Sha W., Malinov S. Titanium Alloys: Modelling of Microstructure, Properties and Applications

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Titanium alloys: modelling of microstructure98

5.2.3 Ti-8Al-1Mo-1V (Ti 8-1-1)

The Ti-8Al-1Mo-1V alloy, similarly to Ti-6Al-2Sn-4Zr-2Mo, is classified as

a near-α alloy and should have similar phase compositions as functions of

the temperature. For this alloy, again, the tendency is that the amount of the

α phase at lower temperature approaches 100% (Fig. 5.3a). The α phase is

stable to higher temperatures as compared to both Ti 6-4 and Ti 6-2-4-2

alloys. The phase composition of 50%α + 50%β is at 995 °C and the β-

transus temperature is 1040 °C, which is in good agreement with the

experimentally measured one. Note should be taken that the β-transus

temperature depends significantly on the oxygen content. The influence of

the oxygen content on the phase equilibria is described in Section 5.2.9.

Beta

Alpha

Phase amounts (%)

100

700 750 800 850 900 950 1000

(°C)

(a)

Al in alpha

Sn in alpha

Zr in alpha

Mo in alpha

Al in beta

Sn in beta

Zr in beta

Mo in beta

Content (wt.%)

700 750 800 850 900 950 1000

(°C)

(b)

5.2

Calculated equilibria versus temperature in the Ti-6Al-2Sn-4Zr-

2Mo alloy: (a) equilibrium amounts of the α and the β phases; (b)

equilibrium concentrations of the alloying elements in the α and the

β phases.

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Thermodynamic modelling 99

The equilibrium chemical compositions of the α and the β phases as

functions of the temperature for this alloy, shown in Fig. 5.3b and c respectively,

suggest that the β phase becomes enriched with molybdenum at lower

temperatures (Fig. 5.3c), as the α phase forms. A similar tendency for

enrichment but to a lesser extent can be seen for the vanadium content. This

result is not surprising as both molybdenum and vanadium are β-stabilising

Phase amounts (wt.%)

100

850 900 950 1000 1050

(°C)

alpha beta

(a)

Content (wt.%)

880 900 920 940 960 980 1000 1020

(°C)

Al Fe Mo O V

(b)

5.3

Calculated equilibria versus temperature in the Ti-8Al-1Mo-1V

alloy: (a) equilibrium amounts of the α and the β phases; (b)

equilibrium concentrations of the alloying elements in the α phase;

phase.

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Titanium alloys: modelling of microstructure100

elements. In this alloy, on slow cooling, the β to α phase transformation will

be controlled by the kinetics of diffusional redistribution of molybdenum

and vanadium between the α and the β phases.

The above four alloys, Ti-6Al-4V, Ti-6Al-2Sn-4Zr-2Mo, Ti-6Al-2Sn-4Zr-

6Mo, and Ti-8Al-1Mo-1V, are among the most widely used near-α and α +

β titanium alloys. In what follows, we shall show thermodynamic calculations

for other alloys from the same classes, as well as for some metastable β

alloys.

5.2.4 Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.35Si (IMI 834)

IMI 834 is a newer near-α titanium alloy, designed to work at high temperature

with high strength and good fatigue resistance. The thermodynamic equilibria

show that this alloy lies between the Ti-6Al-2Sn-4Zr-2Mo and the Ti-8Al-

1Mo-1V alloys in terms of α/β ratios at different temperatures. The amount

of the α phase at lower temperature approaches 100%, the phase composition

of 50%α + 50%β is at about 985 °C and the β-transus temperature is 1020

°C (Fig. 5.4a).

The equilibrium concentrations of the alloying elements, shown in Fig.

5.4b and c for the α and the β phases, respectively, suggest that the diffusional

β to α phase transformation at cooling in IMI 834 alloy is controlled by

redistribution of molybdenum and niobium between α and β phases toward

enrichment of the β phase with these two β-stabilising elements at low

temperature.

Content (wt.%)

880 900 920 940 960 980 1000 1020 1040

(°C)

Al Fe Mo O V

(c)

5.3

Continued

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Thermodynamic modelling 101

5.2.5 Ti-6Al-7Nb (IMI 367)

This alloy, classified as α + β, is also relatively new, and not well studied

yet. It was initially designed as a high-strength titanium alloy with

biocompatibility for surgical implants. In terms of the phase composition,

i.e. α/β ratio, as a function of the temperature, this alloy is similar to Ti-6Al-

Phase amounts (wt.%)

100

900 920 940 960 980 1000 1020

(°C)

(a)

alpha beta

Content (wt.%)

900 920 940 960 980 1000

(°C)

(b)

Al Fe Mo Nb O Si Sn Zr

5.4

Calculated equilibria versus temperature in the Ti-5.8Al-4Sn-3.5Zr-

0.7Nb-0.5Mo-0.35Si (IMI 834) alloy: (a) equilibrium amounts of the α

and the β phases; (b) equilibrium concentrations of the alloying

elements in the α phase; (c) equilibrium concentrations of the

alloying elements in the β phase.

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Titanium alloys: modelling of microstructure102

4V (Fig. 5.5). At room temperature, there is a small equilibrium amount of

the β phase remaining. The 50%α + 50%β phase composition is at 915 °C

and the β-transus temperature is 1010 °C.

The equilibrium concentrations of the alloying elements in the α and the

β phases for this alloy suggest that the β to α transformation on cooling is in

conditions of diffusional redistribution and enrichment of niobium in the β

phase (Fig. 5.5c).

5.2.6 Ti-10V-2Fe-3Al (Ti 10-2-3)

Ti-10V-2Fe-3Al is one of the most popular near-β (or metastable β) titanium

alloys. This alloy is capable of attaining a wide variety of strength levels

depending on the heat treatment. There is still 15–20% of β phase remaining

at low temperatures, below 400 °C, according to the thermodynamic equilibria

for this alloy as functions of the temperature, presented in Fig. 5.6. The β-

transus temperature is 790 °C, identical to the experimentally measured

value. The equilibrium phase compositions for this alloy also show that the

β to α transformation at cooling should be controlled by diffusional

redistribution and enrichment of the β phase mainly with vanadium and to a

lower degree with iron, both β-stabilising elements (Fig. 5.6c).

5.2.7 TIMETAL β21s

This alloy is a metastable β alloy with high-strength. The phase equilibria

for the β21s alloy, shown in Fig. 5.7, are typical for near-β alloys, with

5.4

Continued

Content (wt.%)

920 940 960 980 1000 1020

(°C)

(c)

Al Fe Mo Nb O Si Sn Zr

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Thermodynamic modelling 103

remaining amount of the β phase at room temperature, and a relatively low

β-transus temperature. The temperature at which the phase composition is

50%α + 50%β is in the range of 650–660 °C, similar to that of the Ti 10-2-

3 alloy.

Phase amount (wt.%)

100

700 750 800 850 900 950 1000

(°C)

(a)

alpha beta

Content (wt.%)

800 850 900 950 1000

(°C)

Al Fe Nb O

(b)

5.5

Calculated equilibria versus temperature in the Ti-6Al-7Nb (IMI

367) alloy: (a) equilibrium amounts of the α and the β phases; (b)

equilibrium concentrations of the alloying elements in the α phase;

phase.

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Titanium alloys: modelling of microstructure104

5.2.8 Summary of equilibrium calculations

The equilibrium amount of β phase at room temperature is different for the

different alloys. For the Ti-6Al-2Sn-4Zr-2Mo, the Ti-8Al-1Mo-1V and IMI

834 alloys, the phase composition at room temperature approaches 100% α

phase. For the Ti-6Al-4V and IMI 367, there is a small amount of remaining

β phase at room temperature, and for the Ti-10V-2Fe-3Al and the β21s

alloys, there are higher amounts of remaining β phase. The β-transus

temperature for the different alloys decreases in the order of Ti 8-1-1, IMI

834, Ti 6-2-4-2, IMI 367, Ti 6-4, β21s, Ti 10-2-3.

These modelling data are consistent with the positions of these alloys in

the schematic pseudo binary phase diagram of titanium alloys and their

molybdenum and aluminium equivalents (Table 5.1).

The equilibrium amounts of α and β phases in different titanium alloys as

functions of the temperature can be used to derive unknown parameters for

a simulation model of the microstructure evolution during the course of the

β to α phase transformation, particularly diffusional growth of the α phase

Widmanstätten plates. This will be illustrated in the next few chapters.

5.2.9 Influence of oxygen

In this section, we will analyse the effect of oxygen on the phase equilibria

in titanium alloys. Thermodynamic calculations are performed for oxygen

contents of 0, 0.2, 0.4, …, 1.4 wt.%. The variation of β phase amounts with

Content (wt.%)

800 850 900 950 1000

(°C)

Al Fe Nb O

(c)

5.5

Continued

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Thermodynamic modelling 105

oxygen content in the alloys is shown in Fig. 5.8. Figure 5.9 plots the variation

of transformation temperatures with oxygen content.

Although the calculations from the database may not be accurate at high

oxygen levels, as the database has not been validated above 0.3%, they are

useful to determine the trend of increasing oxygen levels on the phase

transformation. The general result is that increasing the oxygen content stabilises

Phase amounts (wt.%)

100

(a)

400 450 500 550 600 650 700 750 800

(°C)

alpha beta

600 620 640 660 680 700 720 740 760 780

(°C)

Al Fe V O

(b)

Content (wt.%)

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

5.6

Calculated equilibria versus temperature in the Ti-10V-2Fe-3Al

alloy: (a) equilibrium amounts of the α and the β phases; (b)

equilibrium concentrations of the alloying elements in the α phase;

phase.



Titanium alloys: modelling of microstructure106

the α phase, increases the β-transus temperature and so reduces the amount

of β phase present in the alloy at a given temperature. Also, the transformation

temperature range is widened with increasing oxygen content.

5.2.10 Calculation of the driving force

Next, we use the thermodynamic modelling package to calculate the driving

force for formation of α phase for different alloy compositions as a function

of the temperature, reflecting the degree of undercooling below the β-transus

temperature (Fig. 5.10). This is done by fixing the α phase as ‘dormant’ and

calculating the driving force for its formation. The driving force at different

thermodynamic conditions is further used to calculate the activation barrier

for nucleation of α phase. This is necessary for computer simulations of the

phase nucleation process in the model for the morphology of the β to α

phase transformation in titanium alloys (see Chapter 8).

5.3 Titanium aluminides

A reasonable experimental work literature has now been built up on Ti–Al–

x ternary phase diagrams. However, although these basic systems give insight

to phase equilibrium in certain commonly used γ-TiAl based alloys, it is

difficult to interpret phase relationships in multi-component alloys using just

this information. Thermodynamic calculation offers a means by which phase

equilibrium in multi-component alloys can be predicted. With the development

of TiAl-DATA, a database for the calculation of stable and metastable phase

Content (wt.%)

600 650 700 750 800

(°C)

Al Fe V O

(c)

5.6

Continued



Thermodynamic modelling 107

equilibria in multi-component γ-TiAl based alloys (Section 5.3.1), it becomes

possible to quantify the phase type and composition in multi-component γ-

TiAl alloys.

In this section, Thermo-Calc (TC) is used to quantify the phase constitution

and element distribution in some γ-TiAl intermetallics with various additions

Alpha

Beta

450 500 550 600 650 700 750 800

(°C)

(a)

Phase amounts (%)

100

Content (wt.%)

450 500 550 600 650 700 750 800

(°C)

(b)

beta21s

Al in alpha

Mo in alpha

Nb in alpha

Al in beta

Mo in beta

Nb in beta

5.7

Calculated equilibria versus temperature in the β21s alloy: (a)

equilibrium amounts of the α and the β phases; (b) equilibrium

concentrations of the alloying elements in the α and the β phases.

The calculations are performed for composition Al = 3.00, Mo =

14.12, Nb = 3.48, Si = 0.14, Fe = 0.32, C = 0.016, N = 0.024, and

O = 0.15, all in wt.%.

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