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.

Cold dwell fatigue analyses integrating crystal-level strain rate sensitivity and microstructural heterogeneity

“…An agreed definition of these regions of common orientation remains unclear, but they are sometimes considered to be an 'effective structural unit' [28] with given dominant crystallographic orientation with common basal plane or Burgers vector, often with a morphology of high aspect ratio, and with highest dimension of order mm in length. Our former study [27] shows that macrozones of Ti-6Al-4V, which have aspect ratio larger than four and basal poles within 15 o of remote loading direction, lead to much higher stress redistribution (load shedding). In this paper, it is argued that the singlecrystal slip rate sensitivities are needed to better describe local slip localisation, creep accumulation and stress redistribution in polycrystal behaviour.…”

“…Macrozones or microtextured regions (MTRs) are widely observed within titanium components, which are thought to be important for strain localization, dwell debit increase and corresponding crack nucleation [21][22][23][24][25][26][27]. An agreed definition of these regions of common orientation remains unclear, but they are sometimes considered to be an 'effective structural unit' [28] with given dominant crystallographic orientation with common basal plane or Burgers vector, often with a morphology of high aspect ratio, and with highest dimension of order mm in length.…”

Tool for automatic macrozone characterization from EBSD data sets of titanium alloys

“…Ultrasonic attenuation inspection has been successfully used in titanium alloys, where the signal obtained from the ultrasonic response could be correlated with the presence of macrozones (Humbert et al, 2009). A more recent technique is based on a laser ultrasonic method, spatially resolved acoustic spectroscopy, where surface acoustic waves, whose frequency varies with the crystallographic orientations, are propagated on a surface; this technique is able to cover large areas (Li, 2012;Sharples et al, 2007). However, the possibility of data post-processing in these alternative methodologies is limited.…”

Quantifying the effect of macrozones on the cold-dwell fatigue response of UD-rolled Ti-6Al-4V using high-energy X-ray diffraction

“…Even though some work has been done to examine the progression of deformation mechanisms at the interface of macrozones and weak-textured regions [16,17,19], an important consideration, in this case, is that some of the macrozones are not fully or sufficiently enclosed by a weak-textured matrix. A computational study by Liu and Dunne [44] noted that high-aspect macrozones that are fully subsurface are more damaging than those that intersect free surfaces. However, this has not been validated experimentally and based on these experiments, cases where one or more macrozones are found to intersect free surfaces on one or both ends should not be necessarily discounted as less sensitive to dwell fatigue.…”