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INTRODUCTION

IT has

been well established in the literature that microstructural defects significantly affect fatigue crack initiation. TM In fact, it has generally been accepted that in engineering alloys crack initiation invariably occurs at some preexisting defect. 5'6 However, few experimental studies have been performed to determine specifically the relative influences of defect size, shape, and population on the fatigue process. It was therefore the purpose of this investigation to examine the effect of such defect characteristics on crack initiation and the early crack growth behavior of two high strength nickel-base superalloys, AF-115 and AF2-1DA. Both alloys were produced by powder metallurgy (P/M) techniques and contained contrasting populations of preexisting pores and nonmetallic inclusions. The testing conditions included a span of temperatures and strain ranges in order to determine the effect of defects in variable regimes where deformation behavior differed. This study is reported in two parts; in the first paper the details of the crack initiation processes for both alloys over a range of test conditions are reported. These include the identification of the fatigue origins and a description of the early crack growth morphology in the different testing regimes. In the accompanying paper, 7 these data are analyzed along with the results of some additional testing, in order to establish the behavior patterns and the general effect of defect size, shape, and population on the fatigue process of these and similar alloys. II.

A.

MATERIALS

Processing The composition of the AF-115 alloy used in this study is

J. M. HYZAK, formerlywith Metals and CeramicsDivision, Air Force Materials Laboratory, Wright-PattersonAFB, OH, is now Memberof the Technical Staff, Sandia National Laboratories, Livermore, CA 94550. I. M. BERNSTEINis Professor,Department of Metallurgical Engineering and Materials Science, Carnegie-MellonUniversity,Pittsburgh, PA 15213. Manuscript submitted March 6, 1981. METALLURGICALTRANSACTIONSA

shown in Table 1. Powders of this composition were produced by an argon atomization process and were subsequently processed in either of two ways: hot isostatically pressed (HIPed) and heat treated; and HIPed, forged, and heat treated. Specimens from material of both conditions were tested in this program. The HIP parameters were 1190 ~ for two hours at 100 MPa, and the forging conditions were 1120 to 1150 ~ at a press head velocity of 25 to 10 cm per minute using preheated die blocks. A total reduction of approximately 50 pct was accomplished in the forging step. All material was subsequently solution heat treated at 1190 ~ for four hours followed by an aging treatment at 775 ~ for 16 hours. The AF2-1DA powder whose composition is given in Table 2 was also produced using the argon atomization process. The powder was compacted, extruded, and finally superplastically forged, the latter at 1105 ~ The pancakes were then solution treated at 1210 ~ for two hours, stabilized at 1121 ~ for two hours, and fi