CYCLIC STRESS-STRAIN RESPONSE OF POLYCRYSTALLINE NICKEL

Constant plastic strain amplitude fatigue experiments were conducted a t room temperature on polycrystalline nickel (290 mum grain size) at p lastic strain amplitudes from 2.5 x 10(-5) to 2.5 x 10(-3). Cyclic def ormation behavior was characterized by analyzing the cyclic hardening response, evaluating the evolution of the shape of the hysteresis loop , and optical and transmission electron microscopy observations. The r esults indicate that the cyclic stress-strain (CSS) curve has a pronou nced bulge or region of reduced slope extending from a plastic strain amplitude of approximately 2 x 10(-4) to 8 x 10(-4). This region of re duced slope is caused by the localization of plastic strain in persist ent slip bands (PSBs). It is also shown that plastic strain amplitude has little influence on the friction stress of cyclically saturated ni ckel; however, the back stress increases with increasing plastic strai n amplitude. A comparison of the cyclic deformation characteristics of polycrystalline nickel with those of copper indicates that these mate rials exhibit similar fundamental cyclic behavior on both a macroscopi c and microscopic level.