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

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Surface gas nitriding: phase composition and microstructure 443

⇓ TiN ↓ TiO

{002}α

← {210}

1 h

3 h

5 h

30 40 50

(°)

(b)

10000

2500

10000

2500

6400

3600

1600

400

Counts

⇐ {200}

← {111}

⇐ {111}

← {101}

{100}α

{110}β

{101}α

16.18

Continued

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Titanium alloys: modelling of microstructure444

However, Fig. 16.19d reveals the unidirectional nature of such patches. Note

that nitriding was made on both sides (perpendicular to each other) of the sample.

So, the direction of the patches is not determined by the nitriding process,

i.e. not always perpendicular to the nitriding surface as Fig. 16.19a–c would

suggest. These are possibly related with the extrusion of the Timetal 205 rod

⇓ TiN ↓ TiO

{002}α

← {210}

1 h

3 h

5 h

30 40 50

(°)

(c)

10000

2500

10000

2500

6400

3600

1600

400

Counts

⇐ {200}

← {111}

⇐ {111}

← {101}

{100}α

{110}β

{101}α

16.18

Continued



Surface gas nitriding: phase composition and microstructure 445

(a) (b)

50 µm

200 µm

16.19

Optical micrographs of dark layer and patches forming near

the surface of Timetal 205 after gas nitriding at 730 °C. (a) and (d) 1

h; (b) 3 h; (c) 5 h.

from which the samples were prepared. There seems to be an interaction of

the nitriding process with this feature. This feature is erased after nitriding at

830 °C, a temperature above the β-transus of the alloy.

The origin of the ‘banding’ in Fig. 16.19 is most likely due to the presence

of chemistry differences in the Timetal 205, which are both aligned parallel

to the extrusion direction. Since they extend deep into the sample, it is

possibly just a difference in the etching response. Alternatively, the variation

in chemistry promotes a different response to the ingress of nitrogen during

treatment.

After nitriding at 830 °C, the grain structure is completely different (Fig.

16.20), confirming that the samples were at the β region during nitriding at

this higher temperature. After nitriding for 1 hour, an acicular or irregular

needle structure on the nitrided surface is formed (Fig. 16.20a), extending

approximately 50 µm into the depth from the surface. A homogeneous layer

is present beneath this depth. No significant change in the depth of this

nitrogen-effected reaction layer is observed after nitriding for 3 hours (Fig.

16.20b) or 5 hours (Fig. 16.20c) at 830 °C.

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Titanium alloys: modelling of microstructure446

16.8 Ti–Al

In the case of the Ti–Al alloy (Ti-46Al-1.9Cr-3Nb, at. %), colouration is also

observed after nitriding. A silver to golden colour change happens for the

alloy nitrided at 1050 °C.

16.8.1 X-ray diffraction analysis

Phase transformation involves the formation of new compounds after treatment

under N

, which are observed in the diffraction peaks in Fig. 16.21. Nitrogen-

containing compounds are formed during the surface nitriding. The phases

formed on the Ti–Al alloy are strongly influenced by the nitriding process.

For example, Al

is present on the surface after nitriding at 950 °C for

1 hour, but little after 3 or 5 hours (Fig. 16.21a). Also, TiN

0.3

has appeared

in higher abundance after nitriding at the higher temperature of 1050 °C. The

compounds found are TiN, TiN

0.3

, TiO

, and Al

, among which TiN and

TiO

are more dominant.

16.20

Optical micrographs of dark layer forming near the surface of

Timetal 205 after gas nitriding at 830 °C. (a) 1 h; (b) 3 h; (c) 5 h.

100 µm

(c)

(a)

100 µm

(b)

100 µm

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Surface gas nitriding: phase composition and microstructure 447

16.21

X-ray diffraction patterns of the Ti–Al alloy after gas nitriding at

(a) 950 and (b) 1050 °C.

• Al

B2 × TiN

0.3

⇓ TiN ↓ TiO

◊α

1 h

3 h

5 h

◊ {020}

•

{104}

•

{110}

⇐ {111}

◊ {021}

← {101}

× {100}

{110}γ

× {101}

{111}γ

{011}

← {111}

⇐ {200}

•

{113}

← {210}

{002}γ

{020}γ

30 40 50

2θ (°)

(a)

Counts

16.8.2 Microstructure using optical microscopy

Examining the microstructure provides some information towards the formation

of TiN in enhancing the strength of the Ti–Al alloy. There is not a great

difference in grain size and distribution, or changes in the thickness of the

compound layer at the very top of the nitrided surface, after nitriding for

different lengths of time (Fig. 16.22).

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Titanium alloys: modelling of microstructure448

16.21

Continued

• Al

B2 × TiN

0.3

⇓ TiN ↓ TiO

◊α

1 h

3 h

5 h

◊ {020}

•

{104}

•

{110}

⇐ {111}

◊ {021}

← {101}

× {100}

{110}γ

× {101}

{111}γ

{011}

← {111}

⇐ {200}

•

{113}

← {210}

{002}γ

{020}γ

30 40 50

2θ (°)

(b)

Counts



Surface gas nitriding: phase composition and microstructure 449

16.9 Summary of the effect of the parameters

of gas nitriding on the microstructure

The microstructure of nitrided titanium alloys depends on the chemical

composition of the alloy and the processing parameters of gas nitriding.

Temperature and time are the most important parameters of gas nitriding.

There is a tendency of grain growth with increase of the temperature and the

time, as well as new phase formation on the surface of these materials. The

nitrogen diffuses into the metal substrate, forming a compound layer on the

surface of the material which is composed mainly of titanium nitrides Ti

and TiN in the general case, followed by a diffusion zone that consists of an

interstitial solution of nitrogen in the α or β titanium phases. After nitriding

at temperatures below the β-transus temperatures of the titanium alloys, they

have a uniform and homogeneous microstructure, which changes to irregular

and inhomogeneous with increase of the nitriding temperature above their β-

transus temperature. The difference in the microstructure is due to the phase

transformations between α and β that occur during the higher temperature

nitriding or during the heating and cooling stages to and from the nitriding

temperature. The microstructure and thickness of the nitrided layer depend

also on the nitriding time.

The phase composition at the outmost surface layer after surface gas

nitriding is α-Ti (N,O), TiN and TiO

(rutile), the amount of the nitride and

the oxide increasing with nitriding time. Oxidation could not be eliminated

during the nitriding process, though the titanium oxide is formed only on the

surface top. There is not any significant amount of residual surface stress in

the α-Ti(N) phase. The compound layer, in most of the cases, is continuous,

with a thickness varying between 2 and 10 µm, depending on the time and

the temperature of nitriding.

16.22

Optical micrographs of the Ti–Al alloy after gas nitriding at

1050 °C. (a) 1 h; (b) 5 h

100 µm

(a)

100 µm

(b)

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Titanium alloys: modelling of microstructure450

16.10 References

Boriskina N G, Shashkov D P, Kenina E M, Mikhalin V M (1981), ‘Effect of nitriding on

the structure of the surface layers and properties of titanium alloys AT3 and AT6’, Met

Sci Heat Treat, 23 (7–8), 503–06.

Fedirko V M, Pogrelyuk I M, Lopushan’skii V A and Kalyandruk V I (1993), ‘Nitriding

VT35 beta-titanium alloy’, Mater Sci (Russia), 29 (2), 165–69.

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.

Malinov S, Sha W, Guo Z, Tang C C and Long A E (2002), ‘Synchrotron X-ray diffraction

study of the phase transformations in titanium alloys’, Mater Charact, 48 (4), 279–95.

Paransky Y, Gotman I and Gutmanas E Y (2000), ‘Reactive phase formation at AlN–Ti

and AlN–TiAl interfaces’, Mater Sci Eng A, 277A (1–2), 83–94.

Shashkov D P (2001), ‘Effect of nitriding on the mechanical properties and wear-resistance

of titanium alloys’, Metallovedenie i Termicheskaya Obrabotka Metallov, 6, 20–25.

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

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

(2–3), 169–72.

Zhecheva A, Sha W, Malinov S and Long A (2005), ‘Enhancing the microstructure and

properties of titanium alloys through nitriding and other surface engineering methods’,

Surf Coat Technol, 200 (7), 2192–207.

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451

Surface gas nitriding: mechanical properties,

morphology, corrosion

Abstract: Microindentation hardness testing using both Vickers and Knoop

indenters is used to analyse the hardness evolution of nitrided titanium alloys

in relation to the nitriding processing parameters and alloy composition. The

microhardness increases with increase of the temperature and time of

nitriding. This is followed by discussions on tensile properties and fatigue

performance after nitriding, and the surface morphology of the alloys in

relation to the initial surface roughness and nitriding processing parameters.

The corrosion behaviour of the titanium alloys before and after gas nitriding

in response to the corrosive conditions and alloy composition is examined in

the final part of the chapter.

Key words: nitriding, mechanical properties, microhardness, roughness,

corrosion.

17.1 Hardness evolution

This section examines the hardness evolution of titanium alloys in relation to

the processing temperature, time and the alloy composition. The Vickers

indenter is suitable for measuring the hardness from the surface, whereas the

Knoop indenter is for investigating the change of the microhardness across

the depth (distance from the surface towards the substrate core) of the cross-

sections. Optical microscopy assists the study of the difference in the

microhardness of different phases.

The temperature and time of nitriding are the most important processing

parameters of gas nitriding and it is essential to understand their influence on

the hardness behaviour of the titanium alloys.

17.1.1 Near-α Ti-8Al-1Mo-1V

The surface hardness of the alloy nitrided at 950 and 1050 °C increases with

the time prolongation (Fig. 17.1), in agreement with the established effect of

the time of nitriding (Shashkov, 2001). The increase of the hardness is probably

due to the new phases formed on the surface of the materials during the

process of nitriding, such as TiN and TiO

. There is no significant change of

the surface hardness with the increase of the temperature.

The time prolongation and the temperature increase result in increase of

the nitrided layer thickness, which can be estimated from microhardness

profiles because it can be supposed that the nitrided layer ends where the

microhardness value approaches the core microhardness.

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Titanium alloys: modelling of microstructure452

With the increase of the temperature from 850 to 1050 °C, the thickness

of the nitrided layer increases (Fig. 17.2). For this alloy it varies between 50

and 350 µm, depending on the temperature and time of nitriding. It is difficult

to determine the thickness of the nitrided layer from the microstructure

images of the cross-sections because there is no clear boundary between the

diffusion zone and the substrate core.

The hardness values of the surface layer are very high and they decrease

with the depth of the materials through the diffusion zones to approach the

base microhardness of the matrix in the unsaturated core. There is a tendency

of increase in the microhardness values with the time prolongation and the

temperature of nitriding for this alloy. The increase of the microhardness

values and the thickness of the nitrided layer after nitriding at higher

temperatures is due to the increase of the diffusion coefficient. The influence

of the temperature on the microhardness behaviour has been studied by

Galliano et al. (2001).

The maximal value of the microhardness for Ti-8Al-1Mo-1V is 820 HK0.05

for the sample nitrided at 1050 °C for 5 hours. Figure 17.3 presents an

optical micrograph showing the microhardness values from the surface to

the core, associated with the decrease of the nitrogen content.

The variation in the microhardness values for the alloy nitrided at

temperatures higher than its β-transus is due to local differences in the irregular

microstructure (see Fig. 17.4). The microhardness values of the white grains

are higher, possibly because of the different nitrogen content. Nitriding at

temperatures above the β-transus for titanium alloys is not recommended for

950 °C

1050 °C

012 3456

Nitriding time (h)

Hardness (HV0.5)

1000

800

600

400

200

17.1

Surface hardness of Ti-8Al-1Mo-1V after gas nitriding at 950 and

1050 °C for 1, 3 and 5 hours.

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