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FATIGUE OF INTERMETALLIC COMPOUNDS AND THEIR COMPOSITES

N.S. Stoloff, T.R. Smith and A. Castagna Materials Engineering Department Rensselaer Polytechnic Institute, Troy, NY 12180-3590. ABSTRACT Recent studies of fatigue behavior of intermetallic compounds are reviewed. Emphasis is upon fatigue damage leading to crack initiation, effects of environment on crack propagation and behavior of intermetallic matrix composites. Future research directions are suggested.

INTRODUCTION Improvements in low temperature ductility and high temperature creep resistance remain as major goals for most research studies on intermetallic compounds. Nevertheless, use of intermetallics in aerospace or other structural applications will require knowledge of their behavior under cyclic loading conditions. It already has been established that several LI 2 intermetallics, including Ni 3AI + B, display very good resistance to stress controlled fatigue (high cycle or crack growth) at room temperature"Il. Other intermetallics, such as Fe 3A112 1and Ti3AI31, display very rapid crack growth, possibly due to high notch sensitivity or susceptibility to the environment (e.g. moisture or hydrogenl 4 I). Moreover, at elevated temperatures, interactions with the test environment (typically oxygen) or with creep components arising from the load cycle can adversely influence fatigue behavior'5 61 . With respect to strain controlled fatigue, detailed studies of crack 1 initiation have been carried out on crystals of Ni 3AI+B 17 -91 and NiAI "1. Low cycle fatigue behavior of polycrystalline Ni 3AIll 11 and NiAI11 2' 131 alloys also have been reported recently. The purpose of this paper is to review recent progress in understanding of fatigue behavior, especially with respect to crack initiation and growth.

CRACK INITIATION Most work on crack initiation has been carried out on Ni 3AI single crystals. A series of studies in our laboratory have provided evidence that crack initiation in Ni 3 AI is preceded by an accumulation of polygonal loops, Fig. 11141, dislocation dipoles and point defect clusters, as well as the formation of persistent slip bands (PSB) and intrusion/extrusion surface morphology17-91. The dipoles are generated primarily by nonconservative motion of jogged superdislocations and their break-up by pinch-off, see Fig. 21141. Dragging of jogs, impedance of dislocation motion by dipoles and dislocation interactions contribute to cyclic hardening. The resemblance of the strain-controlled cyclic behavior of Ni3 AI to copper with respect to point defect formation and the development of intrusions and extrusions has Mat. Res. Soc. Symp. Proc. Vol. 288. 01993 Materials Research Society

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Fig. 1.Polygonal dislocation loops in fatigued Ni3 AI 4 1.

Fig. 2. Dislocation dipole breakup by pinch off in Ni3 A11141.

been commented upon elsewhere18 I. The most apparent difference between Ni 3A1 and copper crystals is the formation of a "ladder" structure in the latter only1151 . Although studies of crack initiation in other intermetallics are not as d