The effect of aging and cold working on the high-temperature low-cycle fatigue behavior of alloy 800h: part ii: continuo
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THISpaper reports the second,part of a study of the effects of aging and cold working on the low-cycle fatigue (LCF) behavior of alloy 800H in the temperature range 538 to 760 ~ In the first paper) which hereafter is referred to as Part I, the effects of aging and cold working on the yield strength of alloy 800H, as determined during the first cycle of the LCF tests, were reported. In this paper the LCF tests are followed to completion, and, as in Part I, the structure of the alloy is correlated with its mechanical behavior. II. SYNOPSIS OF PART I The specimens that were LCF tested were obtained from the as-received material (solution annealed for one hour at 1149 ~ followed by air cooling) and from material that had undergone the following treatments: aging for 4000 and 8000 h at temperatures of 538, 593, 649, 704, and 760 ~ cold working 5 and 10 pet in tension at ambient temperature; and cold working followed by thermal aging. The solution-annealed and the cold-worked specimens were fatigue tested at the various aging temperatures, while the aged specimens were tested only at their aging temperature. The first stage of Part I was to determine the microstructure of the specimens after the various aging and deformation treatments. The phase identifications were accomplished by X-ray diffraction analysis of electrolytic extractions, while their morphologies were determined using transmission electron microscopy (TEM) and diffraction. In all, six phases were identified by X-ray diffraction: Cr23C6; (M,Ti)C; Ti(C,N); TiN; a 2/' phase that is a Perovskite-type carbide of the form Ni3(A1,Ti) Cx; and an unknown phase, designated x,
R. E. VILLAGRANA, formerly a Staff Scientist at General Atomic Co., is currently a Consultant in San Diego, CA. J. L. KAAE is a Technical Advisor at General Atomic Company, San Diego, CA 92138. J. R. ELLIS is a Member of the Technical Staff at Oak Ridge National Laboratory, Oak Ridge, TN 37831. Manuscript submitted December 17, 1979.
which was neither the G (Nil6SivTi6) phase nor the 0 (FelCr) phase. The results of the X-ray diffraction analysis of the carbide phases are given in Table I. Transmission electron microscopy revealed that the Cr23C 6 w a s almost always located at grain boundaries, while the MC-type carbides appeared every so often as large block-like precipitates within the grains. The 7' observations are summarized in Table II. This table has been revised to reflect new results obtained in the present study (Section 4.1.2). The 0.2 pct offset yield strengths that were measured during the first cycle of fatigue tests are given in Table III. It can be seen that the yield strength of the specimens decreased slightly with increasing testing temperature. Besides the usual thermal softening mechanisms, the softening that occurred above 593 ~ could be partly due to an increase in the mobility of interstitial carbon. (Dynamic strain aging due to carbon is discussed in Section 4.2.2). The effect of cold working was to harden the alloy by the introduction of forest dislocations. The dis
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