**5. Grain boundary α**

Grain boundary alpha (GBα) is found detrimental by serving as a nucleus for crack initiation along α/β interfaces during the monotonic as well as cyclic loading. When the thickness of these GB<sup>α</sup> exceeds several microns, ductility and fatigue crack inititation and propagation are detrimentally affected. Crack is found to propagate with little resistance along the GB<sup>α</sup> in Ti-8Mo-8V-2Fe-3A1 [46]. In addition to tensile ductility, GB<sup>α</sup> also has a strong negative influence over the fatigue behaviour of the β titanium alloys [32, 47]. In fatigue loading, the preferred site for the crack initiation will be the grain boundary decorated with α and inclined at 45° to the axis of loading [48]. This inclined GB<sup>α</sup> provides potential sites for slip localization as well as fracture inititation. Similarly, subsurface crack initiation induced by the well-developed GB<sup>α</sup> is commonly observed in the highly β stabilized Ti alloy β-C [32]. One of the strategies to improve the endurance limit is by properly designing a duplex aging heat treatment step compared to single aging, in order to facilitate more uniform α precipitation. Duplex aging of Ti-15V-3Al-3Cr-3Sn alloy at 250°C/24 h + 500°C/8 h resulted in a microstructure almost free of GBα, and this was also reported as one of the important reasons for the notable increase in fatigue life in high cycle regime after duplex aging [34]. Presence of GB<sup>α</sup> supports the intergranular fracture and reduces the ductility of the material [25, 28, 49, 50]. In aged Ti-10V-2Fe-3Al, soft zones were observed along the grain boundaries due to the GBα; these zones preferentially undergo plastic deformation upon loading [51].

Moreover the fractographic studies have revealed the presence of a band of intense deformation originating from the grain boundary triple point and spreading into the grain interior. This was ascribed to the accommodation deformation required for continous GBα, which has been stopped at triple point leading to high localized stress concentrations.
