Fatigue damage accumulation in nickel modified by ion beam surface microalloying
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INTRODUCTION
IN ductile metals, fatigue crack initiation at low strain amplitudes involves two principal processes: the localization of plastic strain in the interior of the material, in the form of persistent slip bands (PSB's), and the progressive development of stress concentrating geometries at the surface where the slip bands emerge. Whether it is considered at the atomic scale of oxide or contaminant, or taken as a deeper zone in which image forces act and where stress states are specially constrained, the surface region plays a critical role in the crack initiation process. By mediating interactions with the external environment, controlling the egress of mobile dislocations, and altering the operation of nearby dislocation sources, the surface region influences the homogeneity and reversibility of cyclic slip. Therefore, modification of the chemistry, stress state, or defect structure of the surface region has been a traditional approach to the improvement of fatigue performance. In fcc metals, it has been convincingly demonstrated that fatigue cracks nucleate as a result of the formation of intrusion-extrusion (or, "notch-peak") topographies which develop at the edges of persistent slip band structures intersecting the free surface, m Crack nuclei form at the base of the intrusions, or notches, t21 In addition to the notch-peak topography, individual PSB's tend to form net extrusions above the surrounding surface. Together, the notch-peak topography and the net extrusion of the PSB as a whole comprise the primary manifestation of fatigue damage accumulation at the surface and are the precursors to crack formation. The principal factor in the formation of these features is the irreversibility of cyclic slip, which is in turn promoted by cross slip and dislocation annihilation, the egress of point defects at the surface, and environmental D.S. GRUMMON, formerly with The University of Michigan at Ann Arbor, is Assistant Professor, Department of Metallurgy, Mechanics and Materials Science, Michigan State University, East Lansing, MI 48824. J.W. JONES, Associate Professor, Department of Materials Science and Engineering, and G. S. WAS, Associate Professor, Department of Nuclear Engineering, are with The University of Michigan, Ann Arbor, MI 48109. Manuscript submitted September 25, 1987. METALLURGICALTRANSACTIONSA
interactions such as slip step oxidation. These aspects of the fatigue crack initiation process all support the idea that intervention at the surface by microalloying has potential for extending the crack initiation life of metals. Ion beam surface modification, having proven its usefulness for imparting wear and corrosion resistance, has more recently been studied in fatigue experiments. Ion beam processes, whether they employ direct ion implantation or the technique of ion beam mixing, have important advantages over other methods of surface modification, including the ability to form nonequilibrium phases at moderate temperatures. Although traditional means of surface modification, such as shot pee
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