Cyclic Deformation and Fatigue Properties of Ultrafine Grain Size Materials: Current Status and Some Criteria for Improvement of the Fatigue Resistance

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Abstract
In this review, some general conclusions based on studies performed to date on fatigued materials of ultrafine grain (UFG) size produced by equal channel angular pressing (ECAP) are drawn, and open issues of current interest are defined. Important aspects addressed include the apparent discrepancy between improved fatigue strengths in Wöhler (S-N) plots as opposed to inferior fatigue strengths in Manson-Coffin plots, the clarification of the microstructural mechanisms of severe cyclic softening in conjunction with dynamic (local) grain/subgrain coarsening and damaging large-scale catastrophic shear banding. The important roles of the cyclic slip mode, the friction stress, the crystal structure and the temperature of cyclic deformation with respect to stable cyclic deformation behaviour are emphasized. Based on such considerations, criteria are formulated that must be observed, when designing ECAP-processed UFG-materials of superior fatigue strength.