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alloy. The G values measured 13,500 + 500 K / m , were observed to be independent of the growth speed R for the range of experimental conditions investigated. The primary dendrite arm spacing, d, was measured (an average of about 100 dendrites) on photomicrographs taken on sections transverse to the crucible withdrawal direction. Figure 1 shows the change in microstructure of the

Dendrite Spacing in a Directionally Solidified Superalloy S. N. T E W A R I A N D A. M. S R I R A M A M U R T H Y Directionally solidified (DS) superalloys are presently being extensively used as turbine blade and vane components in advanced aeroengines. Mechanical property of these alloys depends on the microstructure whxch is controlled by the various solidification parameters. In this communication the dependence of the primary dendrite spacing (d) on the directional solidification speed (R) in a nickel base superalloy IN-100 ~is presented. For a given alloy composition the dendrite spacing is known to be determined by R and G, the temperature gradient in the liquid at the solid liquid interface. 2 Following are some of the relationships reported in literature by which R and G have been observed to control the primary dendrite spacing. 3-6 d =A

exp B l n ( R G ) + D

d =A(RG)

(a)

[1]

[2]

-~ + B

d = A ( R G ) -~

[3]

d = AR-~

[4]

Hunt 6 has recently analyzed the relationship between d and R from theoretical consideration and predicted the relationship given in Eq. [4] above. He contended that due to experimental scatter and very small range of change in velocity, R , used by previous investigators other emperical relationships (Eqs. [1], [2], [3]) have been shown to be valid. However the same data show equally good a fit to the relationship derived by Hunt. In this investigation investment cast cylindrical bars of IN-100 were remelted and directionally solidified in recrystallized alumina crucible in a modified Bridgman type apparatus. 7 Directional solidification in flowing argon atmosphere was carried out by withdrawing the cylindrical alumina crucible containing the molten alloy through a water-cooled copper radiation-cooling ring at predetermined speeds varying from 3.25 • 10 -6 t o 267 • 10 -6 m/s. DS rods of 0.012 m diam and 0.1 m length were obtained. The temperature gradients, G, were measured using a 0.0002 m diam pt/pt-13 pct Rh thermocouple, protected by 0.001 m thick walled alumina sheath, by immersing it in the alloy melt and letting it traverse along with the melt during directional solidification. Thermocouple outputs were recorded on a strip chart recorder. The temperture profiles were smooth and the temperature gradients were measured by drawing tangents at the liquidus temperature of the S. N. TEWARI and A. M. SRIRAMAMURTHY are Scientists, Defence Metallurgical Research Lab, P.O. Kanchanbagh, Hyderabad, India. Manuscript submitted May 7, 1980. METALLURGICAL TRANSACTIONS A

(b)

(c) Fig. 1--Effect of growth speed on the transverse,microstructure of DSIN-100.(a) 11 x 10-6 m/s, (b) 27.2 • 10 6m/s,(c)267 x 10-6 m/