Rapid directional solidification from highly undercooled Cu-5wt.%Ni alloy melts

Abstract

For decades solidification of highly undercooled alloy melt has been widely studied [1–6]. The study of that is meaningful for two reasons. First, high undercooling (1T ) before solidification can lead to rapid solidification once the melt nucleates, which results in refined microstructures and improved properties. Second, the relative slowly cooled but highly undercooled melt opens up the possibilities of direct observations of the rapid solidification processes under nonequilibrium conditions, which is important both to the consummation of rapid solidification theory and to the optimum of production techniques. If the undercooling of alloy melts and directional solidification technology are combined organically, we will obtain a simple solidification model of rapid solidification of highly undercooled alloy melt. That is valuable both in the quantitative research of solidification of highly undercooled melts and in the production of materials which require aligned crystal orientation with much less time and expenses. Tarshis et al. [7] have studied the microstructures of Cu-Ni alloy solidified at different initial undercoolings. They found that coarse and dendritic microstructures could exist in an undercooling range, from 85 K to 150 K. Beyond this undercooling range equiaxed structures were found. Lux et al. [8] demonstrated the feasibility of producing superalloy castings having directional microstructures from undercooled melt. Ludwig et al. [9] developed a so-called autonomous directional solidification technology and obtained single crystal superalloy turbine blades from undercooled superalloy CMSX-6 melt. Schwarz et al. [10] proposed a mechanism which described the observed transitions in solidification of undercooled melts from a coarse grained dendritic to a grain refined equiaxed microstructure. It was thought that the refinement was caused by remelting and coarsening of primary formed dendrites. Research team of DLR [11, 12] had done much work on the triggered rapid solidification of highly undercooled electromagnetically levitated alloy melt. They observed the microstructures which grew radially from the trigger point. Kiminami and Sahm [13] had studied the undercooling and subsequent solidification of Pd77.5Cu6Si16.5 alloy melt. Also they obtained the radial microstructures which grew from the nucleation point. However, detailed studies about the directional solidification from highly undercooled melts are still scarce. In this paper, directional solidification and rapid solidification from highly undercooled alloy melt are combined using a Cu-5wt.%Ni alloy in a set of selfmade experimental apparatus, therefore, rapid directional solidification from highly undercooled alloy melts are realized, and expected directional solidified structures are obtained. The experiments were divided into two stages. First, undercooling of Cu-5wt.%Ni alloy melt; second, the rapid directional solidification of highly undercooled Cu-5wt%Ni alloy. The 810 mm Cu-5wt %Ni master alloy ingots were prepared from 99.972% Cu and 99.99% Ni in an induction furnace using a graphite crucible under vacuum, then the ingots were cut into pieces with cylindric shape weighing about 5 g (the surface layers of the ingots were cut off in advance). Fig. 1a shows the apparatus used during the first experimental stage. A procedure to get high undercooling, which has been employed effectively in a number of investigations [4–6], was applied by superheating and cooling the molten metal several times whether immersed in a molten high purity B2O3 glass or not (both in a high purity fused silica tube). High frequency induction melting technology was employed here. When the power was off, the sample began to cool and reached its maximum undercooling. The temperature was measured with an infrared thermometer with precision of ±1.0%. Before measurement it was calibrated by standand WRe3-WRe25 thermocouples under the similar experimental conditions as the subsequent undercooling and directional solidification experiments. Fig. 1b shows a sketch of the self-made apparatus used during the directional solidification experiments of undercooled Cu-5wt.%Ni alloy melts. The sample obtained during the first experimental stage was remelted in a funnel-like quartz crucible with 8 4 mm in diameter and formed a liquid cylinder. Because of the electromagnetic levitation force and the surface tension the melt could be held in the crucible. Then when the melt was undercooled to a predetermined undercooling, liquid Ga-In-Sn alloy (it was at room temperature) was used to nucleate the undercooled melt. Then rapid directional solidification would occur in the undercooled melt. The actual undercooling was determined according to the recorded cooling curve. Solidified samples