Microstructure Evolution and Fatigue Properties of Cp-Ti Induced by Cryogenic Low Cycle Fatigue

Abstract

Mechanical twinning plays a crucial role in the plastic deformation of titanium, particularly under cryogenic conditions. The activation of mechanical twinning enhances both the strength and ductility of titanium at these low temperatures. In this study, we investigated the twinning and detwinning behavior of commercially pure titanium (CP-Ti) under cryogenic low cycle fatigue (LCF) loading, comparing the results to those obtained at room temperature (RT). Strain-controlled LCF tests were performed with a strain amplitude of ±2.0%. Ex-situ electron backscatter diffraction analysis revealed a significant increase in twin fraction in specimens subjected to cryogenic temperatures (CT) compared to those tested at RT. Additionally, it was observed that the twinning fraction increased during the early stages of fatigue, with most detwinning occurring during the reverse loading phase of the cycle. Following LCF tests at CT, we observed microstructural evolution, including an increase in residual twinning, associated dislocation density, and grain refinement. The improvement in mechanical properties, such as fatigue stress and fatigue life, due to these microstructural changes was validated using the Hall-Petch relationship and Taylor hardening law. This study confirms the relationship between twinning behavior and microstructural changes in CP-Ti at CT, as well as their impact on fatigue properties.

Keywords: cryogenic deformation, low cycle fatigue, microstructure, twinning, detwinning.