morphology and solute distribution are intrinsically correlated with mechanical properties, extensive work has been done on the directional and rapid solidification processes of intermetallic alloy [1][2][3][4][5][6]. However, most previous investigations were concentrated on the microstructural evolution at small undercooling condition. If an intermetallic compound nucleates and grows from a highly undercooled alloy melt, its formation mechanism will display novel kinetic characteristics because of the extremely nonequilibrium thermodynamic state. During conventional solidification, the heterogeneous nucleation caused by the crucible or mold walls prevents liquid alloys from achieving large undercoolings. Fortunately, glass fluxing [5] method and drop tube processing [1] provide an effective way to eliminate heterogeneous nuclei and subsequently undercool liquid alloys to a great extent. The Ni 3 Sn intermetallic compound has attracted much attention in research because it shows an anomalous increase in yield stress with rising temperature like the Ni 3 Al alloy [2,[7][8][9]. The Ni 3 Sn intermetallic compound has a cubic BiF 3 -type(DO 3 ) crystalline structure above 1193.5 K and transforms into an ordered hexagonal Mg 3 Cd-type (DO 19 ) structure at lower temperatures. Meanwhile, Ni 3 Snbased alloys usually exhibit satisfactory undercoolability to realize bulk rapid solidification. The objective of this paper is to investigate the dendritic growth mechanism and solute trapping effect of the Ni 3 Sn intermetallic compound during the rapid solidification of the highly undercooled ternary Ni-Sn-Ge alloy. Both glass fluxing and drop tube techniques have been applied to achieve high undercoolings.
The phase separation and dendritic growth characteristics of undercooled liquid Fe62.5Cu27.5Sn10 alloy have been investigated by glass fluxing and drop tube techniques. Three critical bulk undercoolings of microstructure evolution are experimentally determined as 7, 65, and 142 K. Equilibrium peritectic solidification proceeds in the small undercooling regime below 7 K. Metastable liquid phase separation takes place if bulk undercooling increases above 65 K. Remarkable macroscopic phase separation is induced providing that bulk undercooling overtakes the third threshold of 142 K. With the continuous increase of bulk undercooling, the solidified microstructure initially appears as well-branched dendrites, then displays microscale segregation morphology, and finally evolves into macrosegregation patterns. If alloy undercooling is smaller than 142 K, the dendritic growth velocity of γFe phase varies with undercooling according to a power function relationship. Once bulk undercooling exceeds 142 K, its dendritic growth velocity increases exponentially with undercooling, which reaches 30.4 m/s at the maximum undercooling of 360 K (0.21TL). As a comparative study, the liquid phase separation of Fe62.5Cu27.5Sn10 alloy droplets is also explored under the free fall condition. Theoretical calculations reveal that the thermal and solutal Marangoni migrations are the dynamic mechanisms responsible for the development of core-shell structure.
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