**3. Development of microstructure during continuous cooling**

Phase composition of titanium alloys after cooling from β phase range is controlled by cooling rate. Kinetics of phase transformations is related to the value of β-phase stability coefficient Kβ resulting from the chemical composition of the alloy [7].

One important characteristic of the alloy is a range of α+β→β phase transformation tempera‐ ture that determines conditions of thermomechanical processing intended for development of suitable microstructure. Start and finish temperatures of α+β→β phase transformation, vary depending on the contents of β stabilizing elements (Table 2).


s – start

Dilatometric tests, microstructure observation and X-ray structural analysis were carried out for cooling rates in the range of 48-0.004°C s-1 and time-temperature-transformation diagrams

The influence of the quantitative parameters of lamellar microstructure on the tensile proper‐ ties and fatigue behaviour of selected two-phase titanium alloys was analysed. Rotational

The materials tested were high strength, two-phase α+β titanium alloys: Ti-6Al-4V,

Ti-6Al-4V 0.3 6.1 − 4.3 − 0.16 0.01 − bal. Ti-6Al-2Mo-2Cr 0.6 6.3 2.6 − 2.1 0.40 0.05 0.2 bal. Ti-6Al-5Mo-5V-1Cr-1Fe 1.2 5.8 5.3 5.1 0.9 0.8 0.05 0.15 bal.

Ti-6Al-4V – martensitic α+β alloy (Kβ = 0.3) – is the most widespread titanium alloy (>60% of all titanium alloys produced in USA and EU). Its high applicability results from good balance of mechanical properties and good castability, plastic workability, heat treatability and weldabil‐ ity. Aluminium addition stabilizes and strengthen α phase, increases α+β↔β transformation temperature and reduces alloy density. Vanadium – β-stabilizer – reduces α+β↔β transforma‐ tion temperature and facilitates hot working (higher volume fraction of β-phase). Depending on required mechanical properties following heat treatment can be applied to Ti-6Al-4V alloy: partial annealing (600÷650ºC / 1h), full annealing (700÷850ºC / furnace cooling to 600ºC / air

Ti-6Al-2Mo-2Cr – martensitic α+β alloy – known as VT3-1, is one of the first widespread hightemperature titanium alloys used in Russia for aircraft engine elements. Amount of βstabilizers is similar to Ti-6Al-4V alloy but β-stabilizing factor is higher (Kβ = 0.6). Mo – stabilises and strengthens β-phase, in the presence of Si increases creep resistance, facilitates plastic working, Cr, Fe – eutectoid elements, stabilise β-phase and strengthen α and β phases

The alloy is processed by forging, stamping, rolling and pressing. Depending on the applica‐ tion and required properties following heat treatment can be applied to the semiproducts: isothermal annealing (870ºC / 1h / furnace cooling to 650ºC / holding for 2 h / air cooling),

cooling) or solutioning (880÷950ºC / water quenching) and ageing (400÷600ºC) [1,3].

**Alloying elements content, wt.%**

K<sup>β</sup> Al Mo V Cr Fe C Si Ti

cycles.

bending tests were carried out to determine high cycle fatigue (HCF) strength at 107

were developed for continuous cooling conditions (CCT).

**2. High strength two-phase titanium alloys**

Ti-6Al-2Mo-2Cr and Ti-6Al-5Mo-5V-1Cr-1Fe (Table 1).

**Table 1.** Chemical composition of the investigated titanium alloys.

in the low and medium temperature range [1].

**factor of βphase**

**Alloy Stability**

70 Titanium Alloys - Advances in Properties Control

f – finish

**Table 2.** Start and finish temperature of the α+β→β phase transformation for selected titanium alloys (vh = vc = 0.08°C s-1)

**Figure 1.** CCT diagram for Ti-6Al-4V alloy.

Cooling of Ti-6Al-2Mo-2Cr and Ti-6Al-4V alloys from above β transus temperature at the rate higher than 18°C s-1 leads to development of martensitic microstructure consisting of α'(α") phases (Fig. 4). Start and finish temperatures of the martensitic transformation β→α'(α") or β→α" do not depend on cooling rate but on β-stabilizing elements content and decrease with increasing Kβ value.

**Figure 2.** CCT diagram for Ti-6Al-2Mo-2Cr alloy.

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73

**Figure 3.** Fig. 3.CCT diagram for Ti-6Al-5Mo-5V-1Cr-1Fe alloy.

For the intermediate cooling rates, down to 3.5°C s-1, martensitic transformation is accompa‐ nied by diffusional transformation β→α + β and the volume fraction of martensitic phases decreases to the benefit of stable α and β phases (Figs 1-2). Cooling rates below 2°C s-1 lead to a diffusion controlled nucleation and growth of stable α and β phases in the shape of colonies of parallel α-phase lamellae in primary β-phase grains (Fig. 5). For extremely low cooling rates precipitations of TiCr2 phase were identified in the Ti-6Al-2Mo-2Cr alloy which were formed in eutectoid transformation.

In the transition alloy Ti-6Al-5Mo-5V-1Cr-1Fe martensitic transformation was not observed at any cooling rate. High cooling rate (>18°C s-1) results in metastable βM microstructure. At lower cooling rates α-phase precipitates as a result of diffusional transformation. At lowest cooling rate, similarly to Ti-6Al-2Mo-2Cr alloy eutectoid transformation occurs and traces of TiCr2 and TiFe2 appears [7].

Microstructure and Mechanical Properties of High Strength Two-Phase Titanium Alloys http://dx.doi.org/10.5772/56197 73

**Figure 2.** CCT diagram for Ti-6Al-2Mo-2Cr alloy.

**Figure 1.** CCT diagram for Ti-6Al-4V alloy.

72 Titanium Alloys - Advances in Properties Control

increasing Kβ value.

in eutectoid transformation.

TiFe2 appears [7].

Cooling of Ti-6Al-2Mo-2Cr and Ti-6Al-4V alloys from above β transus temperature at the rate higher than 18°C s-1 leads to development of martensitic microstructure consisting of α'(α") phases (Fig. 4). Start and finish temperatures of the martensitic transformation β→α'(α") or β→α" do not depend on cooling rate but on β-stabilizing elements content and decrease with

For the intermediate cooling rates, down to 3.5°C s-1, martensitic transformation is accompa‐ nied by diffusional transformation β→α + β and the volume fraction of martensitic phases decreases to the benefit of stable α and β phases (Figs 1-2). Cooling rates below 2°C s-1 lead to a diffusion controlled nucleation and growth of stable α and β phases in the shape of colonies of parallel α-phase lamellae in primary β-phase grains (Fig. 5). For extremely low cooling rates precipitations of TiCr2 phase were identified in the Ti-6Al-2Mo-2Cr alloy which were formed

In the transition alloy Ti-6Al-5Mo-5V-1Cr-1Fe martensitic transformation was not observed at any cooling rate. High cooling rate (>18°C s-1) results in metastable βM microstructure. At lower cooling rates α-phase precipitates as a result of diffusional transformation. At lowest cooling rate, similarly to Ti-6Al-2Mo-2Cr alloy eutectoid transformation occurs and traces of TiCr2 and

**Figure 3.** Fig. 3.CCT diagram for Ti-6Al-5Mo-5V-1Cr-1Fe alloy.


**Table 3.** Phase composition of the selected titanium alloys after controlled cooling from the β-phase range [6,9]

**Figure 4.** Martensitic microstructure of Ti-6Al-2Mo-2Cr alloy after cooling from 1050ºC at a rate of 48°C s-1 (LM–DIC micrograph).

The important parameters for a lamellar microstructure with respect to mechanical properties of the alloy are the β-grain size, size of the colonies of α-phase lamellae, thickness of the αlamellae and the morphology of the interlamellar interface (β-phase) (Fig. 6) [12-13].

> Refinement of the microstructure results in higher yield stress (Fig. 8a). However the increase of yield stress is moderate unless martensitic phase is present. Tensile elongation increases with increasing cooling rate at first (Fig. 8b). However, after reaching maximum the ductility curve declines. Such behaviour was reported earlier and attributed to the change of fracture mode from ductile transcrystalline for low cooling rates to ductile intercrystalline fracture

> **Figure 6.** Stereological parameters of lamellar microstructure: *D* – primary β-phase grain size, *d* – size of the colony of

**Figure 5.** Microstructure of Ti-6Al-2Mo-2Cr alloys after cooling from 1050°C at a rate of 1.2°C s-1: a) LM micrograph, b)

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The size of the colonies of α lamellae having the same crystallographic orientation have significant influence on the mechanical properties of the alloy as it is a measure of effective slip length [8,15]. However transition to the 'basket weave' type of microstructure makes the determination of colonies size even more difficult. Because of that the thickness of α-lamellae was also taken into account as the quantitative parameter illustrating the effect of microstruc‐

along continuous α phase layers at primary β grain boundaries [6,8].

(a) (b)

TEM micrograph.

ture refinement on mechanical properties.

parallel α-lamellae, *t* – thickness of α-lamellae.

Increase in cooling rate leads to refinement of the microstructure – both α colony size and αlamellae thickness are reduced. Additionally new colonies tend to nucleate not only on β-phase boundaries but also on boundaries of other colonies, growing perpendicularly to the existing lamellae. This leads to formation of characteristic microstructure called "basket weave" or Widmanstätten microstructure (Fig. 7) [3].

### **4. Tensile and fatigue properties**

Mechanical properties of two phase titanium alloys strongly depend on morphology of particular phases. In the case of the alloys with lamellar microstructure, the thickness of α lamellae and diameter of their colonies have the most significant influence [3,14].

Microstructure and Mechanical Properties of High Strength Two-Phase Titanium Alloys http://dx.doi.org/10.5772/56197 75

**Cooling rate, °C·s-1 Phase composition of the alloy**

**Table 3.** Phase composition of the selected titanium alloys after controlled cooling from the β-phase range [6,9]

**Figure 4.** Martensitic microstructure of Ti-6Al-2Mo-2Cr alloy after cooling from 1050ºC at a rate of 48°C s-1 (LM–DIC

The important parameters for a lamellar microstructure with respect to mechanical properties of the alloy are the β-grain size, size of the colonies of α-phase lamellae, thickness of the α-

Increase in cooling rate leads to refinement of the microstructure – both α colony size and αlamellae thickness are reduced. Additionally new colonies tend to nucleate not only on β-phase boundaries but also on boundaries of other colonies, growing perpendicularly to the existing lamellae. This leads to formation of characteristic microstructure called "basket weave" or

Mechanical properties of two phase titanium alloys strongly depend on morphology of particular phases. In the case of the alloys with lamellar microstructure, the thickness of α

lamellae and diameter of their colonies have the most significant influence [3,14].

lamellae and the morphology of the interlamellar interface (β-phase) (Fig. 6) [12-13].

α'(α") α + α'(α") α + α'(α") α + α'(α")trace + β α + β α + β

48-18 9 7 3.5 1.2-0.04 0.024-0.004

74 Titanium Alloys - Advances in Properties Control

micrograph).

Widmanstätten microstructure (Fig. 7) [3].

**4. Tensile and fatigue properties**

**Ti-6Al-4V Ti-6Al-2Mo-2Cr Ti-6Al-5Mo-5V-1Cr-1Fe**

β<sup>M</sup> βM + α βM + α βM + α α + β α + β + TiCr2(Fe2)

α'(α") α + α'(α") + β α + α'(α")trace + β α + α'(α")trace + β α + β α + β + TiCr<sup>2</sup>

**Figure 5.** Microstructure of Ti-6Al-2Mo-2Cr alloys after cooling from 1050°C at a rate of 1.2°C s-1: a) LM micrograph, b) TEM micrograph.

**Figure 6.** Stereological parameters of lamellar microstructure: *D* – primary β-phase grain size, *d* – size of the colony of parallel α-lamellae, *t* – thickness of α-lamellae.

Refinement of the microstructure results in higher yield stress (Fig. 8a). However the increase of yield stress is moderate unless martensitic phase is present. Tensile elongation increases with increasing cooling rate at first (Fig. 8b). However, after reaching maximum the ductility curve declines. Such behaviour was reported earlier and attributed to the change of fracture mode from ductile transcrystalline for low cooling rates to ductile intercrystalline fracture along continuous α phase layers at primary β grain boundaries [6,8].

The size of the colonies of α lamellae having the same crystallographic orientation have significant influence on the mechanical properties of the alloy as it is a measure of effective slip length [8,15]. However transition to the 'basket weave' type of microstructure makes the determination of colonies size even more difficult. Because of that the thickness of α-lamellae was also taken into account as the quantitative parameter illustrating the effect of microstruc‐ ture refinement on mechanical properties.

**Average thickness of α-phase lamellae, μm**

σf – fatigue strength at 107 cycles in rotational bending test.

**Table 5.** Mechanical properties of the Ti-6Al-2Mo-2Cr alloy.

**Table 6.** Mechanical properties of the Ti-6Al-5Mo-5V-1Cr-1Fe alloy.

**•** Ti-6Al-2Mo-2Cr *t* = 2 µm, *d* = 20 µm,

**•** Ti-6Al-5Mo-5V-1Cr-1Fe *t* = 1.5 µm, *d* = 35 µm.

**•** Ti-6Al-4V *t* = 3 µm, *d* = 30 µm,

**Table 4.** Mechanical properties of the Ti-6Al-4V alloy

**Average thickness of α-phase lamellae, μm**

**Average thickness of α-phase lamellae, μm**

investigated alloys:

**YS, MPa**

**YS, MPa**

**YS, MPa** **UTS, MPa**

Microstructure and Mechanical Properties of High Strength Two-Phase Titanium Alloys

**UTS, MPa**

**UTS, MPa**

0.8 1235 1305 540 348 1.5 1225 1285 555 340 2.6 1186 1262 535 342 4.3 1160 1236 520 336

Following values of geometrical parameters of lamellar α-phase, i.e. thickness of the α-lamellae (*t*) and diameter of the α-phase lamellae colony (*d*), provided maximum fatigue strength of the

Fatigue fracture surfaces showed transgranular character with typical ductile surroundings of β-phase around α-phase (Fig. 9). Size of the dimples were closely related to thickness of the α-lamellae and size of the colonies of parallel α-lamellae [16]. No pronounced beach markings

1.7 980 1136 550 340 2.0 944 1108 575 338 3.6 924 1055 560 336 6.2 922 1024 540 332

2.4 970 1115 565 326 3.0 928 1068 580 330 5.5 916 1056 570 325 7.6 908 1038 560 336

**σf, MPa**

**σf, MPa**

**σf, MPa** **HV**

77

http://dx.doi.org/10.5772/56197

**HV**

**HV**

**Figure 7.** "Basket-weave" or Widmanstätten microstructure of Ti-6Al-4V alloy after cooling from β-phase range at the rate of 9°C s-1.

**Figure 8.** Yield stress and tensile elongation dependence on the cooling rate from β-phase range for selected titanium alloys.


σf – fatigue strength at 107 cycles in rotational bending test.

**Table 4.** Mechanical properties of the Ti-6Al-4V alloy

**Figure 7.** "Basket-weave" or Widmanstätten microstructure of Ti-6Al-4V alloy after cooling from β-phase range at the

**Figure 8.** Yield stress and tensile elongation dependence on the cooling rate from β-phase range for selected titanium

rate of 9°C s-1.

76 Titanium Alloys - Advances in Properties Control

alloys.


**Table 5.** Mechanical properties of the Ti-6Al-2Mo-2Cr alloy.


**Table 6.** Mechanical properties of the Ti-6Al-5Mo-5V-1Cr-1Fe alloy.

Following values of geometrical parameters of lamellar α-phase, i.e. thickness of the α-lamellae (*t*) and diameter of the α-phase lamellae colony (*d*), provided maximum fatigue strength of the investigated alloys:


Fatigue fracture surfaces showed transgranular character with typical ductile surroundings of β-phase around α-phase (Fig. 9). Size of the dimples were closely related to thickness of the α-lamellae and size of the colonies of parallel α-lamellae [16]. No pronounced beach markings or striations were identified which is an evidence of frequent change of the crack growth direction. This phenomenon along with secondary crack branching are important reasons for advantageous effect of lamellar microstructure on fatigue behaviour.

**Author details**

**References**

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**Figure 9.** Fatigue fracture surfaces of Ti-6Al-5Mo-5V-1Cr-1Fe alloy cooled from 1020°C at a rate of 0.8°C s-1.

The progress of the crack tip through regions of interfacial β-phase is accompanied by the absorption of large amount of energy due to intensive plastic deformation, contributing to lowering the rate of crack propagation. When thickness of β-phase regions decreases, it cannot absorb sufficient amounts of energy and retard the crack propagation.
