Research On The Low-cycle Fatigue Properties Of Fine Nonferrous Metal

Magnesium alloys have been widely used in transportation, aerospace and other fields, as the lightest structural materials. Wrought magnesium alloy, because of its high strength and excellent toughness, is regarded as the development of the next generation of new magnesium alloy. With the process of extrusion, rolling, ECAP (Equal Channel Angular Pressing) and forging, fine-grained magnesium alloy can be obtained, which has excellent mechanical properties. As structural material, magnesium alloys are usually subjected to repeated reverse loading. In order to ensure the material safety, the low-cycle fatigue properties of magnesium alloys must be carried out, which can reveal the law of fatigue behavior and the mechanism of fatigue failure.In this paper, the extrusion AZ31 magnesium alloys are processed by a new roll process called variable-plane-rolling, ECAP with cryogenic treatment and solid solution and aging treatment. The pure coppers are processed by ECAP with cryogenic treatment. To study on the material microstructure (grain size, texture, the precipitates) and the deformation mechanism of the low-cycle fatigue properties of magnesium alloys, the static mechanics experiment, low-cycle fatigue tests and fracture morphology observation are conducted. And to compare the ratcheting behavior in different condition, the AZ80 magnesium are processed by solid solution and aging treatment. From this work, the following conclusions are drawn:(1) The mechanical strength of the extrusion AZ31 magnesium alloys is improved obviously after the variable-plane-rolling. Meanwhile, the variable-plane-rolled magnesium alloys also obtain a certain amount of twins, which can weaken the basal texture. During the low-cycle fatigue experiment, compared with the as extruded, more slip systems are activated, and the phenomena of twin-annealing twin is weakened on the variable-plane-rolled magnesium alloys. But the variable-plane-rolled magnesium alloys still show the cyclic hardening behavior, and its fatigue life is obviously longer than the initial extruded AZ31 magnesium alloys.(2) Compared with the initial extruded AZ31 magnesium alloys, the ECAP and ECAP-cryogenic-treatment samples obtain finer grains and higher hardness. All the ECAP-cryogenic-treatment samples show cyclic hardening behavior. When the total strain amplitude is under 0.6%, the fatigue life of ECAP-cryogenic-treatment samples are significantly improved. When the total strain amplitude is above 0.8%, the fatigue life of ECAP-cryogenic-treatment samples reduce with the improved the tensile mean stress.(3) The grains of copper with ECAP and ECAP-cryogenic-treatment samples are be refined significantly. Both of them show cyclic softening behavior under low-cycle fatigue test. The fatigue fracture of ECAP-cryogenic-treatment samples show more narrow fatigue fatiguestriations, which leads to longer fatigue life.(4) The precipitate in aged samples on AZ80 magnesium can improve the mechanical strength and reduce the tensile mean stress. In ratcheting experiment, the solid-solution samples show cyclic hardening behavior while the aged samples show cyclic softening behavior. And the threshold value of ratcheting is improved in aged samples.