**2. Solution treatment**

Solution treatment comprises of heating the sample from 20 to 30°C above the beta transus temperature (super-transus) or �50°C below the beta transus temperature (sub-transus) for a specified time and rapid cooling of the sample to room temperature. Hence, beta transus temperature (βtrans) plays a vital role in selecting heat treatment temperatures. This beta transus temperature is strongly influenced by the alloying element (i.e., alpha stabilizers will rise the βtrans, beta stabilizers will lower the βtrans, and neutral elements will hardly do the changes in the βtrans). The equation (Eq. (2)) to find the beta transus temperature is given below [23].

$$\begin{array}{l} \text{T}\_{\text{\textquotedblleft}} = \text{882} + 2\text{1.1} \, [\text{Al}] - 9.5 \, [\text{Mo}] + 4.2 \, [\text{Sn}] - 6.9 \, [\text{Zr}] - 11.8 \, [\text{V}] - 12.1 \, [\text{Cr}] - 15.4 \, [\text{Fe}] \\ + 23.3 \, [\text{Si}] + 123 \, [\text{O}] \end{array}$$

$$
\begin{pmatrix} 2 \\ \end{pmatrix}
$$

Beta transus temperature for some of the important beta titanium alloys is listed in **Table 1**.

Solution treatment temperature and the cooling rate strongly influence the mechanical properties realized after subsequent aging treatment. Depending on the requirement, metastable beta alloys such as Ti-13V-l1Cr-3Al and Ti-15Mo-3Al-3Nb-0.2Si are supplied in the solution-treated condition to ease the down-stream cold working operations [4]. Schematic representation of super- and sub-transus solution treatment and the aging process is shown in **Figure 2**. Super-transus solution treatment is done above the βtrans temperature and sub-transus solution treatment below the βtrans temperature. In alloys, such as Ti-5Al-5Mo-5V-3Cr, both super-transus and sub-transus solution treatments were found to be useful in


precipitation hardening. Coherency strains between α precipitates and the β matrix induce a strengthening effect [5]. Comparative examination of the microstructure of solution-treated (800°C/0.5 h) (ST) and microstructure of solution-treated and aged (ST + A) (500°C/8 h) Ti-15-3 samples has clearly revealed the presence of α precipitates in the latter. Precipitation leads to a significant increase in mechanical properties like tensile strength (79%) and hardness (44%) [26]. Age hardening is more effective for beta alloys compared to the α + β alloy owing to the capability of the former to form finer and homogenous α precipitates [27]. The sequence of precipitation of α is dependent on the Moeq of the alloy. The sequence of precipita-

For lower Moeq solute � lean alloy , for example Ti � 10V � 2Fe � 3Al, β �>β þ ɷiso �>β þ ɷiso þ α �>β þ α*:* For higher Moeq solute � rich alloy , for example Ti � 13V � 11Cr � 3Al, β �>β þ β<sup>0</sup> �>β þ β<sup>0</sup> þ α �>β þ α*:*

To produce optimum strength–ductility combination, post solution treatment, aging or soaking the material in the temperature range of 200–650°C (well below the βtrans temperature) for a specified time followed by air or furnace cooling is performed [3]. Single aging comprises of heating to the desired temperature, holding for a specified time, and cooling in air or furnace. In Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe beta alloy, single-step aging at 440°C for 8 h resulted in high tensile strength of 1637 MPa [28]. In Ti-15-3 alloy, aging at 520°C for 10 h yields a good combination of fatigue life and fracture toughness [29]. Similarly, in the same Ti-15-3 alloy, single-step aging led to a significant increase in the microhardness and fatigue life

Dual-step aging or duplex aging unlocks the room for further betterment in the mechanical properties through finer and homogeneous α precipitation compared to single-step aging. Many researchers have reported the advantage of duplex aging over single-step aging of beta Ti alloys; most studied is the low-high combination, that is, a low temperature for first step aging and a somewhat higher temperature for second step aging [31–35]. Enhancement of the material behavior during unidirectional and cyclic/fatigue loading could be achieved through duplex aging. In Ti-3Al-8V-6Cr-4Mo-4Zr alloy (also known as Ti 38-644, Beta C), duplex aging resulted in more homogenous alpha precipitation [3]. Precipitates were found to be finer and microstructure was also free of precipitate-free zones (PFZs) and grain boundary α (GBα); this led to a significant improvement in fatigue life of Ti 38-644 [33]. In Ti-15-3 alloy, finer and more homogenous distribution of α precipitates was achieved through duplex aging compared to the single-step aging [34, 36]. In addition to an increase in the mechanical strength (i.e., YS and UTS), increase in ductility was also achieved by duplex aging of Ti-15-3 alloy [17]. Santhosh et al. [37] reported that duplex aging of Ti-15-3 sample leads to (i) a refined and homogenous distribution of the α precipitates in β matrix; (ii) a higher α phase fraction in β matrix; (iii) freedom from PFZs; and (iv) much less GBα, compared to single-aged Ti-15-3 sample. Similarly, duplex aging of the β-C alloy leads to a substantial

tion is given below:

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

**3.1 Single aging**

**3.2 Duplex aging**

**207**

compared to the solution-treated condition [30].

#### **Table 1.**

*βtrans temperature of the beta titanium alloy [16].*

#### **Figure 2.**

*Schematic representation of solution treatment and the aging process.*

practice [24]. However, prolonged solution treatment above β transus may lead to a remarkable loss of mechanical properties owing to the coarsening of β grains [25]. Selection of the solution treatment temperature will also have a strong influence on the morphology and distribution of the alpha precipitation. For example, in Ti-1Al-8V-5Fe (Ti185), sub-transus solution treatment results in higher yield strength and tensile strength and this enhancement is ascribed to the nanoscale α precipitation in the β matrix [10].

## **3. Aging**

During age hardening, solution-treated alloy will be heat treated in the temperature range of 480–620°C for 2–16 h. This heat treatment leads to precipitation of fine alpha phase in the beta matrix, and these precipitations hinder the movement of dislocations, making deformation difficult. This phenomenon is referred to as

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

precipitation hardening. Coherency strains between α precipitates and the β matrix induce a strengthening effect [5]. Comparative examination of the microstructure of solution-treated (800°C/0.5 h) (ST) and microstructure of solution-treated and aged (ST + A) (500°C/8 h) Ti-15-3 samples has clearly revealed the presence of α precipitates in the latter. Precipitation leads to a significant increase in mechanical properties like tensile strength (79%) and hardness (44%) [26]. Age hardening is more effective for beta alloys compared to the α + β alloy owing to the capability of the former to form finer and homogenous α precipitates [27]. The sequence of precipitation of α is dependent on the Moeq of the alloy. The sequence of precipitation is given below:

For lower Moeq solute � lean alloy , for example Ti � 10V � 2Fe � 3Al, β �>β þ ɷiso �>β þ ɷiso þ α �>β þ α*:*

For higher Moeq solute � rich alloy , for example Ti � 13V � 11Cr � 3Al,

β �>β þ β<sup>0</sup> �>β þ β<sup>0</sup> þ α �>β þ α*:*

#### **3.1 Single aging**

To produce optimum strength–ductility combination, post solution treatment, aging or soaking the material in the temperature range of 200–650°C (well below the βtrans temperature) for a specified time followed by air or furnace cooling is performed [3]. Single aging comprises of heating to the desired temperature, holding for a specified time, and cooling in air or furnace. In Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe beta alloy, single-step aging at 440°C for 8 h resulted in high tensile strength of 1637 MPa [28]. In Ti-15-3 alloy, aging at 520°C for 10 h yields a good combination of fatigue life and fracture toughness [29]. Similarly, in the same Ti-15-3 alloy, single-step aging led to a significant increase in the microhardness and fatigue life compared to the solution-treated condition [30].

#### **3.2 Duplex aging**

Dual-step aging or duplex aging unlocks the room for further betterment in the mechanical properties through finer and homogeneous α precipitation compared to single-step aging. Many researchers have reported the advantage of duplex aging over single-step aging of beta Ti alloys; most studied is the low-high combination, that is, a low temperature for first step aging and a somewhat higher temperature for second step aging [31–35]. Enhancement of the material behavior during unidirectional and cyclic/fatigue loading could be achieved through duplex aging. In Ti-3Al-8V-6Cr-4Mo-4Zr alloy (also known as Ti 38-644, Beta C), duplex aging resulted in more homogenous alpha precipitation [3]. Precipitates were found to be finer and microstructure was also free of precipitate-free zones (PFZs) and grain boundary α (GBα); this led to a significant improvement in fatigue life of Ti 38-644 [33]. In Ti-15-3 alloy, finer and more homogenous distribution of α precipitates was achieved through duplex aging compared to the single-step aging [34, 36]. In addition to an increase in the mechanical strength (i.e., YS and UTS), increase in ductility was also achieved by duplex aging of Ti-15-3 alloy [17]. Santhosh et al. [37] reported that duplex aging of Ti-15-3 sample leads to (i) a refined and homogenous distribution of the α precipitates in β matrix; (ii) a higher α phase fraction in β matrix; (iii) freedom from PFZs; and (iv) much less GBα, compared to single-aged Ti-15-3 sample. Similarly, duplex aging of the β-C alloy leads to a substantial

practice [24]. However, prolonged solution treatment above β transus may lead to a remarkable loss of mechanical properties owing to the coarsening of β grains [25]. Selection of the solution treatment temperature will also have a strong influence on the morphology and distribution of the alpha precipitation. For example, in Ti-1Al-8V-5Fe (Ti185), sub-transus solution treatment results in higher yield strength and tensile strength and this enhancement is ascribed to the nanoscale α precipitation in

**S.No Alloy name Commercial name βtrans temperature (°C)**

 Ti-13V-11Cr-3Al B 120 VCA 650 Ti-3Al-8V-6Cr-4Mo-4Zr Beta C 795 Ti-15V-3Cr-3Sn-3Al Ti 15-3 760 Ti-11.5Mo-6Zr-4.5Sn Beta III 745 Ti-10V-2Fe-3Al Ti 10-2-3 800 Ti-1Al-8V-5Fe Ti 1-8-5 825 Ti-12Mo-6Zr-2Fe TMZF 743 Ti-4.5Fe-6.8Mo-1.5Al TIMETAL LCB 800 Ti-5V-5Mo-1Cr-1Fe-5Al VT22 850 Ti-8V-8Mo-2Fe-3Al Ti 8-8-2-3 775 Ti-6V-6Mo-5.7Fe-2.7Al TIMETAL 125 704

During age hardening, solution-treated alloy will be heat treated in the temperature range of 480–620°C for 2–16 h. This heat treatment leads to precipitation of fine alpha phase in the beta matrix, and these precipitations hinder the movement of dislocations, making deformation difficult. This phenomenon is referred to as

the β matrix [10].

**3. Aging**

**206**

**Figure 2.**

**Table 1.**

*Welding - Modern Topics*

*βtrans temperature of the beta titanium alloy [16].*

*Schematic representation of solution treatment and the aging process.*

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 temperature application [24].

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

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,

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 low-

temperature 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

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

stress concentrations.

**7. Intermediate phases**

condition [55].

**209**

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

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

duplex aging is developed; results are promising [32–34].

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

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 precipitation [42–45]. In Timetal LCB, lower heating rate (0.25 ks<sup>1</sup> ) yielded an optimum combination of strength and ductility with a finer and homogenous α precipitation compared to the faster heating rate (20 ks<sup>1</sup> ) [42]. However, this 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 of grain boundary α.
