Dislocation structure in a single-crystal nickel-base superalloy during low cycle fatigue
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I.
INTRODUCTION
NICKEL-base superalloy undergoes two processes during cycling: the strain hardening and softening and the nucleation and propagation of a microcrack. The cyclic hardening and then softening occur over the whole deformation process. However, the nucleation and propagation of microcrack are superposed in the later stage of the hardening and softening processes. Although every event, which is very complex, within the process is affected by the microstructure, the mechanism of the process is related to the creation, motion, and reaction of dislocations. A number of experimental and theoretical investigations on the dislocation structure in the nickelbase superalloy and ordered materials have been conducted. ~l-~l However, few works on dislocation structure produced during low cycle fatigue (LCF) in the singlecrystal nickel-base superalloy have been reported in detail. As part of a series of an extensive study, in this article, the emphasis will be put on the behavior of dislocations produced during LCF in a single-crystal nickelbase superalloy. II.
MATERIALS AND PROCEDURES
The single-crystal nickel-base superalloy used in this investigation had a composition of (in wt pct) 15Cr, 8.40Co, 5.85W, 3.91A1, 3.98Ti, 1.03Ta, and balance Ni. The alloy was heat-treated as follows: 1100 ~ 8 h AC + 1240 ~ h AC + 1090 ~ h AC + 850 ~ h AC. Low cycle fatigue tests at 760 ~ were carded out after the heat treatment. A full reversed triangular wave with a frequency range of 15 to 25 cycles per minute has been used. The stress ratio (R) is - 1 , the stress amplitude range is from 550 to 800 MPa, and the stress axis is parallel to the crystal growth direction [001]. J.H. ZHANG and Y.B. XU, Associate Professors, and Z.Q. HU and Z.G. WANG, Professors, are with the State Key Laboratory for Fatigue and Fracture of Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, People's Republic of China. Manuscript submitted April 2, 1991. METALLURGICAL TRANSACTIONS A
The thin foils for observation by transmission electron microscopy (TEM) have been prepared by the twin jet polishing method. Dislocation structure has been examined in a PHILIPS* EM 420 microscope using a dou*PHILIPS is a trademark of Philips Electronic Instruments Corp., Mahwah, NJ.
ble tilt goniometer in diffraction and imaging modes. III.
EXPERIMENTAL
RESULTS
A. Initial Structure of the Alloy As shown in Figure 1, the microstructure in the heattreated condition is composed of y matrix and cuboidal y'-precipitates which are completely coherent with the matrix. The misfit (~ = 2(ay, - ay)/a~, + ay) is 0.05 pct. 161The weight fraction of y'-phase with average size of 250 nm in the alloy is 52.4 pct.
B. S-N Curve The relationship between applied stress and life to failure is shown in Figure 2, where the markers 16, 5, 10, 12, and 13 correspond to those at which the foil specimens for TEM examination were prepared.
C. Dislocation Structure in Matrix Transmission electron microscopy observation reveals that strain distribution in LC
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