Elevated temperature creep-rupture behavior of the single crystal nickel-base superalloy NASAIR 100
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I.
INTRODUCTION
ADVANCES in directional solidification to produce single crystal nickel-base superalloys have provided further improvements in creep-rupture capability. Although extensive studies on the creep behavior of polycrystalline alloys have been performed, 1 5 information on single crystal alloys is more limited. Most of the studies on the creep behavior of single crystals have been primarily concerned with the deformation processes at intermediate temperatures. 6-9 In addition, some microstructural aspects of the creep behavior at higher temperatures have been reported. 1~ However, little information is available on the stress and temperature dependencies of the creep properties and the factors controlling the various stages of creep at temperatures near 1000 ~ The purpose of this investigation was to examine in further detail the elevated temperature creep and rupture behavior of a modern single crystal superalloy, NASAIR 100. This alloy is a modification of MAR-M247 which has been designed for single crystal applications. 15This investigation is part of a NASA program to determine the influence of strategic elements in superalloys. 16,17
II.
MATERIALS AND PROCEDURES
Single crystal cylindrical bars were produced by the withdrawal process. Chemical analysis of the bars is presented in Table I. Two separate castings, designated B- 1 and B-2 in Table I, were utilized in this investigation. The bars were given a standard commercial heat treatment, 15 which consisted of solution treatment at 1302 +- 3 ~ for 4 hours followed by forced air quenching, a simulated coating cycle of 982 ~ for 5 hours, and a final age of 871 ~ for 20 hours. 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 June 11, 1984. METALLURGICAL TRANSACTIONS A
Crystal orientations were determined by the Laue back reflection X-ray technique. Specimens whose longitudinal axes were within 10 deg of [001] were used for mechanical testing. All mechanical tests were performed in air at temperatures of 925 and 1000 ~ on specimens with 20 mm gage length and 4.6 mm diameter. Tensile tests were performed at a constant crosshead speed, with an initial strain rate of 2.2 x 10 -4 S-1. Creep-rupture tests were performed using constant load lever arms. Temperature was measured with two Pt/Pt-13 pct Rh thermocouples attached to the specimen, and was controlled to -+ 1 ~ Creep strain was measured with a linear variable differential transformer in conjunction with an extensometer attached to the shoulders of the specimen. Most creep-rupture tests were conducted to failure. However, a few tests were interrupted and cooled under load in order to examine the microstructure after various creep exposures. A few constant crosshead speed compression tests were also performed at 1000 ~ with specimens 10 mm long and 4.6 mm diameter. Specimens for tranmission electron microscopy (
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