Phase selection during solidification of undercooled bulk Bi 95 Sb 5 melts
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Phase selection during solidification of undercooled bulk Bi95Sb5 melts Z. Zhou,a) J. Zhao, and W. Wang Institute of Physics, Chinese Academy of Sciences, P.O. Box 603(34), Beijing 100080, People’s Republic of China (Received 21 May 1999; accepted 13 October 1999)
Relationships between melt heat treatment and undercooling of alloy melts were clarified with Bi95Sb5 through four-factor, three-level orthogonal experiments. The results show that the cooling rate plays the most important role in the undercooling of Bi95Sb5 alloy melts. Undercooling as large as 121 K was obtained in the bulk Bi95Sb5 alloy melt. It is presently the highest undercooling of this alloy system. A metastable phase with tetragonal structure was found in the Bi95Sb5 alloy with undercooling of 121 K.
It is well known that most alloy melts can be undercooled below their equilibrium melting or liquidus temperature before they crystallize into solids. The metastable nature of the undercooled alloy melts provides the possibility of the formation of various kinds of metastable phases. Since many of our modern highperformance materials such as steel products and other alloys are in the form of the metastable state rather than their equilibrium stable state, the understanding of the phase selection during the solidification process is very important. Thermodynamically we know that the metastable phase can be formed only when the melts are undercooled below their equilibrium melting temperature.1–3 So the undercooling of alloy melt plays an important role in the formation of metastable phases. There are many kinds of methods to make alloy melts undercooled, such as the dispersion technique,4 drop tube and drop tower technique,5–7 flux technique,8 levitation technique,9 rapid solidification technique,10 etc. All these methods have a common point, i.e., to avoid the occurrence of heterogeneous nucleation during the cooling process of alloy melts. Generally the undercooling phenomena of the alloy melt can be attributed basically to two reasons: one is the outer factor, i.e., the nucleation behavior—homogeneous or heterogeneous. The other is the inner factor, i.e., the structure relaxation and homogeneity of the melt. The above mentioned methods can mainly be classified into the former one. From experimental views, it was found that appropriate heat treatment of alloy melts could lead to large supercooling of alloy melts before they solidify,11,12 and these could bring significant influence both to the microstructures a)
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J. Mater. Res., Vol. 15, No. 1, Jan 2000 Downloaded: 19 Mar 2015
and to the physical properties of the alloy. This can be attributed to the structure adjustment and homogenization of the melt during the heat treatment process. After appropriate heat treatment, the alloy melt becomes more homogeneous, and the impurities or catalysts can be removed by thawing or vaporizing, which can eliminate the effect of nucleation from the im
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