The effect of defects on the fatigue crack initiation process in two p/m superalloys: part ii. surface-subsurface transi
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INTRODUCTION
IN the preceding paper ~ it was
shown that defects significantly influenced the elevated temperature fatigue initiation processes in two P/M nickel-base superalloys, AF-115 and AF2-1DA. Defects affected both the mode of crack initiation and the location of the failure, either at the surface or in the interior. While defects have previously been shown to reduce the fatigue lives of other nickel-base superalloys,2 as well as other powder metallurgy (P/M) 3'4'5 and cast and wrought materials, 6'7 their influence on the crack initiation process is not totally clear. For example, studies on aluminum s'9 and steel4'1~ alloys have shown generally that larger defects are more effective crack initiators than smaller ones, and that surface inclusions are more detrimental than defects in the interior. These findings, however, contrast with our observation of a surface-subsurface transition in failure location with strain range in AF-115 and AF2-1DA. The primary purpose of this investigation is to determine the influence of defect size, shape, and population on the elevated temperature fatigue processes of two P/M nickelbase superalloys, using the observations cited in the previous paper.1 This will involve considering separately the effect of defect character on the crack initiation and crack propagation phases of fatigue. Additional test results will be described which explain the surface-subsurface transition in terms of defect characteristics.
II. INITIATION TRANSITION AT ELEVATED T E M P E R A T U R E SEM examination of AF-115 and AF2-1DA elevated temperature low cycle fatigue tests has revealed for both alloys a transition in the nucleation site of the dominant crack from a surface or near-surface site to an internal location as the fatigue strain range is reduced (Figures 1 and 2). t These data show that there is a definite strain range where the initiation location changes from the surface to the interior. This J. M. HYZAK, formerly with Metals and Ceramics Division, Air Force Materials Laboratory, Wright-Patterson AFB, OH, is now Member of the Technical Staff, Sandia National Laboratories, Livermore, CA 94550. I. M. BERNSTEIN is Professor, Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon UniVersity, Pittsburgh, PA 15213. Manuscript submitted March 6, 1981. METALLURGICAL TRANSACTIONS A
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