Influence of secondary precipitates and crystallographic orientation on the strength of single crystals of a Ni-based su
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INTRODUCTION
THE anisotropy of the creep strength of single crystals of Ni-based superalloys has been interpreted in terms of the Schmid factors and multiplicity of {111}^112&–type slip systems in the intermediate-temperature ranges.[1,2] For the creep strength of a powder metallurgy RENE´ 95 alloy at 650 8C, the operative mechanism was essentially determined by the mean surface-to-surface spacing between the cooling g8 precipitates.[3] A small mean surface-to-surface spacing between g8 precipitates restricted dislocation motion significantly, whereas a larger mean surface-to-surface spacing between the g8 precipitates promoted homogeneous slip through a/2^110& dislocation bowing and looping.[3] Secondary g8 precipitates in a matrix channel are expected to influence the mean surface-to-surface spacing between g8 precipitates. However, little attention has been paid to the effect of the secondary precipitates on the creep strength and activities of {111}^112&–type slip systems in the intermediate-temperature ranges. Therefore, the primary purpose of the present investigation is to study the effect of heat treatments on the secondary precipitates and to understand the resultant changes of creep behavior in each orientation. The second objective is to investigate the differences between (111)[112] and (111)[211] slip systems.
II. EXPERIMENTAL PROCEDURES The investigation was carried out on an experimental nickel-based superalloy. Its chemical composition is listed in Table I. Single crystals of this alloy were grown from the melt by the modified Bridgman method. After the analysis of the crystallographic orientation by the back-reflection Laue method, specimens for creep rupture tests were cut from the as-grown crystals by a spark cutter. The specimens were solutionized at 1255 8C for 10 hours and then cooled in an air blast. Two kinds of procedures for the aging heat treatment were employed. The single-aged specimens were aged at 1100 8C for 10 hours, and the double-aged specimens were subjected to aging at 1100 8C for 10 hours and, subsequently, to an additional aging at 850 8C for 20 hours. After each aging treatment, the specimens were cooled in an air blast. Creep rupture tests were performed at 700 8C under a nominal stress of 820 MPa. The initial tensile orientations are shown in Figure 1. Dislocation structures were observed by transmission electron microscopy (TEM). The edge lengths of the g8 precipitates were measured by scanning electron microscopy (SEM).
III. EXPERIMENTAL RESULTS A. Microstructures of the Heat-Treated Specimens
K. KAKEHI, Research Associate, is with the Department of Mechanical Engineering, Tokyo Metropolitan University, Tokyo 192-0397, Japan. Manuscript submitted October 23, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
The distributions of the g8 precipitates after aging heat treatment are shown in Figure 2. The average edge lengths were 0.40 mm for the single-aged specimen and 0.39 mm for the double-aged specimen, respectively. The influence VOLUME 30A, MAY 1999—1249
Table I. C
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