**7. Intermediate phases**

The intermediate phases, such as isothermal ɷ phase and β<sup>0</sup> phase, are formed during low-temperature aging, with the aging temperature generally in the range of 200–450°C [3]. Moreover, the omega phase can also form athermally. The ɷ phase provides nuclei for the α precipitation in the subsequent high-temperature aging (second step of duplex aging), thereby promoting the finer and homogenous distribution of the α phase [3]. The above statement is proven in Ti-7333 near beta alloy, isothermal ɷ phase formed during aging has assisted the precipitation of the α phase in the beta matrix [52]. During the first step of the dual-step aging of Ti-5Al-5Mo-5V-3Cr-0.3Fe, �10% volume fraction of ɷ phase was reported by Coakley et al. [53] and this ɷ phase contributed to a �15% hike in microhardness compared to the solution-treated or quenched sample. However, the ɷ phase leads to the embrittlement/loss of ductility in Ti-Mo alloys due to the inhomogeneous slip distribution caused by the interaction of dislocation and ɷ phase/particles upon deformation [54]. Researchers also reported ɷ precipitation during lowtemperature aging of Ti-15-3 alloy [7, 34]. However, the embrittlement effect of the ɷ phase could be efficiently compensated by processing to realize fine β grains [51]. Researchers also reported dynamic precipitation of ɷ phase under cyclic loading condition [55].

Similarly, stress-induced ɷ phase is observed in a metastable beta alloy during the dynamic compression deformation [56]. The ɷ phase is hexagonal in leaner beta alloys and trigonal in heavily stabilized beta alloys [56]. Other than the ɷ phase, the metastable phase β<sup>0</sup> forms as an intermediate phase during the aging of some beta Ti alloy. β<sup>0</sup> phase with a BCC crystal structure forms if the distortion is less due to the higher concentration of alloy. Similarly, ɷ phase with hexagonal crystal structure forms when the distortion in BCC lattice is higher, which is the case with less

concentrated alloys [27]. In line with the preceding discussions, isothermal ɷ is proven to be assisting the alpha nucleation in the Ti-20V [57]. In addition to ɷ phase, <sup>α</sup>″ (martensite) phase was also observed in the initial microstructure of the solution-treated Ti-10V-2Fe-3Al and the author reported that stable α could be formed from this <sup>α</sup>″. This is in addition to the <sup>ɷ</sup> phase serving as the nucleus for <sup>α</sup> formation upon aging [58]. The intermediate martensitic phases <sup>α</sup><sup>0</sup> and <sup>α</sup>″ are suppressed when alloying is done with sufficient beta stabilizer content. This leads to enhancement of the hardenability of the beta alloy [4].

heat treatment, oil/water quenching will not be carried out to avoid insidious residual stress formation. In metastable beta alloys, mostly the heat treatment procedure generally starts with solution treatment and this is followed by aging. In solution treatment, quenching is performed and unwanted residual stresses will get induced due to non-uniformed thermal expansion. Often, the stress-relieving annealing is combined with aging, as the temperature involved is about the same. For example, in Ti-10V-2Fe-3Al, isothermal aging at 495 and 525°C for 8 h leads to precipitation (age hardening) as well as relief of the stresses induced during the solution treatment [24]. In addition to the stress-relieving annealing, recrystallization annealing is also performed to enhance the fabricability of beta titanium alloys, more specifically, if a significant reduction in cross-section is involved, for example sheet formation [4]. Cold workability of Ti-15V-3Cr-3Al-3Sn and Ti-7Mo-5Fe-2A alloys is notably increased by the annealing treatment [59]. In variance to the foregoing discussion, annealing does not increase the cold workability of the Ti-5Mo-5V-5Al-1Fe-1Cr (VT-22) and Ti-7V-4Mo-3Al (TC6) [59]. Cold working is directly related to the formation of sub-grain and cell structure. Little or no influence of the annealing on the cold workability of VT22 and TC6 is attributed to the

poorly defined sub-grain and cell structure [59].

*Heat Treatment of Metastable Beta Titanium Alloys DOI: http://dx.doi.org/10.5772/intechopen.92301*

**9.1 Tensile, microhardness, and impact properties**

property deteriorated [61].

**9.2 Fatigue behavior**

**211**

**9. Mechanical properties influenced by heat treatment**

formly up to 150 mm of thickness [60]. Single-step aging has increased microhardness of Ti-15-3 alloy by 40% compared to the as-received/solutiontreated condition [30]. In a similar way, finer precipitation kinetics associated with duplex aging process yields a higher hardness value in Ti 15-3 alloy [36]. In Ti-5Al-5Mo-5V-3Cr-0.3Fe, duplex aging (300°C/8 h + 500°C/2 h) was adopted; a 15% increase at first stage and 90% increase at second stage in the microhardness was observed. The remarkable increase in the microhardness in the second stage is ascribed to the precipitation of α phase [53]. Aging after α + β solution treatment resulted in a considerable increase in the hardness of β CEZ alloy, but the impact

In beta alloys, precipitate-free zones and grain boundary α also have control over the fatigue behavior [32]. Precipitation-free zones can be a fatigue crack nucleation site and reduce fatigue life. Similarly, the presence of soft zones associated with

The volume fraction of the beta phase in solution-treated alloy plays an important role in determining the tensile strength achieved through heat treatment process [60]. The optimum combination of tensile strength and ductility could be achieved through adequate knowledge of the aging temperature and holding time. For example, in Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe beta alloy, aging at 440°C for 8 h leads to the peak strength of 1697 MPa with 5.6% of ductility. On the other hand, with the same holding time (8 h), 18% ductility along with a considerable decrease in the tensile strength is obtained by increasing the aging temperature to 560°C; the difference is attributed to the variation in the size of the acicular α precipitates [28]. The influence of aging on Young's modulus and ductility of Ti-15-3 alloy was clearly brought out by Naresh Kumar et al. [23]. Hardenability of the beta Ti alloy is proportional to the content of the beta stabilizer. For example, the beta alloy Ti-5Al-2Sn-2Zr-4Mo-4Cr possesses an excellent hardenability; it can be hardened uni-

### **8. Annealing**

In general, annealing is performed to eliminate the deleterious residual stress (stress-relieving annealing) and to ease the fabrication process (recrystallization annealing). Schematic representation of the beta alloy and alpha-beta alloy microstructure in annealed condition is shown in **Figure 3**.

Deleterious tensile residual stresses are induced during various thermomechanical processing steps and fabrication techniques like welding. Sources of residual stresses are given in **Table 2**.

The residual stress gets superimposed on to the service stress, leading to a significant reduction of the life of the component. For example, Ti-5Al-5Mo-5V-3Cr metastable beta alloy was subjected to the stress-relief annealing at 650–750°C for 4 h followed by air cooling [24]. Stress-relief annealing is an intermediate step of thermo-mechanical processing. This annealing is not meant for altering the microstructure. Hence, extreme care should be taken to select the temperature and time combination (i.e., higher temperature annealing should be performed for a shorter time and lower temperature annealing should be performed for a longer time). Post

#### **Figure 3.**

*Schematic representation of typical (a) beta (β) alloy and (b) alpha-beta (α-β) alloy microstructures in annealed condition.*


**Table 2.** *Sources of residual stresses.*

*Heat Treatment of Metastable Beta Titanium Alloys DOI: http://dx.doi.org/10.5772/intechopen.92301*

concentrated alloys [27]. In line with the preceding discussions, isothermal ɷ is proven to be assisting the alpha nucleation in the Ti-20V [57]. In addition to ɷ phase, <sup>α</sup>″ (martensite) phase was also observed in the initial microstructure of the solution-treated Ti-10V-2Fe-3Al and the author reported that stable α could be formed from this <sup>α</sup>″. This is in addition to the <sup>ɷ</sup> phase serving as the nucleus for <sup>α</sup> formation upon aging [58]. The intermediate martensitic phases <sup>α</sup><sup>0</sup> and <sup>α</sup>″ are suppressed when alloying is done with sufficient beta stabilizer content. This leads

In general, annealing is performed to eliminate the deleterious residual stress (stress-relieving annealing) and to ease the fabrication process (recrystallization annealing). Schematic representation of the beta alloy and alpha-beta alloy micro-

Deleterious tensile residual stresses are induced during various thermomechanical processing steps and fabrication techniques like welding. Sources of

The residual stress gets superimposed on to the service stress, leading to a significant reduction of the life of the component. For example, Ti-5Al-5Mo-5V-3Cr metastable beta alloy was subjected to the stress-relief annealing at 650–750°C for 4 h followed by air cooling [24]. Stress-relief annealing is an intermediate step of thermo-mechanical processing. This annealing is not meant for altering the microstructure. Hence, extreme care should be taken to select the temperature and time combination (i.e., higher temperature annealing should be performed for a shorter time and lower temperature annealing should be performed for a longer time). Post

*Schematic representation of typical (a) beta (β) alloy and (b) alpha-beta (α-β) alloy microstructures in*

Aging Solid-state reactions such as precipitation, phase transformation

Quenching Non-uniform thermal expansion or contraction Fabrication Grinding, polishing, milling, and welding

to enhancement of the hardenability of the beta alloy [4].

structure in annealed condition is shown in **Figure 3**.

residual stresses are given in **Table 2**.

**8. Annealing**

*Welding - Modern Topics*

**Figure 3.**

**Table 2.**

**210**

*Sources of residual stresses.*

*annealed condition.*

**Treatment/processes Remarks**

heat treatment, oil/water quenching will not be carried out to avoid insidious residual stress formation. In metastable beta alloys, mostly the heat treatment procedure generally starts with solution treatment and this is followed by aging. In solution treatment, quenching is performed and unwanted residual stresses will get induced due to non-uniformed thermal expansion. Often, the stress-relieving annealing is combined with aging, as the temperature involved is about the same. For example, in Ti-10V-2Fe-3Al, isothermal aging at 495 and 525°C for 8 h leads to precipitation (age hardening) as well as relief of the stresses induced during the solution treatment [24]. In addition to the stress-relieving annealing, recrystallization annealing is also performed to enhance the fabricability of beta titanium alloys, more specifically, if a significant reduction in cross-section is involved, for example sheet formation [4]. Cold workability of Ti-15V-3Cr-3Al-3Sn and Ti-7Mo-5Fe-2A alloys is notably increased by the annealing treatment [59]. In variance to the foregoing discussion, annealing does not increase the cold workability of the Ti-5Mo-5V-5Al-1Fe-1Cr (VT-22) and Ti-7V-4Mo-3Al (TC6) [59]. Cold working is directly related to the formation of sub-grain and cell structure. Little or no influence of the annealing on the cold workability of VT22 and TC6 is attributed to the poorly defined sub-grain and cell structure [59].
