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

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Surface gas nitriding: phase composition and microstructure 423

2Mo (Fig. 16.3). α-Ti(N) has a similar diffraction pattern to TiN

0.3

. TiN

0.3

has the lowest nitrogen content among all titanium nitrides and a similar

structure to α-Ti, and it was detected on the surface of titanium alloys nitrided

under similar processing conditions, as reported in the literature. It is difficult

to distinguish which of these phases is present, due to the similarity of their

diffraction patterns.

Ti-6Al-2Sn-4Zr-2Mo alloy, as well as Ti-8Al-1Mo-1V alloy, has a high

aluminium content and because of that it is supposed that the aluminium-

rich phases Ti

Al or Ti

AlN may exist under certain processing conditions.

XRD analyses (Fig. 16.3), however, do not show Ti

Al or Ti

AlN phases.

The formation of new phases is also evident in the differential scanning

calorimetry and thermo-gravimetry (DSC/TG) curves in Fig. 16.4. It can be

seen that phase transformations took place during heating and cooling for

• α-Ti (N)

⇓ TiN

↓ TiO

30 40 50 60 70

2θ (°)

Intensity (c.p.s)

2000

1500

1000

500

1 h

3 h

5 h

•

⇓•

⇓

•

{103}

← {310}

•

{110}

⇐ {220}

← {220}

← {211}

⇐ {200}

•

{102}

← {200}

•

{101}

← {101}

← {111}

⇓

•

⇓

•

⇓

•

⇓

⇐ {311}

•

⇐ {111}

•

{100}

⇓

•

{002}

← {210}

← {101}

16.3

X-ray diffraction patterns of Ti-6Al-2Sn-4Zr-2Mo after gas

nitriding at 1050 °C for 1, 3 and 5 hours.



Titanium alloys: modelling of microstructure424

the sample nitrided at 950 °C for 5 hours. These curves are from the DSC

with TG, so the increase of the sample mass could also be recorded during

the process of nitriding. An increase of about 0.5% is obtained for the sample

nitrided at 950 °C for 5 hours.

Titanium oxide TiO

is detected for all alloys. Further experiments were

carried out, removing the surface layers of a nitrided sample in steps of

approximately 10 µm, performing X-ray analyses after each removing step.

These experiments were carried out in order to see how the phase composition

changes with depth (Fig. 16.5). From the first scan, it can be seen that the top

layer consists of mainly TiO

. Underneath, TiN and α-Ti(N) are also detected.

The third scan shows the existence of only α-Ti(N). These results prove that

the oxide is present on the surface of the materials and does not penetrate the

sample.

16.3.2 Microstructure

There are differences in the microstructure of the alloy nitrided at different

temperatures and for different times (Fig. 16.6). The nitrided layer after

950 °C is uniform and homogeneous, and the layer thickness can be measured.

The layer thickness is 60, 120, and 180 µm after 1, 3, and 5 hours, respectively,

with an error of ±10 µm. By increasing the nitriding temperature to 1050 °C,

an irregular needle structure is formed under the surface. In this case, it is

difficult to define the layer thickness from the microstructure. The grain

growth at the higher nitriding temperature is because 1050 °C is above the β-

transus temperature for this alloy (995±15 °C). The grain size increases to

TG(%)

100.30

100.20

100.10

100.00

99.90

Time–temperature curve

TG signal

DTA signal

0 50 100 150 200 250 300 350

Time (min)

DTA (µV/mg)

Temperature

(°C)↑ exo

0.02

–0.02

–0.04

–0.06

–0.08

–0.10

–0.12

–0.14

900

800

700

600

500

400

300

200

100

16.4

DSC and TG curves of Ti-6Al-2Sn-4Zr-2Mo nitrided at 950 °C for

5 hours.



Surface gas nitriding: phase composition and microstructure 425

(b)

16.5

(a) X-ray diffraction patterns of Ti-6Al-2Sn-4Zr-2Mo after gas

nitriding at 950 °C for 5 hours and removing the surface layers in

steps of 10 µm; and (b) SEM micrograph of the microstructure and

the positions of the scans.

30 40 50 60 70

2θ (°)

(a)

• α-Ti(N) ⇓ TiN

↓ TiO

Intensity (c.p.s.)

80000

60000

40000

20000

5000

3000

2000

1000

← {110}

← {101}

← {200}

← {210}

← {211}

← {220}

← {310}

← {301}

•

←

•

{100}

•

{002}

⇐

{111}

•

{101}

←

⇐

{200}

•

{102}

←

⇐

{220}

← /

•

{110}

←

← {112}

•

{103}

the order of 400 µm in length and 50 µm in width, after 5 hours nitriding

time, similar to the results for Ti-8Al-1Mo-1V. The alloys are in different

phase conditions during nitriding at 950 and 1050 °C. At 950 °C, the diffusion

zone is mainly in the α phase state, while at 1050 °C, the β phase is the only



Titanium alloys: modelling of microstructure426

16.6

Microstructure of Ti-6Al-2Sn-4Zr-2Mo after gas nitriding at 950

and 1050 °C for 1, 3 and 5 hours. (a) 950 °C, 1 h; (b) 1050 °C, 1 h; (c)

950 °C, 3 h; (d) 1050 °C, 3 h; (e) 950 °C, 5 h; (f) 1050 °C, 5 h. The dark

particles near the surface, most clearly seen in (f), are stains after

etching possibly due to residual etchant near the metal/resin

interface.

200 µm

(a) (b)

200 µm 200 µm

(e) (f)

200 µm



Surface gas nitriding: phase composition and microstructure 427

phase. The mechanisms and kinetics of diffusion would be different, which

would result in the difference in the microstructure morphology.

The microstructure after nitriding at a lower temperature of 850 °C is

similar to the 950 °C microstructure. Both temperatures are below the

β-transus temperature for this alloy.

16.4

αα

α +

ββ

β Ti-6Al-4V

16.4.1 Phase composition

As discussed in the previous sections about Ti-8Al-1Mo-1V and Ti-6Al-

2Sn-4Zr-2Mo, as a result of the diffusion process of gas nitriding, α-Ti(N),

16.7

X-ray diffraction patterns of Ti-6Al-4V after gas nitriding at (a)

950 and (b) 1050 °C for 1, 3 and 5 hours.

200 111 002 311

(°)

N (5 h)

• α-Ti(N)

•α-Ti

⇓ TiN

↓ TiO

5 h

3 h

1 h

0 h

30 40 50 60 70

2θ (°)

(a)

Intensity (c.p.s.)

2500

2000

1500

1000

500

← {101}

⇐ {111}

•

{100}

← {200}

← {111}

⇐ {200}

← {210}

← {211}

← {220}

⇐ {220}

← {002}

← {310}

← {301}

← {112}

⇐ {311}

⇐

•

{002}

•

{101}

•

{102}

•

{110}

•

{103}

⇐

•

⇐

•



Titanium alloys: modelling of microstructure428

•α-Ti

• α-Ti(N)

⇓ TiN

↓ TiO

5 h

3 h

1 h

0 h

30 40 50 60 70

2θ (°)

(b)

Intensity (c.p.s.)

2500

2000

1500

1000

500

← {101}

⇐ {111}

← {200}

← {111}

⇐ {200}

← {210}

← {211}

← {220}

⇐ {220}

← {310}

← {301}

← {112}

⇐

•

{002}

•

{101}

⇐

•

←

•

⇐ {311}

•

{103}

⇐

•

{110}

•

{102}

•

{100}

←

16.7

Continued

cubic titanium nitride TiN and TiO

phases are formed on the surface of Ti-

6Al-4V alloy (Fig. 16.7). With the increase of the time to 10 hours and for

the alloy nitrided at 850 °C, the nitrided surface layer has the same phase

composition.

The coloration of the compound layer is different after nitriding at 950

and 1050 °C for different periods of time. After 1 hour of nitriding Ti-6Al-

4V has a golden colour, corresponding to the TiN compound. The red colour

of Ti-6Al-4V nitrided for 3 hours indicates the simultaneous formation of

titanium nitrides. Compounds identified by XRD are TiN and possibly Ti

at 950 °C, and TiN with a small amount of TiO

at 1050 °C. After 5 hours,

the metal colour is white, and the compounds are TiO

and TiN. The coloration

depends on the process parameters and the appearance of the new phases,

and is, by and large, consistent with the X-ray diffraction data.

As mentioned in the previous section about Ti-6Al-2Sn-4Zr-2Mo, the



Surface gas nitriding: phase composition and microstructure 429

phase transformations are also evident in the DSC curves obtained during

nitriding (Fig. 16.8). The peaks that appear during heating and cooling are

probably caused by the new phase formation.

16.4.2 Microstructure

The microstructure of Ti-6Al-4V, being an α + β alloy, is different from the

microstructure of the near-α alloys (Fig. 16.9, the microstructure of the alloy

nitrided for 10 hours, is similar).

16.8

DSC curves of Ti-6Al-4V nitrided at 950 (curve 1) and 1050

(curve 2) °C for 3 hours

exo

DSC (µV/mg)

0.300

0.250

0.200

0.150

0.100

0.050

–0.050

Time–temperature curves

DTA signals

0 50 100 150 200

Time (min)

[1][2]

Temperature

(°C)

1000

800

600

400

200

200 µm

(a) (b)

16.9

Microstructure of Ti-6Al-4V after gas nitriding at 950 and 1050 °C

for 1, 3 and 5 hours. (a) 950 °C, 1 h; (b) 1050 °C, 1 h; (c) 950 °C, 3 h;

(d) 1050 °C, 3 h; (e) 950 °C, 5 h; (f) 1050 °C, 5 h.



Titanium alloys: modelling of microstructure430

200 µm

(e) (f)

16.9

Continued

(a)

Compound layer

16.10

SEM images of compound layer of Ti-6Al-4V after gas nitriding

at (a) 950 and (b) 1050 °C for 3 hours.



Surface gas nitriding: phase composition and microstructure 431

(b)

Compound layer

16.10

Continued

The thickness of the compound layer increases with the increase of

temperature (Fig. 16.10).

16.5 Near-

ββ

β Ti-10V-2Fe-3Al

16.5.1 Phase composition

Ti-10V-2Fe-3Al forms the same phases as the titanium alloys in the previous

sections during nitriding at the same processing parameters (Fig. 16.11). The

initial alloy condition consists of α-Ti and β-Ti, and the reaction during the

process of nitriding is as follows: α-Ti + β-Ti → α-Ti(N) + TiN + TiO

. The

same phase constitution is observed after nitriding at the lower temperatures

of 750 and 850 °C.

The coloration of the compound layer is different after nitriding, grey

after 1 hour of at 950 and 1050 °C, turning red after 3 and 5 hours at same

temperature regimes.

Ti-10V-2Fe-3Al contains 10% vanadium, but no nitrides of vanadium are

detected on the surface of the metal after the nitriding.

The lattice parameters a and c of the α phase increase after nitriding for

1 hour at 950 and 1050 °C. There is no significant change of both a and c

with further increase of the nitriding time to 3 and 5 hours at the temperature

of nitriding of 950 °C. There is no clear tendency for the lattice parameter a

after nitriding for different periods of time at 1050 °C. The values of the

lattice parameters a and c of the α phase are higher for Ti-10V-2Fe-3Al after

nitriding at the same processing parameters, though lower for the initial

samples as compared to these for Ti-8Al-1Mo-1V, possibly related with the

difference in the chemical composition of these two titanium alloys.



Titanium alloys: modelling of microstructure432

16.5.2 Microstructure

The compound layer in most cases is continuous and its thickness for the

alloy nitrided at 950 and 1050 °C for 3 and 5 hours varies between 5 and 10

µm, as seen from the cross-section.

The microstructure of the alloy nitrided at 750 °C is homogeneous, because

this temperature is below the β-transus temperature for this alloy (see Fig.

16.12a,c,e). Similar results are obtained for the other three alloys above. The

16.11

X-ray diffraction patterns of Ti-10V-2Fe-3Al after gas nitriding at

(a) 950 and (b) 1050 °C for 1, 3 and 5 hours.

•α-Ti • α-Ti (N) ↓ TiO

¤ β-Ti ⇓ TiN

5 h

3 h

1 h

0 h

30 35 40 45 50 55 60 65 70 75

2θ (°)

(a)

Intensity (c.p.s.)

4000

3500

3000

2500

2000

1500

1000

500

← {101}

⇐ {111}

•

{002}

← {111}

⇐ {200}

← {211}

← {220}

⇐ {220}

⇐

•

{100}

⇐

•

⇐

¤ {211}

←

¤ {220}

•

{101}

•

{102}

•

{110}/←

•

{103}

⇐ {311}

•

/ ←{002}

•

⇐

•

/ ←

•

¤ {110}

•

←

•

←

•

