Low Cycle Fatigue of an Ultrafine Grained AA5083 Aluminum Alloy Composite Produced by Cryomilling

Low cycle fatigue (LCF) properties were investigated for a novel cryomilled AA5083 aluminum composite with duplex coarse and ultrafine grain sizes and reinforced with boron carbide particulates, referred to as trimodal material. Fully reversed cyclic tests were conducted under plastic strain control at plastic strain amplitudes from 0.15 to 0.6 pct using a constant plastic strain rate in a servo-hydraulic testing system. A nonlinear elastic modulus was used to calculate the elastic contribution to the measured total strain. The LCF performance of this trimodal material is compared to previous results for unreinforced AA5083 aluminum alloy with bimodal grain size (85/15 pct CM/UM) and its coarse-grained wrought counterpart, AA5083-H131. Stress response curves for the trimodal material revealed slow hardening until failure associated with the presence of particulate reinforcements. The very small asymmetry between tension and compression stresses reflects a lack of strain localization beyond the initial cycles. The trimodal and 85/15 pct CM/UM alloys have similar and superior low cycle fatigue strength compared to AA5083-H131. From the Coffin-Manson plot, the trimodal material has a shorter fatigue life than 85/15 pct CM/UM alloy and AA5083-H131 for high plastic strain amplitudes, but nearly identical life at low amplitudes. Microcracks were observed near the dominant crack on trimodal specimen surfaces at failure. Back-scattered images revealed that particulates altered the crack propagation direction; cracks nearly always propagated around particulates.