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

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Aluminising: fabrication of Al and Ti–Al coatings by mechanical 533

19.2 As-synthesised state

The thickness of the titanium/aluminium MA-coating is about 200 µm; it has

a high apparent density and is free of porosity and cracks (Fig. 19.2a,b). The

coating consists of titanium particles, with non-uniform sizes, in an aluminium

matrix. The maximum iron content in the titanium/aluminium coating is

about 0.3 at.% near the coating/substrate interface. It decreases in the direction

of the coating surface. The reason for such low content of iron is that the

balls and the chamber inner walls are being coated by titanium/aluminium

layer in the early stages of MA processing, preventing the release of iron into

the coating. There are no traces of nitrogen and oxygen in the titanium/

aluminium coating. The magnitude of contamination during MA depends on

the time and intensity of milling, nature of the powder, ball-to-powder weight

ratio, and seal integrity (Suryanarayana, 2001). If the container is not properly

sealed, the atmosphere surrounding the container leaks into the container

and contaminates the powder. Thus, when reactive metals such as titanium

are milled in improperly sealed containers, the powders are contaminated

with nitrogen and oxygen. By careful control and improvements of the seal

quality, the interstitial contaminants in the titanium alloy can be reduced to

as low as 100 ppm of oxygen and 15 ppm of nitrogen (Suryanarayana, 2001).

The microstructure is rough, with no clear grain structure on the coating

surface (Fig. 19.2c). The typical surface morphology obtained by atomic

force microscopy (AFM) appears similar to a ductile fractured surface, the

nodules in the image showing flow of material at fracture (Fig. 19.2d).

Furthermore, the dimple structure in some areas of the titanium/aluminium

19.1

Schematic illustration of the mechanical alloying method.

Al coating

Ti+Al

coating

Sample

Balls

Powder

Mechano-reactor

Vibration

chamber

Vibration

frequency

50 Hz

Thermocouple

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Titanium alloys: modelling of microstructure534

coating at high magnification may provide evidence of the ductile behaviour

of the material during MA processing (Fig. 19.2e).

The thickness of the aluminium coating is about 50 µm, without a clear

interface between the substrate and aluminium layer (Fig. 19.3a,b). The

200 µm

(c)

(d)

µm

1.2

0.8

0.6

0.4

0.2

µm

100 µm

(a)

Interface

Ti+Al coating

Substrate

× 10

Intensity of Kα1

0 50 100 150 200

µm

(b)

(e)

1 µm

19.2

As-synthesised titanium and aluminium coating produced by

MA method: (a) secondary electron image of cross-section

microstructure; (b) concentration profile; (c) surface microstructure;

(d) morphology; (e) dimple structure.

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Aluminising: fabrication of Al and Ti–Al coatings by mechanical 535

coating is continuous and very dense (Fig. 19.3c). The surface of the aluminium

coating is flatter and more homogenous than that of the titanium/aluminium

coating (Fig. 19.2). The morphology is again similar to a fracture surface

(Fig. 19.3d). The dimple structure and microcracks in some areas of the

aluminium coating at high magnification are observed (Fig. 19.3e). The

µm

1.2

0.6

µm

0 5 10 15 20 25 30 35 40 45

µm

(d)

200 µm

(c)

1 µm

(e)

0 50 100 150

µm

(b)

Intensity of Kα1

Al coating

Substrate

× 10

1.5

0.5

100 µm

(a)

19.3

As-synthesised aluminium coating produced by MA method: (a)

secondary electron image of cross-section microstructure; (b)

concentration profile; (c) surface microstructure; (d) morphology; (e)

dimple structure.

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Titanium alloys: modelling of microstructure536

absolute amount of aluminium in the titanium/aluminium coating is more

than that in the aluminium coating because the former is thicker.

During MA processing, aluminium is better deposited on the surface in

the presence of titanium particles. The microstructure of the coating surface

provides evidence of ductile behaviour of the material during MA (Figs. 19.2

and 19.3). This behaviour would indicate that the local temperature under

the ball impacts is higher than the temperature of the chamber walls. The

stress generated during MA may be rather high as well (Suryanarayana,

2001). The titanium particles are in the aluminium matrix (Fig. 19.2a). Titanium

appears as a dispersive material and aluminium serves as a bonding one.

Cold welding between particles and substrate under repeated ball collisions

leads to the formation of a composite coating. Aluminium flows into the

pores between the titanium particles under the impact of the balls. As a

result, a dense titanium and aluminium coating grows. Cold welding occurs

non-uniformly. Therefore, a very rough surface on the titanium/aluminium

coating is formed (Figs. 19.1 and 19.2c). In the case of using the powder of

pure aluminium, an aluminium layer with some thickness is coated, but

extensive cold welding between aluminium particles and the aluminium layer

cannot happen. Cold welding may occur in the early stages of MA, when the

aluminium particles are soft, and their tendency to weld is high. Under

deformation, the particles and the aluminium layer become harder and more

brittle (Suryanarayana, 2001). This is confirmed by the microcrack in Fig.

19.3d. With increase of MA-time, cold welding is restrained. Under the ball

collisions, the aluminium coating starts flaking and becomes smoother in

comparison with the titanium/aluminium one (Figs. 19.1, 19.2 and 19.3). In

the case of the titanium/aluminium coating, alloying may occur rapidly in

the presence of titanium particles. There is no time for the coating to become

brittle.

19.3 Annealing treatment of aluminium coating

Table 19.1 summarises the phase composition, without including the phases

of the substrate, α-Ti and β-Ti. During annealing, on the surface of the

sample different aluminide phases are formed as the result of interdiffusion

and reaction between titanium and aluminium. Aluminium reacts with titanium

on heating up to 600 °C, and forms Al

Ti. No diffraction peaks of pure

aluminium are in the X-ray pattern (Fig. 19.4a), but it completely reacts on

the formation of Al

Ti.

The contrast difference in Fig. 19.5a, a back-scattered electron image,

reflects different compositions. On heating up to 600 °C, aluminium diffuses

into the substrate. The grey dark layer from the start of the substrate is

enriched by aluminium (Fig. 19.5b). The thickness of the diffusion layer is

about 80 µm. The coating itself becomes porous after annealing treatment at

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Aluminising: fabrication of Al and Ti–Al coatings by mechanical 537

600 °C, with microcracks between substrate and coating (Fig. 19.5a). The

coating surface is porous and rough as well. Upon heating up to 600 °C, pits

are formed on the surface (Fig. 19.5c).

A second anneal at 700 °C after annealing treatment at 600 °C leads to the

formation of Ti

Al on the surface (Table 19.1 and Fig. 19.4a). In addition,

Intensity (rel. un.)

(a) Al coating (b) Ti+Al coating

αTi

βTi

αTi

βTi

αTi

d=2.15

d=2.48

αTi

βTi

αTi

d=2.15

d=2.48

αTi

βTi

d=2.15

d=2.48

αTi

d=2.15

d=2.48

d=2.40

d=2.48

αTi

d=2.15

d=2.08

2θ (deg.)

45 40 35 30 25 20 45 40 35 30 25 20

2θ (deg.)

Ti Ti

Al Al

TiAl

-phase

as-synthesised

annealing

T=600 °C

T=700 °C

T=800 °C

T=900 °C

T=1000 °C

T=1100 °C

as-synthesised

annealing

T=600 °C

T=700 °C

T=800 °C

T=900 °C

T=1000 °C

T=1100 °C

αTi

d=2.08

d=2.40

d=2.48

αTi

d=2.15

d=2.08

d=2.40

d=2.48

d=2.15

Intensity (rel. un.)

βTi

19.4

X-ray diffraction patterns (CuK

) of MA-coatings after annealing

at temperatures ranging between 600 and 1100 °C: (a) aluminium

coating; (b) titanium/aluminium coating.

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Titanium alloys: modelling of microstructure538

Substrate

Start of

substrate

Coating

Substrate

Al coating

0 50 100 150 200 250

µm

Intensity of Kα1

2.5

1.5

0.5

×10

(b)

19.5

Aluminium coating after annealing at 600 °C: (a) back-scattered

electron image showing cross-section microstructure;

(b) concentration profile; (c) surface microstructure.

10 µm

(c)

100 µm

Substrate

Start of

substrate

Coating

(a)

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Aluminising: fabrication of Al and Ti–Al coatings by mechanical 539

new diffraction lines with d = 2.48 and 2.15 Å are in the X-ray pattern (Fig.

19.4a). In Table 19.1 and Fig. 19.4, this phase is denoted as X-phase. After

subsequent increase of the annealing temperature from 700 to 1000 °C, the

intensity of the X-phase diffraction lines increases while the intensity of the

Al ones decreases. Increase of annealing temperature increases the volume

fraction of X-phase at the expense of Ti

Al. After annealing treatment at

1100 °C, only slight asymmetry of the α-Ti diffraction peaks at the high-

angle side may indicate the presence of the Ti

Al phase (Fig. 19.4a). The

intensity of the X-phase lines with d = 2.48 and 2.15 Å considerably decreases

after annealing at 1100 °C, in comparison with treatment at 1000 °C, while

the other strong peaks with d = 2.40 and 2.08 Å appear in the X-ray diffraction

pattern (Fig. 19.4a).

The lines with d = 2.48 and 2.15 Å could be attributed to TiC, and the

ones with d = 2.40 and 2.08 Å could be attributed to Ti

AlC. The above

sequence of peaks appearing means that Ti

AlC could be formed from TiC

(Zhou et al., 2003). Formation of carbide phases during annealing treatments

can be due to carbon contamination of the coating during MA. The X-phase

completely disappears after the elimination of the aluminised layer.

Moreover, formation of Al

Ti and TiAl compounds on the titanium surface

coated by the aluminium film precedes formation of Ti

Al (Romankov et al.,

2006, 2007). In the case of the aluminium coating produced by MA, Al

and TiAl phases are not present. This can be a result of the stress and

structural features of MA-coating. During MA processing, the substrate surface

and coating are plastically deformed, leading to grain refinement into the

nanometre scale and an extremely high density of dislocations (Wang et al.,

2003). The large amount of grain boundaries and defects may provide rapid

atomic diffusion in the surface layers. As a result, diffusion transformation

on the substrate with the aluminium coating produced by MA can occur very

fast, and Al

Ti and TiAl phases will not remain after 2 hours annealing.

These phases may be skipped altogether. Defect structure formed during MA

processing can have a great effect on the thermodynamics of the interdiffusion

process.

Table 19.1

The principal temperature ranges of the aluminide phase formation and

the phase composition on Ti-4%Al-3%Mo-1%V alloy of the mechanical alloying

deposited coatings

Temperature (°C) Aluminium coating Titanium/aluminium coating

600 Al

Ti Al

700–900 Ti

Al, X-phase Al

Ti, Al

Ti, TiAl, Ti

1000 Ti

Al, X-phase Al

Ti, TiAl, Ti

Al, X-phase

1100 Ti

Al (trace), X-phase TiAl, Ti

Al, X-phase

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Titanium alloys: modelling of microstructure540

With an increase of annealing temperature, a gradual levelling of coating

happens (Fig. 19.6a,b). Annealing at 1000 °C gives a homogeneous and even

microstructure on the aluminium coating (Fig. 19.6b). In the high magnification

micrograph of the aluminium coating, micropits are present after annealing

(a) (b)

200 µm 200 µm

10 µm 10 µm

10 µm

(e) (f)

µm

4.5

1.5

µm

19.6

Surface microstructural evolution of aluminium coating during

annealing treatments: (a) 800 °C; (b) 1000 °C; (c) 900 °C; (d) 1000 °C;

(e) 1100 °C; (f) morphology after 1100 °C.

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Aluminising: fabrication of Al and Ti–Al coatings by mechanical 541

at 900 °C. The sizes of the micropits increase, and the coating becomes

porous as the annealing temperature rises to 1000 °C (Fig. 19.6c,d). With

subsequent increase of temperature up to 1100 °C, the coating structure

coalesces (Fig. 19.6e), and the surface morphology becomes flat (Fig. 19.6f).

Thus, the annealing treatment results in the gradual levelling of the MA-

coating surface, which is very rough in the as-synthesised condition (Fig.

19.3d).

After annealing at 1100 °C, the MA-coating itself shows a structure with

high apparent density (Fig. 19.7a). However, there are some pores and peeling

in some areas along the coating/substrate interface (Fig. 19.7a,b). Figure

19.7b shows a different composition of aluminium coating. The jump in

0 50 100 150 200 µm

(c)

coating

Substrate

×10

Intensity of Kα1

1.5

0.5

100 µm

(a)

100 µm

(b)

19.7

Cross-section microstructure of aluminium coating after

annealing at 1100 °C: (a) secondary electron image; (b) back-

scattered electron image; (c) concentration profile.

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Titanium alloys: modelling of microstructure542

aluminium concentration is at the interface (Fig. 19.7c), consistent with the

clear contrast in the back-scattered electron image (Fig. 19.7b). The diffusion

layer in the coating adjacent to the substrate is formed. The substrate itself

has a coarse grain lamellar structure. The concentration of aluminium in the

near surface region of the substrate is about 16 at.% after annealing at 1100

°C, twice the aluminium concentration in the substrate.

Thus, significant changes in the structure of the aluminium coating occur

after annealing at temperatures above 800 °C. At about 880 °C, α-Ti (hcp)

to β-Ti (bcc) transformation in the substrate should occur. The intense

interdiffusion between substrate and coating leads to recrystallisation of the

coating structure. Moreover, at temperatures exceeding 900 °C, the growth

of β-grains in the substrate occur. The concentration of aluminium at the

near-interface region increases significantly upon heating. As a result, α-Ti

to β-Ti transformation in the substrate and near-surface region of the substrate

does not occur simultaneously. Annealing at temperatures above 900 °C

leads to recrystallisation of the near-surface region and stabilising α-Ti at

the interface. Then, the lines of β-phase are not in the X-ray diffraction

patterns of the aluminium coating after annealing treatment above 900 °C

(Fig. 19.4a).

19.4 Annealing treatment of the

titanium/aluminium coating

Annealing of titanium/aluminium coatings at 600 °C induces the formation

of Al

Ti (Table 19.1 and Fig. 19.4b). Aluminium completely reacts on the

formation of Al

Ti. The peaks of α-Ti are in the X-ray diffraction pattern

after annealing at 600 °C (Fig. 19.4b), showing that particles of pure titanium

are present in the coating. The coating is porous (Fig. 19.8a), consisting of

non-uniform particles. A layer with a thickness of about 12 µm from the start

of the substrate is enriched by aluminium (Fig. 19.8b). However, in comparison

with the aluminium coating, the diffusion layer is substantially thinner (Fig.

19.5a,b). After annealing at 600 °C, the surface microstructure of the titanium/

aluminium coating is porous and rough, along with the formation of very

fine particles on the surface (Fig. 19.8c). A comparison of the two types of

coatings might indicate that in the case of the aluminium coating, formation

of Al

Ti results from interdiffusion between the titanium substrate and the

aluminium layer. In the case of the titanium/aluminium coating, Al

Ti is

formed mainly as a result of reaction between titanium particles and the

aluminium matrix in the coating, which also leads to the porosity.

Annealing the titanium/aluminium coating at 700 °C reduces the intensity

of the Al

Ti and α-Ti diffraction peaks in the X-ray pattern (Fig. 19.4b).

Al lines along with the wide asymmetric peaks of the Al

Ti phase are

present. The Al

Ti and TiAl compounds have close lattice plane spacing.

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