The influence of alloying on slip intermittency and the implications for dwell fatigue in titanium

TL;DR

This study investigates how alloying elements influence slip behavior and stress bursts in titanium alloys, revealing that certain alloying strategies can mitigate dwell fatigue by promoting slip homogeneity and reducing localized slip bursts.

Contribution

It provides new in situ observations of slip bursts at the grain scale in titanium alloys and links alloying elements to slip intermittency and dwell fatigue susceptibility.

Findings

Trace oxygen reduces slip burst magnitude and promotes slip homogeneity.

Precipitation of Ti_3Al increases slip localization and stress bursts.

Avoiding long basal slip paths can improve dwell fatigue resistance.

Abstract

Dwell fatigue, the reduction in fatigue life experienced by titanium alloys due to holds at stresses as low as 60% of yield, has been implicated in several uncontained jet engine failures. Dislocation slip has long been observed to be an intermittent, scale-bridging phenomenon, similar to that seen in earthquakes but at the nanoscale, leading to the speculation that large stress bursts might promote the initial opening of a crack. Here we observe such stress bursts at the scale of individual grains in situ, using high energy X-ray diffraction microscopy in Ti-7Al-O alloys. This shows that the detrimental effect of precipitation of ordered Ti_3Al is to increase the magnitude of rare pri<a> and bas<a> slip bursts associated with slip localisation. In contrast, the addition of trace O interstitials is beneficial, reducing the magnitude of bas<a> slip bursts and increasing the homogeneity between basal and prismatic <a> slip. This is further evidence that the formation of long paths for easy basal plane slip localisation should be avoided when engineering titanium alloys against dwell fatigue.