The Mechanistic Link between Macrozones and Dwell Fatigue in Titanium Alloys

Abstract This paper addresses the role of macrozone crystallography and morphology in dwell fatigue in titanium alloy Ti-6Al-4V. Until now, the relationship between macrozones and dwell fatigue damage has remained mechanistically uncertain, but this paper establishes a mechanistic link between macrozones and dwell fatigue damage, and explains the preference for dwell facets to be sub-surface. It also outlines the criteria which are important in a potential definition of a macrozone (or microtextured region). High aspect ratio (>∼4) macrozones are particularly damaging when their long-axes are orientated near-normal to the principal loading direction, such that their basal planes are oriented to within ∼15 degrees to the principal stress direction. These criteria may be useful in guiding the development of a diagnostic experimental measurement tool (based on EBSD or ultrasonics for example) for macrozone detection in components.