Cyclic creep and stress rupture of a mechanically alloyed oxide dispersion and precipitation strengthened nickel-base su
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I.
INTRODUCTION
BOTHcyclic creep acceleration and deceleration have been reported for pure metals and microstructurally simple systems.~-6 The cyclic creep acceleration has been attributed to the recovery of work hardened structures during off-load periods, resulting in a faster creep rate during the subsequent on-load periods. 2-s The cyclic deceleration has been observed below a critical threshold stress and has been attributed to additional hardening induced by stress cycling.' Cyclic creep behavior of some steels has also been studied. 7-~~Once again, both cyclic creep deceleration and acceleration were observed. The deceleration in one case was explained in terms of recoverable anelastic strain behavior, 8 while the acceleration was again attributed to the recovery of dislocation substructures under the driving force of an internal stress. ~0 In this paper we report and discuss the cyclic creep behavior of Inconel* alloy MA 6000E, an ODS nickel-base *Trademark of International Nickel Company.
superalloy. The major test variable was the cyclic loading frequency, as in previous work performed on a simpler ODS alloy, Inconel MA 754,1~ which is a nickel-chromium solid solution strengthened by fine Y203 oxide dispersoids.
II. MATERIAL AND EXPERIMENTAL PROCEDURE
Inconel alloy MA 6000E is a mechanically alloyed oxide dispersion strengthened (ODS) and y' precipitation strengthened nickel-base alloy. The nominal composition of the alloy in weight percent is 15Cr-4.5A1-2.5Ti-2Mo-4W2Ta-0.5C-0.15Zr-0.1 B- l. 1YzO3. Thermomechanical processing results in a coarse, elongated grain structure with a (110) texture (see Figure 1). The transverse dimensions of the grains are from 1 to 2 mm with the average grain aspect ratio being greater than 10. V.C. NARDONE and D. E. MATEJCZYK, Graduate Research Assistants, and J.K. TIEN, Professor, are all with Henry Krnmb School of Mines, Columbia University, New York, NY 10027. Manuscript submitted July 30, 1982. METALLURGICALTRANSACTIONS A
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Fig. 1--Photomicrograph of the grain structure of Inconel MA 6000E.
The alloy contains a uniform distribution of both the 7' precipitates and the inert oxide dispersoids. The volume fraction of the Y' is approximately 50 pct and their size is about 0.2/xm in diameter. A dark-field image of the Y' is shown in Figure 2. The yttrium oxide dispersion (see Figure 3) is approximately 2.5 vol pct with the size of the particles ranging from 5 to 100 nm, the average, determined statistically, being 28 nm. ~ The uniform distribution of the dispersoids is within both the matrix and the 7'. Detailed discussion on MA 6000E's creep behavior and microstructure can be found elsewhere. '2 The cyclic creep experiments were carried out on a modified constant load Satec JE-20,000 pound lever beam creep testing machine. The load was cycled via a motor-driven lifting linkage, with power to the motor being controlled by a timing device. A damped spring arrangement was used to avoid load overshoot during load changes. The need to eliminate load overshoot is necess
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