Effect of strain rate on the high-temperature low-cycle fatigue properties of a nimonic PE-16 superalloy
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I.
INTRODUCTION
T H E fatigue life of superalloys at elevated temperatures is significantly affected by such testing parameters as strain rate, hold time, and wave shape. This arises primarily because of changes in deformation and damage mechanisms caused by differences in slip mode, oxidation, phase instabilities, creep, and dynamic strain aging (DSA), and has been reviewed in detail by Sidey and Coffin, t~] Wells e t a l . I2] and Bhanu Sankara Rao. t31These damage processes are reported to lead to premature failure when compared with fatigue failure under timeindependent conditions. Further, the time-dependent mechanisms mentioned above may influence fatigue life either synergetically or independently. Hence, the essential prerequisite for accurate prediction of low-cycle fatigue (LCF) life at elevated temperatures is determination of the rate-controlling damage process that influences cyclic deformation under the appropriate combination of experimental stress/strain, temperature, strain rate, environment, and prior metallurgical condition of the material. NIMONIC* PE-16, chosen for the present investiga*NIMONIC is a trademark of Inco Alloys International, Inc., Huntington, WV.
M. VALSAN, Scientific Officer, K. BHANU SANKARA RAO, Scientific Officer, and S.L. MANNAN, Head, are with the Materials Development Division, lndira Gandhi Centre for Atomic Research, Kalpakkam-603102, TN, India. D.H. SASTRY, Professor, is with the Metallurgy Department, Indian Institute of Science, Bangalore560012, India. Manuscript submitted February 2, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
tion, is a nickel-iron-base superalloy that has wide application at high temperatures as a material for aircraft ducting systems; gas turbine flame tubes; missile hot components; superheater tubes; and wrapper tubes, clad tubes, and tie rods in nuclear reactors. The use of this alloy at high temperatures necessitates a comprehensive understanding of its overall deformation and fracture behavior under hot tension, creep, fatigue, and creep-fatigue conditions. Information regarding high-temperature LCF properties is relatively limited, [4-7] although some information exists on room-temperature properties.t8'91 The objective of the present work is to determine the effect of individual and interactive time-dependent processes on the LCF properties of NIMONIC PE-16 over a range of strain rates at elevated temperatures. This article is part of a detailed investigation aimed at understanding the effect of prior microstructure on the LCF behavior of this alloy. I~~
II.
EXPERIMENTAL PROCEDURE
The chemical composition of NIMONIC PE-16 alloy is provided in Table I. The alloy, in the form of 20-mmdiameter rods, was solution-annealed at 1313 K for 4 hours. A number of these samples were then subjected to a double-aging treatment at 1073 K for 2 hours plus 973 K for 16 hours. The solution-annealed prior microstructure is designated as microstructure A and the doubleaged one as microstrncture B. Low-cycle fatigue tests were conducted on axial cylindr
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