Quantitative investigation of slip band activities in a bimodal titanium alloy under pure fatigue and dwell-fatigue loadings

Abstract

This study investigates the slip system activities in primary α grains of a forged Ti–6Al–4V alloy under pure fatigue and dwell-fatigue conditions, using high-resolution digital image correlation (HR-DIC) analysis obtained through interrupted mechanical tests. A quantitative examination of operating slip systems and strain localization on large-field HR-DIC data is conducted using an automated analysis framework developed in a previous study. The findings reveal higher slip activities under dwell-fatigue conditions, evidenced by parameters such as slip band density, area fraction of grains activated with slips, and mean equivalent plastic strain intensities along slip bands through statistically sound analysis. A notable shift in dominant slip activity from basal slips to prismatic slips is observed under pure fatigue conditions, while dominant prismatic slip activity persists throughout dwell-fatigue processes. Furthermore, slip bands with early-developed high strain localization, referred to as slip markings, are identified as prerequisites for surface crack initiation, and three types of slip markings are discovered. Finally, possible reasons for dwell-life debits, and potential anisotropy in the two major slip systems, basal and prismatic slips, are discussed.