ECAP加工を行ったZK60Aマグネシウム合金の微視的組織と低サイクル疲労特性

<p>Severe plastic deformation such as the equal channel angular pressing (ECAP) has been received attention in the field of grain refinement strengthening. There are several studies on low-cycle fatigue of ECAPed materials having fcc and bcc lattice structures, but low-cycle fatigue behavior has not been understood in an ECAPed materials having a hcp lattice structure yet. In the present study, a hot-extruded ZK60A magnesium alloy was processed by the ECAP for 1 to 8 passes at a die temperature of 200°C. Microstructure, tensile strength, and low cycle fatigue property of the ECAPed ZK60A alloy were investigated. The low-cycle fatigue test were conducted under a constant plastic strain amplitude of ε_pl=10^-3. In tensile deformation tests, the ECAPed samples processed for 1 and 2 passes revealed high tensile strengths. However, after 4 and 8 ECAP passes, tensile strengths decreased in comparison with the 1 and 2 pass samples. In low-cycle fatigue tests, the initial stress amplitudes of ECAPed samples were certainly higher than that of the as-extruded sample. Among the ECAPed samples, the 2-pass sample showed the highest intial stress amplitude. The initial stress amplitudes of the other ECAPed sample were almost equal, even though the 1-pass sample showed a higher tensile strength. Until 2000 cycles, all the ECAPed samples exhibited no significant cyclic softening behavior, while rapid softening has been observed in ECAPed samples including copper and strainless steel. The absence of the cyclic softening seems to be of advantage in an application for engineering purpose. In the 4- and 8-pass samples, stress amplitudes decreased gradually with increasing cycles. The fatigue life cycle of the 1-pass sample was less than 2000 cycles, which was shorter than the as-exruded sample. However, for the 4- and 8-pass samles, the fatigue life cycles were extended up to 6000 cycles.</p>