Cyclic creep and anelastic relaxation analysis of an ODS superalloy
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I.
INTRODUCTION
RECENTLY, we reported a very dramatic effect that accompanies load cycling for two oxide dispersion strengthened (ODS) alloys, INCONEL MA 6000 j and INCONEL MA 754. 2 The major variable in both cyclic creep (load controlled low cycle fatigue, square wave loading with an R value near zero) studies was the loading frequency. A decrease in the cyclic minimum strain rate and increase in rupture life were shown to occur as a result of cyclic creep loading. The strengthening effect became more pronounced as the frequency of load application increased. The frequency dependence of the cyclic creep behavior was shown to result from the storage and recovery of anelastic strain during load cycling and specifically how this affected nonrecoverable creep. In this paper, we report and discuss the effect of two other test variables on the cyclic creep behavior of the ODS superalloy INCONEL MA 6000, namely, temperature and maximum load. The observed effect of temperature and maximum load on the cyclic minimum strain rate is shown to be consistent with the anelastic mechanism used to explain the frequency effect. In addition, since the storage and recovery of anelastic strain has been shown to play an important role in the cyclic creep of INCONEL MA 6000, this strain was analyzed in detail. Specifically, the temperature dependence of the anelastic recovery was determined in order to elucidate the physical basis of the relaxation process.
*INCONEL is a trademark of the INCO family of companies. VINCENT C. NARDONE, formerly Associate Research Scientist, Henry Krumb School of Mines, Columbia University, is Associate Research Scientist at United Technologies Research Center, East Hartford, CT 06108. WILLIAM L. KIMMERLE, Graduate Research Assistant, and JOHN K. TIEN, Howe Professor, are with Henry Krumb School of Mines, Columbia University, New York, NY 10027. Manuscript submitted July 23, 1984.
METALLURGICALTRANSACTIONS A
II. EXPERIMENTAL PROCEDURE AND RESULTS
A. Cyclic Creep Testing The alloy INCONEL MA 6000 was provided by NASALewis Research Center in the form of a 1.5 cm by 3.0 cm cross-section bar. The nominal composition of the alloy in weight percent is given in Table I. The table shows that, in addition to the solid solution strengtheners Cr, Mo, W, and Ta, INCONEL MA 6000 also contains the 3~' formers A1 and Ti as well as the inert Y203 dispersoids. Thus, INCONEL MA 6000 is both a precipitation and an oxide dispersion strengthened nickel-base alloy. INCONEL MA 6000 is initially mechanically alloyed followed with consolidation by hot extrusion.3 The extruded material is then zone annealed to produce a coarse elongated recrystallized grain structure. This thermomechanical processing results in a (110) texture, with the grains having transverse dimensions from 1 to 2 mm and an average grain aspect ratio greater than ten. 4 After recrystallization the following heat treatment is performed: 1232 ~ for one-half hour/air cooled (AC), 954 ~ for two hours/AC, 843 ~ for 24 hours/AC. The microstructure that results f
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