ECAP加工および熱処理されたニッケルの低サイクル疲労特性

抄録

Low-cycle fatigue tests were carried out on pure nickel samples having a wide range of grain sizes. Nickel bullets having 4×4×40 mm³ shape were processed by the equal-channel angular pressing (ECAP) technique for four passes. Then, the ECAPed samples were annealed for 12 hours at various temperatures ranging from 423K to 1073K. The average grain sizes of the samples annealed at 423K and 1073K were 0.8 μm and 105 μm, respectively: the ECAP process and the subsequent annealing treatment enabled us to prepare the samples that differed in grain size by a factor of one hundred. The annealed samples were shaped to strip samples having a gauge shape of 1.5×1.8×2.0 mm³. The low-cycle fatigue tests were conducted under a constant plastic strain amplitude of ε_pl=10^-3. The specimen annealed at 423K showed cyclic softening from beginning of the fatigue test. At an intermediate annealing temperature (753K), the cyclic softening became moderated, while the samples annealed at 773K and 1073K showed cyclic hardening. From SEM observations after the fatigue tests, shear bands generated across many grains were detected at the sample annealed at 423K, while slip bands were developed inside individual grains in the sample annealed at 773K. Hence, it is likely that the cyclic softening and hardening are associated with the shear banding and the slip band development, respectively.