**6. Precipitation-free zones (PFZs)**

increase of fatigue behaviour by producing the completely recrystallized β microstructure with homogenously distributed α precipitates and reducing the grain boundary α [32]. Finer and homogenous α precipitation resulting from duplex aging enhanced the fatigue limit of beta alloy Ti-5Al-5Mo-5V-3Cr [38]. In a pre-strained Ti-10Mo-8V-1Fe-3.5Al, two-step aging was found more effective and yielded higher strength than conventional aging [39]. In contrast to the proceeding instances, Kazanjian et al. reported that multi-step aging made little difference to fatigue crack growth compared to the single-step aging [40]. In addition to the single and duplex aging, triplex aging or aging performed in three steps was attempted by some researchers on Ti-15-3 beta alloy; they found no significant benefit in either tensile strength or ductility of the material [41]. Duplex aging was also found to result in an enhancement of thermal stability during the elevated

**4. Influence of the rate of heating to the aging temperature**

precipitation [42–45]. In Timetal LCB, lower heating rate (0.25 ks<sup>1</sup>

precipitation compared to the faster heating rate (20 ks<sup>1</sup>

During the heat treatment of metastable beta titanium alloys, heating rate adopted to attain the desired aging temperature has an influential role in the α

optimum combination of strength and ductility with a finer and homogenous α

heating rate will vary from alloy to alloy. For example, a similar heating rate of 0.25 ks<sup>1</sup> produced coarser and non-uniform alpha precipitation in the Ti-15-3 alloy and the same authors reported 0.01 ks<sup>1</sup> as the optimum heating rate for this alloy [42]. Wu et al. [45] reported a significant increase in microhardness of the Ti-15-3 alloy when a lower heating rate was used. They attributed it to the homogenous alpha precipitation. In addition, the lower heating rate yielded a microstructure free

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].

) yielded an

) [42]. However, this

temperature application [24].

*Welding - Modern Topics*

of grain boundary α.

**5. Grain boundary α**

**208**

Non-uniform distribution of precipitates can occur during certain heat treatment conditions forming regions in microstructure free of precipitates usually near proximity of grain boundary. Uneven precipitation of α upon certain aging conditions may result in such zones where precipitation will not occur, and such zones are termed as precipitation-free zones (PFZs). The preferential α phase nucleation along beta grain boundaries can result in depletion of solute atoms near grain beta boundary region eventually resulting in the formation of PFZs. The hardness of this PFZ is less than the precipitation-hardened surrounding matrix. Hence, PFZs act as sites for strain localization during loading and reduce the tensile strength and ductility as the strength difference between PFZs and aged matrix is higher [34, 50]. In the case of fatigue loading, the presence of PFZs can act as crack nucleation sites imposing a deleterious effect in Ti-3Al-8V-6Cr-4Mo-4Zr [33] and Ti-15-3 [34] by slip localization leading to early crack initiation. To avoid the formation of PFZs and to improve the monotonic and fatigue loading behaviour, duplex aging is developed; results are promising [32–34].
