Low Cycle Fatigue of an Ultra-Fine Grained 5083 Aluminum Alloy Composite Produced by Cryomilling

Low cycle fatigue properties were investigated for a cryomilled 5083 Al-matrix alloy with duplex grain size and reinforced with Al-coated boron carbide particulates, referred to as trimodal material. Fully-reversed tests were conducted under plastic strain control at plastic strain amplitudes (PSA) from 0.15%--0.6% at a constant plastic strain rate of 10-3 s-1. A nonlinear elastic modulus was used to calculate the elastic contribution to the measured total strain. Trimodal was compared to unreinforced cryomilled 5083 aluminum alloy with duplex grain size, 85/15% CM/UM (cryomilled/unmilled), and its coarse-grained counterpart, AA5083-H131.;Fatigue tests showed that the hysteresis loops for trimodal were nearly symmetrical. Stress response curves revealed slow hardening until failure. Hardening was due to increasing mutual interaction of dislocations and interaction of mobile dislocations with particulates. The asymmetry due to differences between tension and compression stresses showed a positive and high value during the initial cycle at low PSA, and decreased and leveled off as the PSA increased. Asymmetry at half-life was lower than its initial value and near zero for all PSA. Asymmetry provides evidence of strain localization.;The cyclic stress-strain curves showed that trimodal and 85/15% CM/UM have similar and superior low cycle fatigue strength compared to H131. The Holloman power law relationship fits well for both cryomilled materials, but not H131. Cyclic hardening observed in the trimodal material was due to the presence of particulate reinforcements. Particulates lead to the development of localized stresses in the matrix around the reinforcing particles. From the Coffin-Manson plot, trimodal had a lower fatigue life than 85/15% CM/UM and H131 for high PSA, but near identical for low PSA.;SEM analysis showed that trimodal material failed abruptly in a similar manner as did 85/15% CM/UM. High magnification images showed that fast fracture regions are similar at all PSA with ridges throughout the surface. The slow fatigue regions had flat ridges and fine microscopic voids. Polished surfaces showed that microcracks developed near the dominant crack. Back-scattered images showed that particulates altered crack propagation direction; cracks nearly always propagated around particulates. Examination of trimodal at half-life showed minimal fatigue damage on specimens' gage section.