**Abstract**

Heat treatment of metastable beta titanium alloys involves essentially two steps—solution treatment in beta or alpha+beta phase field and aging at appropriate lower temperatures. High strength in beta titanium alloys can be developed via solution treatment followed by aging by precipitating fine alpha (α) particles in a beta (β) matrix. Volume fraction and morphology of α determine the strength whereas ductility is dependent on the β grain size. Solution treatment in (α + β) range can give rise to a better combination of mechanical properties, compared to solution treatment in the β range. However, aging at some temperatures may lead to a low/nil-ductility situation and this has to be taken into account while designing the aging step. Heating rate to aging temperature also has a significant effect on the microstructure and mechanical properties obtained after aging. In addition to α, formation of intermediate phases such as omega, beta prime during decomposition of beta phase has been a subject of detailed studies. In addition to covering these issues, the review pays special attention to heat treatment of beta titanium alloys for biomedical applications, in view of the growing interest this class of alloys have been receiving.

**Keywords:** beta titanium alloys, heat treatment, duplex aging, precipitation hardening, intermediate phases, fatigue behavior

### **1. Introduction**

High specific strength and excellent corrosion resistance of titanium-based materials make them an attractive choice for application in various industries such as aerospace, biomaterials, and automotive [1, 2]. Alloying of pure titanium opens a new horizon to develop a variety of products with exceptional properties. Based on the alloying elements and phases present at room temperature, Ti alloys are broadly classified into α, α + β, and β alloys. Compared to α and α + β alloys, β alloys have advantages such as excellent higher specific strength, sufficient toughness, excellent corrosion resistance, better biocompatibility, good fatigue resistance, and good formability.

Moeq is a well-accepted measure to characterize the β-phase stability for a given composition and equation for the deriving the Moeq is shown in Eq. (1) [3, 4].

Moeq*:* ¼ 1*:*0 wt ð Þþ *:*% Mo 0*:*67 wt ð Þþ *:*% V 0*:*44 wt ð Þþ *:*% W 0*:*28 wt% ð Þ Nb þ 0*:*22 wt ð Þþ *:*% Ta 2*:*9 wt ð Þþ *:*% Fe 1*:*6 wt ð Þ� *:*% Cr 1*:*0 wt ð Þ *:*% Al (1)

Beta titanium alloys with a Moeq between 10 and 30 are metastable and hence heat treatable and deeply hardenable [4]. Moeq of the various beta titanium alloys along with their commercial name is shown in **Figure 1**.

temperature). In beta alloys, Ti-6Cr-5Mo-5V-4Al and Ti-5Al-5Mo-5V-3Cr, solution treatment in α + β range followed by aging yielded a better strength–ductility combination compared to solution treatment in the β range followed by aging [11, 12]. Further, if the α + β solution treatment is preceded by rolling in the α + β range even better strength–ductility combination was obtained; this was attributed to the formation of finer β grains in Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe [13]. Similarly, α + β rolling followed by α + β solution treatment of Ti-10V-2Fe-3Al resulted in improvement of fracture toughness [14]. Deformation or cold working in-between solution treatment and aging could lead a path for obtaining homogeneous precipitation because the dislocations serve as a precursor for precipitation [9]. Zhan et al. also reported that dislocations formed during the high strain rate deformation of the metastable β alloy Ti-6Cr-5Mo-5V-4Al have acted as nucleation sites for the α laths/ precipitation at elevated temperature [15]. Similarly, in Ti-15-3 alloy cold working before duplex aging is found to be advantageous in forming finer precipitates [16]. Ti-15-3 alloy exhibits lower precipitation kinetics compared to Timetal LCB and VT22 alloy [17]. Cold working before aging or two-step/duplex aging can be used to increase the precipitation kinetics in Ti-15-3. However, intervening cold work also leads to a significant loss in ductility. Aging of Ti-15-3 alloy with deformed microstructure for prolonged times leads to dislocation rearrangement and formation of subgrains [18]. Grain boundary α and precipitation free zones may occur in aged condition; they play an important role in degrading the tensile and fatigue properties. The authors have reviewed various processing techniques of the beta titanium alloys elsewhere [19]. The heat treatment of beta titanium alloy for biomedical application [20, 21] and heat treatment of additively manufactured beta Ti alloy

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].

T<sup>β</sup> ¼ 882 þ 21*:*1 Al ½ �� 9*:*5 Mo ½ �þ 4*:*2 Sn ½ �� 6*:*9 Zr ½ �� 11*:*8 V½ �� 12*:*1 Cr ½ �� 15*:*4 Fe ½ �

Beta transus temperature for some of the important beta titanium alloys is listed

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

(2)

[22] have also been reported in the literature.

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

**2. Solution treatment**

þ 23*:*3 Si ½ �þ 123 O½ �

in **Table 1**.

**205**

Unlike α + β alloys, in β alloys, higher beta stabilizer content results in complete retention of beta phase upon air cooling or water quenching from solution treatment temperature (above β transus temperature). This difference in transformation can be related to the difference in electron density; the α + β alloys have <4 el/atom, while β alloys have higher electron density, for example, it is 4.148 el/atom for Ti-15-3 alloy [5]. Beta alloys are more workable because of the higher stacking fault energy of the BCC phase, which supports the formation of multiple and cross slips upon deformation, thereby preventing crack formation [6].

In most of the commercial beta alloys, metastable or thermodynamically unstable β phase with BCC crystal structure is formed upon quenching after solution treatment. Hence, subsequent aging leads to the precipitation of the α phase from the beta matrix. To understand the transformation in detail, modeling of precipitation of α by decomposition of β is performed under the framework of Johnson-Mehl-Avrami for Ti-15-3 a metastable beta alloy [7] and Ti-5Mo-2.6Nb-3Al-0.2S/ β21s [8], both being β alloys. The transformation of β to α + β upon aging is a slow diffusion-controlled growth of alpha plates in the beta matrix. Hence, the aging time decides the α precipitation fraction [7]. Various microstructures and correspondingly mechanical properties are feasible through heat treatment of β titanium alloys, thereby making them a wide spectrum of candidate materials for a wide range of applications. Beta Ti alloys with less beta stabilizing element were found to have faster precipitation reactions. For example, VT22 alloy exhibited higher precipitation kinetics compared to the Ti-15-3 and Timetal LCB [9]. Devaraj et al. reported that superior strength is achieved through the micro and nanoscale precipitation of α phase in a beta matrix of Ti-1Al-8V-5Fe [10]. As already mentioned, heat treatment of β titanium alloys is comprised of two steps, that is, solution treatment and aging. The solution treatment can be subdivided into α + β and β solution treatment based on the temperature (i.e., α + β solution treatment T < β transus temperature and β solution treatment temperature T > β transus

**Figure 1.** *Moeq of various beta Ti alloys.*

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

Beta titanium alloys with a Moeq between 10 and 30 are metastable and hence heat treatable and deeply hardenable [4]. Moeq of the various beta titanium alloys

Unlike α + β alloys, in β alloys, higher beta stabilizer content results in complete retention of beta phase upon air cooling or water quenching from solution treatment temperature (above β transus temperature). This difference in transformation can be related to the difference in electron density; the α + β alloys have <4 el/atom, while β alloys have higher electron density, for example, it is 4.148 el/atom for Ti-15-3 alloy [5]. Beta alloys are more workable because of the higher stacking fault energy of the BCC phase, which supports the formation of multiple and cross slips

In most of the commercial beta alloys, metastable or thermodynamically unstable β phase with BCC crystal structure is formed upon quenching after solution treatment. Hence, subsequent aging leads to the precipitation of the α phase from the beta matrix. To understand the transformation in detail, modeling of precipitation of α by decomposition of β is performed under the framework of Johnson-Mehl-Avrami for Ti-15-3 a metastable beta alloy [7] and Ti-5Mo-2.6Nb-3Al-0.2S/ β21s [8], both being β alloys. The transformation of β to α + β upon aging is a slow diffusion-controlled growth of alpha plates in the beta matrix. Hence, the aging time decides the α precipitation fraction [7]. Various microstructures and correspondingly mechanical properties are feasible through heat treatment of β titanium alloys, thereby making them a wide spectrum of candidate materials for a wide range of applications. Beta Ti alloys with less beta stabilizing element were found to have faster precipitation reactions. For example, VT22 alloy exhibited higher precipitation kinetics compared to the Ti-15-3 and Timetal LCB [9]. Devaraj et al. reported that superior strength is achieved through the micro and nanoscale precipitation of α phase in a beta matrix of Ti-1Al-8V-5Fe [10]. As already mentioned, heat treatment of β titanium alloys is comprised of two steps, that is, solution treatment and aging. The solution treatment can be subdivided into α + β and β solution treatment based on the temperature (i.e., α + β solution treatment T < β

transus temperature and β solution treatment temperature T > β transus

**Figure 1.**

**204**

*Moeq of various beta Ti alloys.*

along with their commercial name is shown in **Figure 1**.

*Welding - Modern Topics*

upon deformation, thereby preventing crack formation [6].

temperature). In beta alloys, Ti-6Cr-5Mo-5V-4Al and Ti-5Al-5Mo-5V-3Cr, solution treatment in α + β range followed by aging yielded a better strength–ductility combination compared to solution treatment in the β range followed by aging [11, 12]. Further, if the α + β solution treatment is preceded by rolling in the α + β range even better strength–ductility combination was obtained; this was attributed to the formation of finer β grains in Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe [13]. Similarly, α + β rolling followed by α + β solution treatment of Ti-10V-2Fe-3Al resulted in improvement of fracture toughness [14]. Deformation or cold working in-between solution treatment and aging could lead a path for obtaining homogeneous precipitation because the dislocations serve as a precursor for precipitation [9]. Zhan et al. also reported that dislocations formed during the high strain rate deformation of the metastable β alloy Ti-6Cr-5Mo-5V-4Al have acted as nucleation sites for the α laths/ precipitation at elevated temperature [15]. Similarly, in Ti-15-3 alloy cold working before duplex aging is found to be advantageous in forming finer precipitates [16]. Ti-15-3 alloy exhibits lower precipitation kinetics compared to Timetal LCB and VT22 alloy [17]. Cold working before aging or two-step/duplex aging can be used to increase the precipitation kinetics in Ti-15-3. However, intervening cold work also leads to a significant loss in ductility. Aging of Ti-15-3 alloy with deformed microstructure for prolonged times leads to dislocation rearrangement and formation of subgrains [18]. Grain boundary α and precipitation free zones may occur in aged condition; they play an important role in degrading the tensile and fatigue properties. The authors have reviewed various processing techniques of the beta titanium alloys elsewhere [19]. The heat treatment of beta titanium alloy for biomedical application [20, 21] and heat treatment of additively manufactured beta Ti alloy [22] have also been reported in the literature.
