Effect of grain size and testing temperature on low-cycle fatigue behavior and plastic deformation mode of Ti-2Al-2.5Zr

Symmetrical push-pull low-cycle fatigue (LCF) tests were performed on Ti-2Al-2.5Zr samples with different grain sizes (5 and 40 μm) at room temperature (RT) and low temperature (77 K). The results show that the coarse-grained samples of 40 μm exhibit a higher ductility and LCF life than the fine-grained ones at RT. Meanwhile, the fine-grained samples of 5 μm displayed improved ductility and LCF life at 77 K compared with those at RT. Microstructural observations using optical microscopy (OM), scanning electron microscopy, and transmission electron microscopy (TEM) revealed that a transition occurred in the plastic deformation mode, from twinning with slip to slip alone, as the grain size decreased from 40 to 5 μm at RT. Conversely, in the fine-grained samples fatigued at 77 K, twinning was activated and became one of the dominant plastic deformation modes. The improvement in the LCF life of the coarsegrained samples at RT and the fine-grained ones at 77 K could be attributed to the activation of deformation twinning. In addition, the cyclic stress response curves showed that cyclic stress saturation was exhibited in the fine-grained samples at all strain ranges. An initial cyclic hardening followed by cyclic softening was displayed ahead of the cyclic stress saturation in coarse-grained samples at high strain amplitudes. When the testing temperature decreased to 77 K, cyclic stress hardening prior to cyclic stress saturation also appeared in the fine-grained samples. The relation among the grain size, testing temperature, plastic deformation modes, and LCF life in Ti-2Al-2.5Zr was subsequently discussed.