On morphologies, microsegregation, and mechanical behavior of directionally solidified cobalt-base superalloy at medium
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I.
INTRODUCTION
YEARS of research have led to the belief that mechanical properties of directionally solidified alloys can be significantly improved by increasing cooling rate and refining the dendrite structure. I1"2"31However, because of limited laboratory measures at present, investigations on directional-solidification technology are generally limited to the condition of low-temperature gradient ( 120 /xm, A2 > 40 /zm) of those obtained by conventional high rate solidification (HRS) directional-solidification processes with the temperature gradient and growth velocity being about 50 K/cm and 300/xm/s, respectively. This structural change of cobalt-base superalloy K10 may be attributed to the high-temperature gradient and the rapid directional-solidification speed in the ZMLMC directional-solidification process.
The principle figure of the ZMLMC directionalsolidification apparatus shows that the result of induction-forced heating makes the solid/liquid interface stay steadily a little above the upper surface of the cooling alloy in the whole process, ensuring the heat-flow direction on the interface of liquid and solid to be coincident with the direction of "drawing and pulling." In addition, the high-temperature gradient is beneficial to acquiring the straight colunmar dendrite structure, tS~ In this experiment, the cooling rate of directional solidification is hundreds of times higher than that of the traditional directional-solidification process, resulting in the highly refined dendrite structures. Meanwhile, since the development of the side branches and coarsening process needs time and space, on the contrary, the rapid solidification rate shortens the local solidification time and decreases the length and width of the mushy zone;t9] therefore, the development of side branches is limited, and the coarsening process does not have enough time to proceed. Table II shows dendrite-arm spacings of directionally solidified K10 at the different cooling rate. The primary and secondary dendrite-arm spacings decrease with the increase of the cooling rate (G. V) of directional solidification (Figure 3), which can be expressed as follows: Al = 1.428 • 103 (G" V) -I
[1]
0.312 x 10 3 (G" V)-1
[2]
/~2 =
where A~ is the primary dendrite-arm spacing and A2 is the secondary one, and G and V are the temperature gradient and solidification rate, respectively. The relationship between the primary and secondary arm spacings and the cooling rate are decided synthetically by alloy properties and solidification parameters. In this experiment, the rather steep temperature gradients and high-solidification rates, resulting in a short local solidification time (0, produce very small initial perturbations and primary spacings and, furthermore, prevent dendrite coarsening. As a result, the dendrite spacings are very small and decrease with the decrease of the
Fig. 2--Directional-solidification microstructures of K10 superalloy at medium cooling rates. (a) V = 558/xm/s, G~ = 806.5 k/cm; (b) V = 870/zm/s, G~ = 763.1 k/cm; and (c)
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