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

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Introduction to titanium alloys 3

processing parameters to achieve these properties are sometimes critical.

The metastable β alloys are heat treatable to very high strength usually by

solution treatment plus quenching followed by ageing. The highest strength

among titanium alloys can be reached with such heat treatment combined

with cold deformation.

1.2.2 The most popular alloys

Over the recent decades, many titanium-based materials have been developed

for different ranges of applications.

Ti-6Al-4V presently is the most widely used, high-strength titanium alloy.

It finds a large application in the aerospace industry and in medicine, for

medical prostheses. It is also used in the automotive, marine and chemical

industries (Boyer et al., 1994). The control and optimisation of the morphology

of the α phase is one of the important issues in the use of the alloy.

Thermomechanical processing is a very useful method of improving the

microstructure, e.g. controlling the size and the aspect ratio of the α lamellar

phase, optimising the phase ratio of α to β phases, and controlling the

morphology of the β phase. Hot deformation of the Ti-6Al-4V alloy in the β

phase field results in relatively large prior-β grains, α phase is formed during

cooling and martensite may be formed on rapid quenching. When processing

in the α + β phase field, a much finer α + β structure can be obtained,

because the primary α phase limits the growth of β phase. When processing

in the β phase field, the microstructural evolution may include the mechanical

deformation of the equiaxed β grains and possible dynamic or metadynamic

recrystallisation during processing, and the β to α phase transformation

when cooling after deformation.

Commercial titanium alloy Ti-6Al-2Sn-4Zr-2Mo (usually in its modified

version Ti-6Al-2Sn-4Zr-2Mo-0.08Si) is one of the most commonly used

high temperature titanium alloys. It has an attractive combination of tensile

strength, creep strength, toughness and stability for long-term applications at

temperatures up to 425 °C. It is used for gas turbine components and in other

applications where this good combination of properties is required. At the

same time, it has poor tribological properties that are typical of most of the

titanium alloys. It has low surface hardness and wear resistance. These

disadvantages of the material limit its application.

Ti-6Al-2Sn-4Zr-6Mo is becoming increasingly important for the aero engine

industry. This heat-treatable α-β alloy is designed to combine the elevated

temperature characteristics of Ti-6Al-2Sn-4Zr-2Mo with higher strength levels.

Ti-8Al-1Mo-1V was developed for gas turbine engine applications and

especially for compressor blades and wheels. It has the highest modulus of

all the commercial titanium alloys.

Ti-10V-2Fe-3Al is a high-strength titanium alloy. Its major advantages

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Titanium alloys: modelling of microstructure4

are its excellent forgeability, high toughness in air and saltwater environments

and its high hardenability. It is used in the aerospace industry.

The commercial alloy β21s has a higher strength to weight ratio (specific

strength) over other common engineering materials. It is specifically designed

for oxidation resistance, elevated temperature strength, creep resistance and

thermal stability. This alloy is also resistant to corrosion from aircraft hydraulic

fluids (e.g. Skydrol) at all temperatures. It is applicable for metal matrix

composites because it can be rolled to foil, is compatible with fibres, and is

stable until 816 °C. Commercial applications include aerospace components

and prosthetic devices. For the latter application, with appropriate thermal

mechanical processing, the modulus of β21s is comparable to that of bone.

β21s also possesses excellent hydrogen embrittlement resistance, which has

contributed to its chemical and offshore oil uses.

Other titanium alloys described in this book include Timetal 205. The

chemical compositions of the alloys are shown in Table 1.1.

1.3 Titanium aluminides

The aluminides of titanium (titanium aluminides) are ordered intermetallic

alloys, and are one special class of titanium alloys. Ti–Al alloys are different

compared to conventional titanium alloys because they are principally chemical

compounds alloyed to enhance strength and formability. Aluminides have

higher operational temperatures than conventional titanium alloys, but are

more expensive and generally have lower ductility and formability. With the

increasing demands for high temperature structural materials with high strength-

to-weight ratios, there has been a great increase in the research and development

of alloys based on titanium aluminides such as TiAl, Ti

Al and Ti

AlNb,

because of their attractive and unique set of specific properties, such as high

melting temperature, exceptionally high proof strength at elevated temperatures

(Winstone, 2001), hardness, high modulus and modulus retention with

temperature, creep resistance at high temperatures, and good oxidation

resistance (Sun et al., 2002). These alloys are characterised by their high

oxidation and corrosion resistance of at least up to 800 °C. However, the

practical use of these titanium aluminides has been limited. The main reasons

for this are their lack of ductility and toughness, and poor fracture resistance,

at ambient temperatures. In addition, cost remains a negative factor.

Nevertheless, they offer the promise of a light, strong, and chemically stable

material for structural applications at high temperatures, in the aerospace,

automobile and turbine power generation industries. Intermetallic compound

Al (α

-phase) with superstructure is the main component of a number of

single- or two-phase alloys, for example Ti

Al/TiAl (α

/γ-phases).

The γ-based titanium aluminide intermetallics have been considered as

one of the most promising candidate materials, for examples in advanced

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Introduction to titanium alloys 5

Table 1.1

Characteristics and typical chemical composition in weight percent of some popular titanium alloys

Alloy Type β-transus Al Mo V Nb Zr Fe Cr Sn O Si H C N

(°C)

Ti-8Al-1Mo-1V near α 1040 8.02 1.04 0.98 – – 0.08 – – 0.085 – 0.0035 0.01 0.004

Ti-6Al-2Sn- near α 995±15 6.13 1.95 – – 3.97 0.07 – 1.93 0.065 0.11 0.0045 – 0.002

4Zr-2Mo

Ti-6Al-2Sn- α + β 932 6 6 – – 4 0.15 – 2 0.1 – 0.0125 – 0.015

4Zr-6Mo

Ti-6Al-4V α + β 1000±15 6.59 – 4.1 – – 0.18 – – 0.19 – 0.002 – 0.005

Ti-10V-2Fe-3Al near β 790–805 3.06 – 9.95 – – 1.93 – – 0.09 – 0.002 0.02 0.009

β21s meta- 807 3 14.12 – 3.48 – 0.32 – – 0.15 0.14 0.0963 0.016 0.024

stable β

Timetal 205 β 780 – 14–16 ––––––– –– – –

Ti-46Al- γ – 31.3 – – 7.0 – – 2.5 – 0.07 – 0.0024 – –

1.9Cr-3Nb

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Titanium alloys: modelling of microstructure6

aerospace engine and airframe components. The remaining part of this section

will focus on this type of aluminide. Efforts to enhance ductility and improve

other mechanical properties have been concentrated on microstructural control

with a variety of micro-alloying additions including B, Cr, Mn, Mo, Nb, Si,

V, Ta, and W. TiAl alloys usually have the form of lamellar structures composed

of γ-TiAl and α

-Ti

Al phases. They have higher specific strength than Ni

based superalloys (Lipsitt et al., 2001), but their tensile strength is not as

high and further improvement of their strength is necessary for them to

replace Ni based alloys. It is known that refinement of the lamellar size is a

promising way of improving the yield strength of TiAl alloys.

Industrial multiphase γ-TiAl alloys have a wide set of phases, which

include disordered α (ordinary hcp structure) and β (ordinary bcc structure)

phases, ordered α

(Ti

Al with the DO

structure based on hcp lattice) and

γ (near-cubic face-centred tetragonal L1

crystal structure), and ordered body-

centred cubic B2 phase.

TiAl-based alloys generally start solidification as the β phase and go

through the α single-phase region and α to α + γ and α + γ to α

+ γ reactions,

producing an α

+ γ two-phase structure. Thus, different two-phase structures

can be obtained by manipulating these phase transformation reactions.

Mechanical properties can be tailored to meet the needs for a special application

component by controlling the alloy microstructure.

Duplex (DP) and fully lamellar (FL) microstructures are two typical

microstructures. Gamma titanium aluminides usually have a lamellar

microstructure that consists of equiaxed polycrystalline grains and densely

packed lamellae within the grains. The lamellae consist of two phases, α

and γ. The α

plates are thinner than the γ plates and are interspersed between

the many γ plates. Because they have slower diffusion rates than conventional

titanium alloys, the titanium aluminides feature enhanced high-temperature

properties, in strength retention, creep, stress rupture, and fatigue resistance.

The phase transformation involving the ordering of disordered α-Ti phase

with formation of ordered α

(Ti

Al) and γ (TiAl) phases at different

temperatures in TiAl-based alloys is of major importance in manufacturing

these materials, as it controls the grain size and spacing of γ and α

lamellae,

and hence the yield strength and other mechanical properties. Because of its

importance, the α to α

+ γ transformation has been studied in great depth

from both technological (Hu, 2001) and scientific (Yamaguchi et al., 2000)

points of view.

The fully lamellar microstructure is currently regarded as the most attractive

because the overall balance of properties is reasonably good. The presence

of lamellae can have a significant effect on the mechanical properties due to

the high density of semi-coherent lamellar boundaries, which affects subsequent

dislocation motion and fracture behaviour. The mechanical behaviour of γ

alloys changes with both the volume percentage of the various phases and

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Introduction to titanium alloys 7

grain morphologies. Two main microstructural parameters – grain size and

lamellar thickness, of both α

and γ phases – compete to contribute to the

tensile and toughness properties. The influence of lamellar thickness on

yield strength (Cao et al., 2000), deformation and fracture, and creep behaviour

(Wen et al., 2000) has proved to be crucial.

Oxygen and nitrogen have significant effect on alloy strength properties.

As the amount of oxygen and nitrogen increases, the toughness decreases

until the material eventually becomes quite brittle.

Much improved service properties, including ductility, may be achieved

with titanium aluminides by forging and extrusion at temperatures high enough

for the alloys to have the α structure, thermomechanical processing and heat

treatment. Development of the new methods of such a treatment should be

based on scientific data about phase transformations in these materials.

1.4 Modelling

The development of titanium alloys, including titanium aluminides targeted

for specific applications, has been an important task in the manufacturing

industry. Traditionally, such alloys and processes are designed empirically,

i.e. with a mixture of experience and intuition. This is both costly and time

consuming. It is now highly desirable to develop advanced computer-based

models for the design of alloy compositions in an increasingly theoretical

and less empirical manner. These models will be based upon the general

theories of thermodynamics and phase transformation.

Accurate prediction of microstructural evolution is essential to control the

microstructure, to understand processing–microstructure–property

relationships, and to tailor manufacturing conditions. It is important to

understand the correlation among processing parameters, microstructure and

properties, which would allow the optimisation of the processing parameters

and alloy composition in order to achieve the desired combination of properties

for any particular application. At present, this is one of the most challenging

areas in the field of research on titanium alloys. The relationship between the

processing parameters and the mechanical properties for titanium alloys has

so far mainly been studied empirically. The idea of computer modelling and

simulation of the different processes taking place during processing of titanium

alloys and of the correlation between processing parameters and mechanical

properties is very attractive and highly desirable. Computer modelling of

these alloys is a promising way to optimise the processing parameters.

Describing and understanding phenomena and correlations in materials

by modelling and simulation is becoming popular and effective. Materials

modelling and simulation have proven their ability to address a variety of

problems ranging from development of fundamental knowledge to solving

real practical and industrial problems. There is a huge amount of work

devoted to materials modelling (Guo et al., 2004).

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Titanium alloys: modelling of microstructure8

The titanium manufacturing industry will benefit from improvement in

microstructure control leading to more consistent and less scattering of

properties, therefore gaining improved competitiveness in worldwide markets.

The aerospace and offshore industries will benefit from improved materials

properties and a wider selection of materials due to more advanced processing

technology. Microstructure modelling will contribute to the scientific

understanding of microstructural evolution as a whole, which will also have

long-term benefit to the improvement of titanium products for other applications

including both aerospace and non-aerospace, the latter including applications

in civil engineering and a whole range of other applications. Therefore,

mechanical and construction industries will also benefit. Moreover, the

established modelling methods are also applicable to other alloy systems.

1.5 References

Boyer R, Welsh G and Collings E W (1994), Materials Properties Handbook: Titanium

Alloys, Materials Park, OH: ASM International.

Cao G X, Fu L F, Lin J G, Zhang Y G and Chen C Q (2000), ‘The relationships of

microstructure and properties of a fully lamellar TiAl alloy’, Intermetallics, 8 (5–6),

647–53.

Guleryuz H and Cimenoglu H (2005), ‘Surface modification of a Ti–6Al–4V alloy by

thermal oxidation’, Surf Coat Technol, 192 (2–3), 164–70.

Guo X, Pettifor D, Kubin L and Kostorz G (eds) (2004), Multiscale Materials Modelling,

Mater Sci Eng A, 365 (1–2), 1–354.

Hu D (2001), ‘Effect of grain refinement on continuous cooling phase transformation in

some TiAl-based alloys’, in: Winstone M R (ed), Titanium Alloys at Elevated

Temperature: Structural Development and Service Behaviour, London: IoM

Communications, 263–75.

Leyens C and Peters M (2003), Titanium and Titanium Alloys: Fundamentals and

Application, Weinheim: Wiley.

Lipsitt H A, Blackburn M J and Dimiduk D M (2001), ‘Commercializing intermetallic

alloys: Seeking a complete technology’, in: Hemker K J, Dimiduk D M, Clemens H,

Darolia R, Inui H, Larsen J M, Sikka V K, Thomas M, Whittenberger J D (eds),

Structural Intermetallics, Warrendale, PA: TMS, 73–82.

Škorić B, Kakaš D, Rakita M, Bibić N and Peruškob D (2004), ‘Structure, hardness and

adhesion of thin coatings deposited by PVD, IBAD on nitrided steels’, Vacuum, 76

(2–3), 169–72.

Sun J, Wu J S, Zhao B and Wang F (2002), ‘Microstructure, wear and high temperature

oxidation resistance of nitrided TiAl based alloys’, Mater Sci Eng A, 329–331, 713–

17.

Wen C E, Yasue K, Lin J G, Zhang Y G and Chen C Q (2000), ‘The effect of lamellar

spacing on the creep behavior of a fully lamellar TiAl alloy’, Intermetallics, 8 (5–6),

525–29.

Winstone M R (ed) (2001), Titanium Alloys at Elevated Temperature: Structural Development

and Service Behaviour, London: IoM Communications.

Yamaguchi M, Inui H and Ito K (2000), ‘High-temperature structural intermetallics’,

Acta Mater, 48 (1), 307–22.

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Microscopy

Abstract: The first part of the chapter discusses in situ high temperature

microscopy, using two popular commercial titanium alloys as examples. An

optical microscope equipped with a high temperature stage is used with both

normal and florescence lighting. The second part describes optical

microscopy and scanning electron microscopy conducted on Ti-46Al-1.9Cr-

3Nb alloy before and after heat treatments. Heat treatment has led to the

formation of a fully lamellar microstructure. The third and final part extends

to transmission electron microscopy and energy-dispersive spectroscopy of

this alloy before and after heat treatment.

Key words: high temperature, optical microscopy, titanium aluminides based

on TiAl, microstructure, scanning/transmission electron microscopy.

2.1 High temperature microscopy of surface

oxidation and transformations

Titanium alloys are desirable and essential for many structural applications

at elevated temperatures (Winstone, 2001). It is important, especially for

high-temperature applications, to understand the nature of the phase

transformations taking place under such conditions. Most of the research on

the phase transformations in titanium alloys is performed at room temperature

on samples quenched after heat treatment. There have been some studies

dedicated to in situ experimental studies and modelling of the phase

transformations in titanium alloys (Chapters 6 and 7). These studies, however,

are concerned with the quantitative study of the kinetics and the

thermodynamics of the phase transformations but not with the morphology

and geometry of the phases present.

It is possible to study the material’s microstructures by applying in situ

microscopic investigations. Kobayashi et al. (2001) studied the phase trans-

formations in Ti-10%Zr alloy during continuous cooling and isothermal holding

using hot-stage optical microscopy. High-temperature microscopy was also

applied to study the reverse martensite transition in titanium–zirconium alloys.

In this section, we will introduce and describe in situ study of the phase

transformations and changes on the titanium alloys’ surface during high

temperature exposure using high-temperature microscopy.

2.1.1 Surface oxidation and deoxidation

Figure 2.1 shows how the surface microstructure of the Ti-6Al-2Sn-4Zr-

2Mo-0.08Si alloy changes during continuous heating at different temperatures.

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Titanium alloys: modelling of microstructure12

The sample was etched chemically before the experiment and the microstructure

was developed. The initial microstructure was typical for the alloy, containing

α lamellae colonies. There is no change in the microscope image when the

alloy was heated to 600 °C (Fig. 2.1a). The images at room temperature and

at 600 °C are identical for both Ti-6Al-2Sn-4Zr-2Mo-0.08Si and Ti-6Al-4V

alloys. With the continuing heating process, a gradual change of the brightness

of the samples is observed. This implies that processes of oxidation took

place on the samples’ surface. The most oxidised surface is observed at

about 750 °C (Fig. 2.1b). However, further increase of the temperature leads

to disappearance of the surface oxidised layer and the microscopic image

returns to the initial image (see Fig. 2.1c). This effect has been observed in

many samples and mainly when a standard purity helium (99.996%) atmosphere

was used for reducing sample oxidation. The oxidation usually starts at

around 650 °C and disappears at around 900 °C. In most of the samples,

there are no changes upon cooling.

The process of oxidation takes place because the high-temperature optical

microscope stage is not a vacuum device and the helium purity is not sufficient

to completely prevent the oxidation. Two possible reasons for the disappearance

of the surface oxidation are:

• The increase in temperature leads to an increase of the diffusivity of

oxygen. Hence, when the temperature is high (> 850 °C), oxygen from

the surface diffuses to the sample core, tending towards homogenisation.

This effect would result in a decrease of the oxygen level in the surface

layer, if there is not an adequate oxygen supply from the environment.

• The oxides formed on the sample surface in the temperature range of

650–750 °C are Ti

O and Ti

O. The ranges of existence of these oxides

are not well determined even for the binary Ti-O system. However, it is

known that they are not stable at high temperatures. These ordered oxides

100 µm

(a) (c)(b)

100 µm

2.1

Microscopic images taken at different temperatures during

continuous heating (25 °C/min) of etched Ti-6Al-2Sn-4Zr-2Mo alloy,

showing the appearance of an oxidised surface layer at about 750 °C,

which disappears upon further heating to 950 °C.

(a) 600 °C; (b) 750 °C; (c) 950 °C.

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Microscopy 13

are formed and decomposed through second order transitions (α Ti ↔

O and Ti

O ↔ Ti

O). The temperatures for these transitions are not

yet well established. For the binary Ti-O alloys, these temperatures are

suggested at around 600 °C. For the titanium alloys, the temperature

may be increased due to the alloying elements.

2.1.2 Thermal etching

α ↔ β phase transformations were not detected during continuous heating

and cooling when etched samples were used, even when oxidation was not

apparent. Further experiments were conducted starting with polished but

unetched samples. Because the samples were unetched, their microstructure

was not observable at room temperature (Fig. 2.2a). However, when these

samples were heated to about 680–750 °C, their microstructure gradually

became visible (Fig. 2.2b). This effect was observed in all unetched samples.

Usually the microstructure was developed at around 680–720 °C when a

heating rate of 25 °C/min was applied, and at higher temperatures when

faster heating rates were used.

The alloy microstructure was observed due to the effect of thermal etching.

The thermal etching is based on the difference in the thermal expansions of

the phases present. As a result, the topography of the sample surface is

changed and the microstructure becomes visible. A small effect is also possible

from concurrent surface evaporation. However, the surface evaporation would

result in irreversible changes on the sample topography, whereas in the

present case a tendency to reverse flattening of the sample surface upon

further heating is observed (see next section).

100 µm

(a)

100 µm

(b)

2.2

Microscopic images at (a) room temperature and (b) 680 °C of

unetched Ti-6Al-4V alloy, showing the development of the

microstructure at high temperatures as a result of thermal etching.

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Titanium alloys: modelling of microstructure14

2.1.3 Phase transformations

Figure 2.3 traces one case when in the Ti-6Al-4V alloy the α ↔ β phase

transformations were observed and monitored in situ. Such observations are

experimentally possible when surface oxidation is avoided. The oxidation

can be avoided by (i) use of helium with very high purity (> 99.9999%); (ii)

protection of the sample surface with heat-resistant glass and (iii) applying

higher heating/cooling rates (60 °C/min). Following these experimental

conditions, a number of successful cases when the phase transformations

were detectable were obtained.

Starting with an initial unetched structure (Fig. 2.3a), when the sample

was heated up to 800 °C (Fig. 2.3b), the α lamellae microstructure was

detectable due to the thermal etching. Further increases in temperature led to

100 µm

(a)

100 µm

(b)

100 µm

(c)

100 µm

(d)

2.3

Microscopic images taken at different temperatures during

continuous heating and cooling (60 °C/min) of an unetched Ti-6Al-4V

alloy, showing the change of the sample surface topography as a

result of α ↔ β phase transformations. (a) 20 °C (initial structure); (b)

800 °C (during heating); (c) 1100 °C (maximum temperature);

(d) 800 °C (during cooling) and 20 °C (final structure).

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