Environmental damage of a cast nickel base superalloy
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which produced fine ~,' and promoted planar slip accentuated the ductility minimum in air tests. In oxygen containing environments, the frequent observation of enhanced cavitation implies either that actual penetration of the gas in some form is involved or that internal voids grow in response to external oxidation. A time-dependent process of damage penetration is supported by tests in which the oxygen partial pressure was suddenly increased; the associated increased creep rate did not occur instantaneously but required a lapse of several hours. 7 This observation implies that the generally small effects of environment reported in laboratory creep tests m a y not give an accurate indication of the effect on service lives if damage is time dependent. A similar problem exists in the second stream of investigation of environmental effects which has studied the influence of environment on fatigue. A review of some of the extensive literature on this subject is given in Ref. 2. As with creep, both air strengthening and v a c u u m strengthening have been reported, although generally, as with Snowden's work on lead, 8 a dramatic loss in endurance results when testing in air or oxygen relative to vacuum. Reducing the oxygen partial pressure during fatigue crack propagation in stainless steel at 800 ~ was shown by Smith e t a l 9 to lead to a lower crack growth rate after about 1 h. During this time, the crack grew about 80/~ which was taken to imply oxygen penetration to this depth ahead of the crack tip. Detailed studies of the influence of cyclic frequency on fatigue lives of a nickel base superalloy by Solomon and Coffin ~~and Woodford and Coffin 11 at 538 ~ showed that most of the loss in fatigue life with decreasing test frequency was due to environmental influence rather than time dependent creep damage processes. The major unanswered question is whether the hold-time/ frequency effects saturate, which might be expected on an oxygen penetration model. Indirect support for saturation derives from thermal fatigue testing l~ and from air vs vacuum tests over a range of temperature. ~3 Saturation of the frequency effect due to test environment might be expected when damage has penetrated to a depth equal to the m a x i m u m crack growth
I S S N 0360-2133 / 81 / 0211-0299500.75/0 METALLURGICAL TRANSACTIONS A 9 1981 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME
VOLUME 12A, FEBRUARY 1981--299
increment possible on the next cycle for crack propagation through the damaged material. The third stream of investigation is the least extensive but m a y well be that which leads to a confluence since it addresses directly the question of penetration of the damaging species. Douglass TM and HancocM s have shown that the creep rupture life and tensile ductility of nickel are drastically reduced by prior exposure in air. H a n c o c k is has attributed this damage to vacancy injection and associated internal void formation. He also reported an initial decrease in creep rate following prior oxidation. Thus, th
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