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 543

The wide asymmetric peaks of Al

Ti in the X-ray pattern are overlapping

diffraction lines from Al

Ti and TiAl. Al

Ti, TiAl and Ti

Al are formed as a

result of the reaction between Al

Ti and Ti particles on heating up to 700 °C.

After subsequent increase of annealing temperature from 700 to 900 °C, the

volume fraction of Al

Ti gradually decreases while the volume fraction of

Al, Al

Ti and TiAl increases. Titanium in the titanium/aluminium coating

0 50 100 150 200 µm

(b)

Ti–Al coating

Substrate

×10

Intensity of Kα1

1.5

0.5

100 µm

(a)

Substrate

Start of substrate

Coating

19.8

Titanium and aluminium coating after annealing at 600 °C: (a)

back-scattered electron image showing cross-section microstructure;

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

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Titanium alloys: modelling of microstructure544

completely reacts on the formation of these compounds after treatment at

800 °C (Fig. 19.4b). Al

Ti decomposes only on heating up to 1000 °C.

Annealing at 1000 °C gives Al

Ti, TiAl and Ti

Al on the titanium/aluminium

coating (Table 19.1 and Fig. 19.4b). In addition, the peaks of the X-phase are

in the X-ray diffraction pattern after treatment at 1000 °C. What is important

is that the diffraction peaks of α-Ti are again present after annealing at

1000 °C (Fig. 19.4b). Then, with an increase of annealing temperature to

1100 °C, the intensity of α-Ti peaks increases dramatically, while the intensity

of the Ti

Al lines decreases. This can be related to the rapid release of

aluminium into the substrate when the mobility of the atoms increases. The

TiAl compound on the titanium/aluminium coating does not decompose

completely after annealing at 1100 °C (Table 19.1 and Fig. 19.4b).

With an increase of temperature from 800 to 1000 °C, the microstructure

of the coating becomes very inhomogeneous, with a very rough and irregular

hillock-like structure (Fig. 19.9a,b). Large microcracks appear on the smooth

areas (Fig. 19.9b). With subsequent increase of annealing temperature from

1000 to 1100 °C, the microstructure of the titanium/aluminium coating starts

levelling (Fig. 19.9c). The surface is porous (Fig. 19.9d). The coating structure

coalesces as the annealing temperature rises from 1000 to 1100 °C. The

microcracks start healing (Fig. 19.9e). During the annealing treatment, the

surface morphology of the titanium/aluminium coating becomes less rough

compared to the as-synthesised MA-coating (Figs. 19.2d and 19.9f).

There are no peelings, microcracks or pores (Fig. 19.10a). The coating is

very dense, and has a good apparent adherence to the substrate after treatment

at 1100 °C. Back-scattered electron imaging shows a multilayered structure

of coating, with different thicknesses (Fig. 19.10b). Instead of a smooth

change in concentration from α-Ti to TiAl, there are the jumps in the

concentration (Fig. 19.10c). The discontinuity is consistent with the clear

10 µm

(c)

19.8

Continued



Aluminising: fabrication of Al and Ti–Al coatings by mechanical 545

200 µm

(a) (b)

200 µm

10 µm

(e) (f)

µm

19.9

Surface microstructural evolution of titanium/aluminium coating

during annealing treatments: (a) 800 °C; (b) 1000 °C; (c) 1100 °C;

(d) 900 °C; (e) 1100 °C; (f) morphology after 1100 °C.

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Titanium alloys: modelling of microstructure546

contrast difference seen in the back-scattered electron image (Fig. 19.10b,c).

Figure 19.10d shows a diffusion layer from the start of the substrate. The

substrate itself has a coarse grain lamellar structure, which may be formed

from the coarse grains of β-Ti upon cooling. The columnar structure is

formed at the near interface region, with a concentration of aluminium about

50 at.% after annealing at 1100 °C. There is a layer with fine lamellar

structure between the columnar and coarse lamellar grains.

Thus, recrystallisation of the titanium/aluminium coating also starts at

temperatures above 880 °C, as α-Ti to β-Ti transformation in the substrate

19.10

Cross-section microstructure of titanium/aluminium coating

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

scattered electron image; (c) concentration profile; (d) back-scattered

electron image of the substrate at interface.

(a)

Substrate

Start of substrate

Coating

100 µm

(b)

50 µm

(d)

0 50 100 150 200 250

µm

(c)

Ti–Al coating

Substrate

×10

Intensity of Kα1

1.5

0.5



Aluminising: fabrication of Al and Ti–Al coatings by mechanical 547

happens (Fig. 19.9). Formation of microcracks on heating up to 1000 °C can

be a result of a strong stress being developed because of crystallographic

differences between the multilayered coating and the substrate. Recrystallisation

of the near surface region of the substrate, caused by the change in aluminium

concentration, can lead to partial stress relaxation. Increase of atom mobility

with an increase of annealing temperature results in healing microcracks.

The distinction between the aluminium and the titanium/aluminium coatings

produced by MA is that the aluminide phases in the titanium/aluminium

coating are formed successively during the annealing treatment (Table 19.1),

similar to the titanium samples coated by the aluminium film (Romankov et

al., 2006, 2007). However, in the case of Romankov et al. (2006, 2007),

Al starts decomposing only after the decomposition of TiAl. In the case

of the titanium/aluminium coating here, the volume fraction of Ti

Al decreases

dramatically, while TiAl does not decompose completely (Fig. 19.4b). The

differences in structural formation and its transformation between aluminium

and titanium/aluminium coatings during annealing treatments may be related

to the thickness of the coatings. On the other hand, the presence of titanium

particles in the as-synthesised titanium/aluminium coating may have some

effect on the thermodynamics of the reactions. The powders of pure titanium

and aluminium are mixed in the proportion for TiAl compound. Upon heating,

aluminium mainly reacted with titanium in the titanium/aluminium coating

forming aluminide phases. As a result, release of aluminium and its diffusion

into the substrate may be restrained. TiAl compound is stabilised up to 1100

°C. Interdiffusion growth of aluminide phases can dominate at the substrate/

coating interface. Interdiffusion gives a good apparent adherence to the substrate

on heating up to 1100 °C (Fig. 19.10). However, the dynamics of interdiffusion

growth may slow down in the direction of the coating surface. Also, after

treatment at 1100 °C, adherence to the substrate is better for the titanium/

aluminium coating than for the aluminium one (Figs. 19.5 and 19.10), which

can be related with the thickness and structures of the coatings.

19.5 Summary

By means of the mechanical alloying method, aluminium and titanium/

aluminium coatings can be deposited on titanium alloys. During MA processing,

aluminium is better deposited on the surface in the presence of titanium

particles. The MA technique produces titanium/aluminium coatings with a

thickness of 200 µm and aluminium ones with a thickness of 50 µm after

two-hours milling at room temperature. The titanium and aluminium coating

consists of titanium particles in the aluminium matrix. As-synthesised coatings

show structures with high apparent density and free of porosity. However,

the surface morphology of the MA-coatings is very rough. Annealing treatment

leads to levelling of the surface microstructure. Recrystallisation of the coatings



Titanium alloys: modelling of microstructure548

starts at temperatures above 880 °C, as α-Ti to β-Ti transformation in the

substrate happens. Annealing at temperatures ranging between 600 and

1100 °C gives different aluminide phases on the surface. In the case of the

aluminium coating, Al

Ti and Ti

Al are formed upon heating up to 1100 °C.

In the case of the titanium/aluminium coating, Al

Ti, Al

Ti, TiAl and Ti

are formed on the surface. After heating at 1100 °C, the titanium and aluminium

coating shows a multilayered structure with high apparent density, free of

porosity, and a good adherence to the substrate.

Thus, the MA method allows the production of very thick Ti–Al coatings

after a relatively short time at room temperature. No special milling atmosphere

is required. Ti–Al intermetallic coatings can be produced by subsequent

annealing treatment. The MA method is likely to be useful for the fabrication

of other diffusion coatings as well.

19.6 References

Romankov S, Sha W, Ermakov E and Mamaeva A (2006), ‘Characterization of aluminized

layer formation during annealing of Ti coated with an Al film’, J Alloy Compd, 420

(1–2), 63–70.

Romankov S, Sha W, Ermakov E and Mamaeva A (2007), ‘Characterization of interdiffusion

growth of aluminized layer on Ti alloys’, J Alloy Compd, 429 (1–2), 143–55.

Suryanarayana C (2001), ‘Mechanical alloying and milling’, Prog Mater Sci, 46 (1–2),

1–184.

Tao N, Zhang H, Lu J and Lu K (2003), ‘Development of nanostructures in metallic

materials with low stacking fault energies during surface mechanical attrition treatment

(SMAT)’, Mater Trans, 44 (10), 1919–25.

Wang Z B, Tao N R, Tong W P, Lu J and Lu K (2003), ‘Diffusion of chromium in

nanocrystalline iron produced by means of surface mechanical attrition treatment’,

Acta Mater, 51 (14), 4319–29.

Zhou A, Wang C and Huang Y (2003), ‘A possible mechanism on synthesis of Ti

AlC

’,

Mater Sci Eng A, 352 (1–2), 333–39.



549

α lamellar phase

final microstructural morphology

characterisation, 261

phase transformation stages, 261

α/β interface localisation

Eulerian scheme, 213

Lagrangian scheme, 213

volume of fluid method, 213

acicular α phase, 206

aluminising, 530–46

fabrication of coating, 530–46

Al coating annealing treatment,

534–5, 537–40

as-synthesised state, 531–4

Ti/Al coating annealing treatment

of, 540–2, 544–5

aluminium coating

after annealing, 536

annealing treatment, 534–5, 537–40

as-synthesised microstructure, 533

cross-section microstructure post

annealing, 539

fabrication by mechanical alloying,

530–46

surface microstructural evolution, 538

vs Ti/Al coating, 545

annealing

Al coating, 534–5, 537–40

Ti/Al coating, 540–2, 544–5

anti-phase boundary, 239, 282, 287

energy, 297

‘architecture of the neural network,’

311–12

Arrhenius-type equation, 218

artificial neural network, 301–28

β-transus temperature, 331–43

definition, 306

features, 308

influence of number of neurons on

performance, 499

layout of neurons within a network,

306

list of alloys for training and testing,

410

MatLab program code example for

training, 361–3

microhardness profiles modelling,

496–512

Mo and Al interaction in

Ti–xAl–2Sn–4Zr–yMo–0.08Si

system, 342

model description, 497–501

input parameters, 497–8

performance and test, 501

training, 498–501

models and applications in phase

transformation studies, 331–63

models for different correlations, 303

simulations and predictions, 501–3,

508–9

feed-forward hierarchical networks

model, 497

software products GUI, 304–5

special feature, 302

statistical analysis, 336

TTT diagrams, 343–61

atomic force microscopy, 472

‘averaging the integral intensities’

principle, 130

Avrami theory, 140

Index



Index550

β to α phase transformation

FEM for morphology prediction,

204–36

1-D model, 218–23

2-D model, 223–4, 227–8, 230,

232–3

experimental and modelling

methodology, 205–6

experimental observation, 206

extending to other alloys, 233–4

Ti-6Al-4V microstructure model,

206–18

Ti-6Al-4V models, 233

Bayesian regularisation, 308, 316, 498

post-training linear regression

analysis, 312–13

regression coefficients, 315

Boltzmann constant, 190, 210, 241

bond order potential, 292, 293, 295, 297

Brillouin zone, 245

β21s alloy, 4

advantages and disadvantage, 83

after surface gas nitriding, 430–5, 438

high magnification SEM after

nitriding at 850 °C, 440

high magnification SEM after

nitriding at 950 °C, 441

microstructure using optical

microscopy, 433–4

microstructure using SEM, 435,

438

optical micrographs after nitriding

at 850 °C, 438

optical micrographs after nitriding

at 950 °C, 439

physical appearance change, 430

X-ray diffraction analysis, 431–3

XRD pattern after nitriding at 850

°C, 436

XRD pattern nitriding at 950 °C,

437

β to α + β transformation kinetics,

122, 158

calculated equilibria vs temperature,

107

calculated TTT diagrams, 162

diffusion distance of Mo, 155

hydrogen penetration effects, 86–91

results of hydrogen analysis, 90

XRD pattern, 90

JMA parameters derivation, 157

mechanical properties, 83–5

elongation after ageing at different

temperatures, 86

heat treatments, 84

microstructure after water

quenching and ageing, 85

tensile strength after ageing at

different temperatures, 86

Vickers hardness after water

quenching, 85

microstructure after isothermal

exposure, 124

α phase amounts vs temperature, 139

surface gas nitriding

hardness evolution, 462

microhardness profiles, 463

microhardness profiles after

nitriding at 850 °C, 463

microhardness profiles after

nitriding at 950 °C, 463

synchrotron radiation X-ray

diffraction

β phase fractions vs temperature

for different oxygen levels, 47

diffraction patterns at different

temperatures, 45

diffraction patterns at room

temperature, 36

measurements at elevated

temperatures, 52–4

measurements at room

temperature, 41–2

thermodynamic equilibria, 138–40

transformation kinetics, 154–6

TTT diagrams, 161–2

for start of β to α + β

transformation, 356

typical resistivity curve, 119

XRD patterns

in the range of 30 to 75° 2θ, 129

in the range of 36.5 to 41.5° 2θ,

131

β-transus temperature, 2, 15, 42, 53, 206,

222, 331–43

alloying elements interaction in

multicomponent systems, 341

distribution of input data set, 334



Index 551

linear regression analysis, 342

model description, 332–6

database construction and analysis,

332–3

input and output parameters, 332

training, 333–6

model performance, 336

phase diagram calculation of Ti–X

systems, 336, 339–41

statistical analysis of variables, 333

testing and whole datasets

performance, 338

Ti–xAl alloy, 339

Ti–xMo alloy, 339

Ti–xO alloy, 340

Ti–xV alloy, 340

training and validation performance,

337

Burgers vector, 271, 282, 284, 287, 288,

289, 294, 296

Cahn–Hilliard diffusion equation, 242

‘Cambridge Engineering Selector,’ 323

‘Cambridge Materials Selector,’ 323

Cauchy pressures, 297

CCT see continuous-cooling-

transformation

cellular automaton method, 258, 259,

263, 268–9

microstructural evolution modelling,

258–70

Ti–6Al–4V alloy during

thermomechanical processing,

259–63

dynamic crystallisation, 262–3

flow stress–strain curves, 259–60

phase transformation, 261

coefficients of thermal expansion, 59–60,

cold welding, 534

commercially pure titanium, 467, 468

fatigue data in reference condition

after nitriding, 472

fatigue strength, 471

influence of nitriding on tensile

strength, 470

tensile properties and fatigue

performance after nitriding,

467–70

complex stacking fault, 282, 284, 288,

297

computer-based models

classification, 301–2

physical, 301

statistical, 301–2

continuous-cooling-transformation, 166,

168, 170, 187

calculations, 179–84

diagram, 343

Ti–6Al–2Sn–4Zr–2Mo, 171

simulation and monitoring, 198–201

corrosion behaviour, 486–95

alloy composition and surface gas

nitriding effects, 490, 492

basic principles, 487–9

corrosion resistance after treatment,

494–5

corrosive medium effects, 492

media temperature effects, 492, 494

CP-Ti see commercially pure titanium

crystallographic behaviour

titanium aluminide, 271–90

crack path analyses, 274, 278–80

model for microcracks nucleation

in basal slip, 282–4, 287–9

single crystal characteristic,

272–4

transmission electron microscopy,

280, 282

differential scanning calorimetry, 70–91,

165, 205

hydrogen penetration effects, 86–91

mechanical properties of β21s alloy,

83–5

phase and structural transformations,

70–83

and thermo-gravimetry

curves of Ti–6Al–2Sn–4Zr–2Mo,

424

differential thermal analysis, 165

diffusion equation, 514, 516, 521, 526

diffusion theory, fundamental, 503, 512,

526

superstructure, 287, 288

stereographic projection, 279

DRX see dynamic recrystallisation

DSC curves, 430



Index552

DSC/TG. see differential scanning

calorimetry

dynamic recrystallisation, 258, 259, 262,

263–5, 267–8

micrographs, 263

EDX see energy dispersive X-ray

elastic modulus tensor, 245

embedded-atom method, 297

energy dispersive X-ray, 27

elemental composition of α

and γ

phases, 25

Eulerian scheme, 213

extrapolation, 311

face-centered cubic, 239

FEM see fine-element method

Fick’s equation, 522

Fick’s law, 522

fine-element method

morphology of β to α phase

transformation, 204–36

1-D model, 218–23

2-D model, 223–4, 227–8, 230,

232–3

experimental and modelling

methodology, 205–6

experimental observation, 206

extending to other alloys, 233–4

Ti-6Al-4V microstructure model,

206–18

Ti-6Al-4V of models, 233

Finnis–Sinclair type many-body

potentials, 297

flow softening

hot-pressing at lower temperatures,

260

mechanisms, 260

fluctuation–dissipation theorem, 241

Fourier transformation, 245

fracture behaviour, of titanium aluminide,

271–90

crack path analyses, 274, 278–80

model for microcracks nucleation in

basal slip, 282–4, 287–9

single crystal characteristic,

272–4

transmission electron microscopy,

280, 282

γ titanium aluminide, 4, 6, 16–25, 54–68.