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INTRODUCTION

INHOMOGENEOUS deformation in ductile materials is a precursor to failure under both monotonic and cyclic loadings. In INCONEL* 718 and similar superalloys, *INCONEL is a trademark of Inco Alloys International, Inc., Huntington, WV.

cracks initiate and propagate along planar deformation bands, tl-8~ These bands lie on {111} planes and are regularly spaced across the grain. The bands form during either monotonic Esm or cyclic loading, r~ 81 It has been reasoned that the formation of these bands and the restriction of dislocation movement to these bands cause cyclic softening, t~-s~ Strain rate and material microstructure influence the degree of coarseness of deformation.tZ-4'6'sk Environmental effects on fatigue crack initiation and propagation are influenced by deformation microstructure. [~-8] INCONEL 718 is a precipitate-hardened alloy in which the primary strengthening agents are the y" precipitates (D022 structure and disc-shaped, with a diameter of 30 nm and a thickness of 6 nm).m Additional strengthening is

D.W. WORTHEM, formerly Graduate Student, Department of Theoretical and Applied Mechanics and Materials Research Laboratory, University of Illinois, is with Sverdrup Technology, Inc., NASA Lewis Research Center Group, Cleveland, OH 44135. I.M. ROBERTSON, D.F. SOCIE, and C.J. ALTSTETTER, Professors, are with the Department of Materials Science and Engineering and Materials Research Laboratory, University of Illinois, Urbana, IL 61801. F.A. LECKIE, formerly Professor, Department of Theoretical and Applied Mechanics and Materials Research Laboratory, University of Illinois, is Professor, Department of Mechanical Engineering, University of California, Santa Barbara, CA. Manuscript submitted January 23, 1989. METALLURGICAL TRANSACTIONS A

obtained from the y' precipitates (L12 structure and spherical, with a diameter of 20 nm). tgl Uniaxial fatigue tests have shown that the cyclic deformation response of INCONEL 718 is divided into two regimes. In the first regime, cyclic hardening occurs. The extent of the regime depends on the plastic strain amplitude, extending for 100 cycles at low plastic strain amplitudes and decreasing as the plastic strain amplitude increases. In the second regime, which extends throughout the remainder of the life, cyclic softening is observed, tl-Sj Microstructural examination of failed samples revealed that the deformation structure consisted of a regularly spaced array of deformation bands, t~-8] No strengthening precipitates were observed in these bands. Conflicting reports as to the nature of these bands have been presented; in one case, all bands were claimed to be twins, ~21while in another case, no twins were reported. I8] This type of coarse deformation structure was formed at temperatures from 25 ~ to 649 ~ From the microstructural observations, it has been suggested that the initial hardening was due to the initial interaction between the dislocations and the precipitates. Softening occurred when the dislocations cut the precipitates, reducing their interaction