The influence of cobalt, tantalum, and tungsten on the elevated temperature mechanical properties of single crystal nick
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I.
Table I.
INTRODUCTION
TRENDSin nickel-base superalloy development have resuited in alloys with as many as a dozen elemental additions, each with its own purpose, to produce a balance of good mechanical properties and environmental resistance. ~ Some recent investigations have involved the roles of strategic materials such as Co 2'3'4 and Ta 5 in a number of superalloys. The present investigation is directed toward examining the influence of both Co and Ta on the elevated temperature tensile and creep-rupture properties of nickel-base superalloy single crystals. These properties will be discussed in relation to the observed microstructural features which were presented in the previous paper.6 Among the microstructural variables considered were the volume fraction, distribution, morphology, and coarsening rate of the y ' phase, the y - y ' lattice mismatch, and the composition of both y and 3".
II.
MATERIALS AND PROCEDURES
Table I presents the nominal concentrations of the intentionally varied elements for the eight single crystal compositions. A more complete description of the compositions, heat treatments, and microstructures can be found in the previous paper. 6 These compositions are based on MARM247 (Alloy G), which contains 3 pct Ta, 10 pct W, and 10 pct Co. Alloys B, E, and G are 0, 5, and 10 pct Co alloys at the baseline refractory metal level of 3 pct Ta and 10 pct W. Nickel was substituted for Ta to form alloys with 0 pct Ta and 9 pct W at each Co level: Alloys A, D, and F. Finally, 2 pct W was substituted for 3 pct Ta to form alloys with 0 pct Ta and 12 pct W at 0 and 10 pct Co: Alloys C and H.
Cobalt 0 0 0 5 5 10 10 10
M. V. NATHAL, Research Metallurgist, is with NASA Lewis Research Center, Cleveland, OH 44135. L. J. EBERT, Professor of Metallurgy and Materials Science, is with Case Western Reserve University, Cleveland, OH 44106. Manuscript submitted November 15, 1984.
Tantalum 0 3 0 0 3 0 3 0
Tungsten 9 10 12 9 10 9 10 12
Note: *All values in weight percent. The following elements remained constant at levels appropriate for MAR-M247: AI = 5.5, Cr = 8.5, Ti = 1.0, Mo = 0.7, Ni = balance. Actual compositions are presented in Reference 6.
The single crystal castings were produced by the withdrawal process in the production facilities of TRW, Inc. All mechanical tests were performed on specimens with gage dimensions of 20 mm length and 4.5 mm diameter. The longitudinal axes of all specimens were oriented within 10 deg of [001]. Tensile tests were performed in air at 1000 ~ The tensile tests were performed with an Instron testing machine at constant crosshead speed, with an initial strain rate of 2.2 x 10 -4 S - l . Creep-rupture tests were performed in air at 925 and 1000 ~ Temperature was controlled to --- 1 ~ with the aid of two Pt/Pt-13Rh thermocouples attached to the specimens. Loads were applied using 10:1 or 20:1 constant load lever arms. Creep strain was measured using linear variable differential transformers in conjunction with extensometers. The knife edges of the extensometers wer
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