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

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Synchrotron radiation X-ray diffraction 65

1450 °C. At elevated temperatures, above 1100 °C, the amounts of α and γ

are affected by the presence of the oxides and the oxygen in solid solution.

A number of scans at each temperature reveal the kinetics of the phase

transformations. Figure 3.20 shows the scans measured in 10 min. steps each

for two temperatures. The diffraction patterns of the several scans at each

Mole % phase

100

Liq

600 700 800 900 1000 1100 1200 1300 1400 1500 1600

(°C)

(a)

Mole % phase

100

α/α

600 700 800 900 1000 1100 1200 1300 1400 1500 1600

(°C)

(b)

3.19

Mole percent versus temperature plot, representing the entire

set of the phases in the Ti-46Al-1.9Cr-3Nb alloy (a) calculated using

the Thermo-Calc software, and (b) derived after the fitting procedure

of the diffraction patterns.

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Titanium alloys: modelling of microstructure66

Intensity (c.p.s.)

3000

2500

2000

1500

1000

500

29 30 31 32 33 34 35 36 37 38

2θ (°)

(a)

{111}γ

{011}B2

{021}α

{002}γ

{020}γ

1: 0 min

2: 10 min

3: 20 min

800 °C

Intensity (c.p.s.)

10000

8000

6000

4000

2000

29 30 31 32 33 34 35 36 37 38

2θ (°)

(b)

{111}γ

1: 0 min

2: 10 min

3: 20 min

1200 °C

{104}Al

{002}α

{100}Ti

{002}Ti

{011}B2

{101}Ti

{021}α

{012}α

+ {113}Al

{002}γ

{020} γ

3.20

X-ray diffraction patterns for Ti-46Al-1.9Cr-3Nb alloy, showing

the kinetics of the transformation during isothermal exposure at (a)

800 and (b) 1200 °C.

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Synchrotron radiation X-ray diffraction 67

temperature are similar. The phase transformations are very fast and

thermodynamic equilibria have been achieved in minutes or shorter. No

phase transformations are observed with prolonged time.

Metallography and scanning electron microscopy

The microstructure after high-temperature exposure is of a duplex nature as

in the forged state (Chapter 2) (Fig. 3.21).

In Fig. 3.22, the oxidised layer is shown. The depth of the X-ray penetration

is 5–6.5 µm for scan in the range of 30–40° in 2

, and 12 µm at 70° in 2

3.4.3 Summary

The full-profile quantitative phase analysis of the set of X-ray diffraction

patterns collected at high temperatures allows measurement of the molar

fraction versus temperature for every phase in the Ti-46Al-1.9Cr-3Nb alloy.

A thermodynamically calculated phase diagram of the Ti-46Al-1.9Cr-3Nb

alloy corresponds well to the results of the molar phase fraction derived after

the fitting procedure of the X-ray diffraction patterns. The coefficients of

thermal expansion in the temperature range of 20–1450 °C are derived for

both γ and α /α

phases independently. X-ray diffraction and microscopy

100 µm

3.21

Optical micrograph, showing duplex microstructure of Ti-46Al-

1.9Cr-3Nb alloy after high-temperature measurements.

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Titanium alloys: modelling of microstructure68

data prove the presence of a titanium and aluminium oxide layer at the alloy

surface. The Ti

O oxide forms in the surface of the gamma titanium aluminide

alloy at high temperature.

3.5 References

Berberich F, Matz W, Richter E, Schell N, Kreissig U and Moeller W (2000), ‘Structural

mechanisms of the mechanical degradation of Ti–Al–V alloys: In situ study during

annealing’, Surf Coat Technol, 128–129, 450–54.

Daróczi L, Beke D L, Lexcellent C and Mertinger V (2000), ‘Effect of hydrostatic

pressure on the martensitic transformation in CuZnAl(Mn) shape memory alloys’,

Scripta Mater, 43 (8), 691–97.

Ducher R, Viguier B and Lacaze J (2002), ‘Modification of the crystallographic structure

of γ-TiAl alloyed with iron’, Scripta Mater, 47 (5), 307–13.

Guo F A, Ji V, Zhang Y G and Chen C Q (2001), ‘A study of mechanical properties and

microscopic stress of a two-phase TiAl-based intermetallic alloy’, Mater Sci Eng A,

315 (1–2), 195–201.

Hao Y L, Yang R, Cui Y Y and Li D (2000), ‘The influence of alloying on the α

/(α

+γ)/

γ phase boundaries in TiAl based systems’, Acta Mater, 48 (6), 1313–24.

Li X Y, Taniguchi S, Matsunaga Y, Nakagawa K and Fujita K (2003), ‘Influence of

siliconizing on the oxidation behavior of a γ-TiAl based alloy’, Intermetallics, 11 (2),

143–50.

3.22

Scanning electron microscopy image, showing the surface

oxidised layer (arrow) of Ti-46Al-1.9Cr-3Nb alloy after high-

temperature measurements.



Synchrotron radiation X-ray diffraction 69

Li Z, Gao W, Yoshihara M and He Y (2003), ‘Improving oxidation resistance of Ti

Al and

TiAl intermetallic compounds with electro-spark deposit coatings’, Mater Sci Eng A,

347 (1–2), 243–52.

MacLean E J, Millington H F F, Neild A A and Tang C C (2000), ‘A versatile diffraction

instrument on Station 2.3 of the Daresbury Laboratory’, Mater Sci Forum, 321–324,

212–14.

Palm M, Zhang L C, Stein F and Sauthoff G (2002), ‘Phases and phase equilibria in the

Al-rich part of the Al–Ti system above 900 °C’, Intermetallics, 10 (6), 523–40.

Qin G W, Smith G D W, Inkson B J and Dunin-Borkowski R (2000), ‘Distribution

behaviour of alloying elements in α

(α)/γ lamellae of TiAl-based alloy’, Intermetallics,

8 (8), 945–51.

Tang Z, Niewolak L, Shemet V, Singheiser L, Quadakkers W J, Wang F, Wu W and Gil

A (2002), ‘Development of oxidation resistant coatings for γ-TiAl based alloys’,

Mater Sci Eng A, 328 (1–2), 297–301.

Zhang D, Dehm G and Clemens H (2000), ‘Effect of heat-treatments and hot-isostatic

pressing on phase transformation and microstructure in a β/B2 containing γ-TiAl

based alloy’, Scripta Mater, 42 (11), 1065–70.

Zhang W J, Reddy B V and Deevi S C (2001), ‘Physical properties of TiAl-base alloys’,

Scripta Mater, 45 (6), 645–51.

Zupan M and Hemker K J (2001), ‘High temperature microsample tensile testing of

γ-TiAl’, Mater Sci Eng A, 319–321, 810–14.



Differential scanning calorimetry and

property measurements

Abstract: Differential scanning calorimetry can reveal reproducibly the

thermal effects upon heating of titanium alloys and aluminides. The main

part of this chapter describes the kinetics of the γ + α to α phase

transformation in the Ti-46Al-1.9Cr-3Nb alloy, derived quantitatively from

the calorimetry data, giving phase compositions at any point during the

transformation upon continuous heating. This is followed by discussions of

mechanical property testing, and hydrogen penetration measurement. For the

latter purpose, alloys are cathodically charged with hydrogen in acid

solutions for various times at different current densities and temperature,

followed by X-ray diffraction to determine the nature of the microstructural

change.

Key words: phase transformation, microstructure, intermetallics, hydrogen

absorbing materials, high-temperature alloys.

4.1 Phase and structural transformations

This section explains the transformation behaviour in Ti-46Al-1.9Cr-3Nb

alloy, combining differential scanning calorimetry, microscopy, X-ray

diffraction, and thermodynamic calculations.

4.1.1 Forged alloy

The forged microstructure (Fig. 2.5) is as expected, considering the history

of heat and thermomechanical processing of the forged alloy (Chapter 2).

This duplex microstructure should result in good ductility.

The X-ray diffraction analysis of the forged alloy shows the presence of

γ, α

and a small amount of B2 phases (Fig. 4.1). The B2 phase is clearly

detectable, especially with high-resolution synchrotron radiation diffraction.

More detailed interpretation of the X-ray results is given in Chapter 3.

4.1.2 Differential scanning calorimetry

There are many peaks in the differential scanning calorimetry curves for

continuous heating (Fig. 4.2), with different character and appearance. We

now denote the different thermal effects present as TE1, TE2, TE3, TE4,

TE5, TE6 and TE7. All these thermal effects are reproducible during repeated

experiments.

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Differential scanning calorimetry and property measurements 71

In the temperature range of 900–1150 °C, there are two thermal effects –

TE1 and TE2 (Fig. 4.2). These are quite well separated when heating with

slow heating rates (5, 10 and 20 °C/min). Thermal effect TE1 is sharp and

well defined. Its start temperature, however, is not clearly measurable. TE2

is very broad and ranges from 980 to 1160 °C at a heating rate of 5 °C/min.

TE1 is highly influenced by the heating rate and shifts significantly to higher

temperatures when faster heating rates are applied (Figs. 4.2 and 4.3). At

high heating rates, the two thermal effects, TE1 and TE2, are overlapped.

At a further increase of the temperature to about 1200 °C, there are two

small peaks, TE3 and TE4. TE4 is clearly observed at all heating rates. TE3

is clear at the higher heating rates (20 °C/min or higher) but is hardly visible

at the heating rates of 5 and 10 °C/min. There is a perfect reproducibility for

both peaks at different runs with error bar within a range of ±1.5 °C. Moreover,

both peaks appear at constant temperatures and are not influenced by the

heating rate (Figs. 4.2 and 4.3). The peak temperature of TE3 varies within

a narrow temperature interval of 1177–1179 °C and the peak temperature of

TE4 varies in the temperature interval of 1203–1207 °C, for different heating

rates (Fig. 4.3).

The major thermal effect, TE5, appears in the temperature interval from

1200 to 1350 °C. Its start temperature is not well defined, while the peak and

Intensity (c.p.s.)

3500

3000

2500

2000

1500

1000

500

30 35 40 45 50 55 60 65 70

2θ (°)

{111}γ

{020}α

{011}B2

{021}α

{002/020}γ

{021}γ

{022}α

{121}γ

{002}B2

{022}γ

{221}γ

{023}α

{112}B2

{222}γ

{113/131}γ

4.1

Synchrotron radiation X-ray diffraction pattern of forged Ti-46Al-

1.9Cr-3Nb alloy.

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Titanium alloys: modelling of microstructure72

endo ⇒

50 °C/min

40 °C/min

30 °C/min

20 °C/min

10 °C/min

05 °C/min

TE1

TE2

TE7

TE6

TE3

TE4

TE5

Heat flow (W/g)

2.5

2.0

1.5

1.0

0.5

700 800 900 1000 1100 1200 1300 1400

(°C)

4.2

Differential scanning calorimetry curves for Ti-46Al-1.9Cr-3Nb

alloy employing different heating rates. For clarity, the calorimetry

curves are shifted along the vertical axis with respect to each other.

5 1015202530 35404550

Heating rate (°C/min)

(°C)

1400

1350

1300

1250

1200

1000

950

TE5 - End temperature

TE5 - Peak temperature

TE4 - Peak temperature

TE3 - Peak temperature

TE1 - End temperature

TE1 - Peak temperature

4.3

Influence of the heating rate on the different thermal effects

observed at continuous heating of Ti-46Al-1.9Cr-3Nb alloy.



Differential scanning calorimetry and property measurements 73

end temperatures are easily measurable and are influenced by the heating

rate. There is a small shift to higher temperatures when faster heating is

applied (Fig. 4.3).

TE6 is pronounced at heating rates of 10 and 20 °C/min. At higher heating

rates (30, 40 and 50 °C/min), this thermal effect is not completed. At a lower

heating rate (5 °C/min), TE6 is not clearly seen with only some signs, given

by the presence of an inflection point in the curve, at about 1340 °C. At

further increase of the temperature above 1440 °C, there is some indication

for the presence of another thermal effect, only at heating rates of 5 and

10 °C/min.

4.1.3 Interpretation of the calorimetry data

The variety of peaks and their appearances indicate several phase and structural

changes during heating. Such variety of peaks has not been observed during

differential thermal analysis (Ohnuma et al., 2000) of other gamma titanium

aluminides. Careful analysis is necessary in order to correlate the thermal

effects with the corresponding phase and structural changes. In order to

correctly interpret the calorimetry curves, additional theoretical and

experimental study is necessary involving: (i) thermodynamic calculations

of the phase equilibria in the Ti–Al–Cr–Nb system; (ii) additional experiments

involving microstructure investigations of alloy heated to different

temperatures; and (iii) calorimetry runs on repeat, or secondary, heating.

Thermodynamic calculations of the phase equilibria in the quaternary Ti–

Al–Cr–Nb system are made with Thermo-Calc using the TiAl-database. This

database can be used for the prediction of stable and metastable phase equilibria

in multi-component gamma titanium aluminides. The calculations are for the

actual alloy composition for both taking and not taking into account the

oxygen content of 700 ppm (0.07 wt.% or 0.17 at.%). The calculation results

are similar, with and without taking into account the oxygen content. The

extent of similarity can be assessed by comparing the calculated equilibrium

mole% of phases versus temperature plots with oxygen (this chapter) and

without oxygen (Chapter 3). Though there are five elements in the alloy

compositions, the term quaternary is used because the content of oxygen as

an impurity in these calculations is fixed and small.

Figure 4.4 shows the phase equilibrium diagram at different temperatures,

obtained by keeping the chromium concentration constant and varying the

aluminium and niobium contents. The isothermal diagrams are plotted for

ranges of aluminium and niobium that are of practical interest for gamma

titanium aluminides. The actual alloy composition is also depicted in the

diagrams. The thermodynamic calculations suggest the following conclusions:

• The B2 phase, which does not exist in the binary Ti–Al diagram, is an

equilibrium phase in the quaternary Ti–Al–Cr–Nb phase diagram at certain



Titanium alloys: modelling of microstructure74

thermodynamic conditions. It exists in a wide concentration range,

including the composition of the alloy here, and forms a variety of two-

phase and three-phase equilibria involving the α, γ, and α

phases,

depending on the temperature.

• The alloy composition falls into different phase equilibrium fields,

depending on the temperature, that include α

+ γ + B2 at 1000 °C, α +

γ + B2 at 1100 °C, α + γ at 1200 °C and homogeneous α at 1350 °C (see

Fig. 4.4). This suggests that at heating/cooling, the alloy phase composition

would undergo a variety of phase transformations that are more complicated

as compared to those in the binary Ti–Al phase diagram. α + γ to α is the

major phase transformation and, on the calorimetry curve, should appear

as a prominent wide peak.

✽

30 35 40 45 50 55 60 65 70

at.% Al

30 35 40 45 50 55 60 65 70

at.% Al

30 35 40 45 50 55 60 65 70

at.% Al

30 35 40 45 50 55 60 65 70

at.% Al

at.% Nb

(a) 1000 °C

(b) 1100 °C

(d) 1350 °C

at.% Nb

+ B2

γ + B2

+ γ + B2

+ γ

+ B2

α + α

+ B2

α + B2

α + γ + B2

γ + B2

+ γ

α + γ

α + α

α + B2

+ γ + B2

α + γ

α + β

γ + B2

4.4

Calculated isothermal sections of the quaternary Ti–Al–Cr–Nb

system with the composition of the alloy plotted on them. The

calculations are for Cr = 1.9 at.%. For clarity, only the range of

compositional interest for gamma titanium aluminides is plotted.

α + α

+ γ

