Structure evolution of amorphous TiAlSi during annealing

] O U R N A 1, O F

F.I.REVIER

Journal of Non-Crystalline Solids 215 (1997) 140-145

Structure evolution of amorphous Ti-A1-Si during annealing K.W. Liu *, J.S. Zhang, J.G. Wang, G.L. Chen State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, 100083 Beijing, People's Republic of China Received 29 October 1996; revised 29 January 1997

Abstract

Structure evolution of amorphous phase obtained by mechanical alloying for Ti-A1-Si during the annealing treatment was investigated. Microstructure development was monitored by X-ray diffraction, differential thermal analysis and transmission electron microscopy. The structure evolution of the amorphous phase treatment can be classified into the first stage partial crystallization of amorphous phase producing TisSi 3 phase, the second stage crystallization of the remaining amorphous phase producing the corresponding titanium aluminides, and last grain growth of all. Crystallization results in the formation of TiaA1, TiA1 and Al3Ti according to the relative amount of elemental mixtures of Ti and Al. TisSi 3 is the only titanium silicide produced by crystallization. Annealing at less than 800°C produced nanocrystailine composites of AlaTi, TiAl, TiaAl and TisSi 3 with a grain size less than 20 nm. At high annealing temperature crystalline sizes of all phases increase. © 1997 Elsevier Science B.V.

1. Introduction

Mechanical alloying (MA) process is a high-energy ball milling operation which involves the repeatedly welding, fracturing, and rewelding of powder particles. Mechanical alloying is regarded as an effective tool to synthesize supersaturated solid solutions, amorphous alloys and nanocrystalline materials [1,2]. This metastable material can then be used as a precursor to obtain the desired chemical constitution a n d / o r microstructure by crystallization [3]. The intermetallic compounds (~/-TiA1 and (ct 2Ti3A1) are very useful structural materials due to their high oxidation resistance, low density and high melting point [4,5]. However, the application of these intermetallic alloys is limited by the poor ductility

* Corresponding author. Tel.: +86-1 6233 2508; fax: +86-1 6232 7283; e-mail: [email protected].

and fracture toughness at room temperature and poor elevated temperature strength. To improve these properties, the compounds have been fabricated as composite materials containing a secondary phase such as boride, carbide, or oxide [6]. Some titanium aluminides have been successfully synthesized by crystallization. For example, the amorphous phase produced by mechanically alloying Ti-50 and 55 at.% Al powder transformed to the TiAl(~) phase on annealing at 615°C for 168 h [7]. Silicon is an important alloying addition to titanium alloys and as silicide compounds provide dispersion strengthening and improve microstructure stability at elevated temperature. Homogeneously distributed fine silicide particles can also act as preferred nucleation sites during recrystallization [8]. In this research, we try to study the structure evolution of the amorphous phase during the annealing in Ti-AI-Si.

0022-3093/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 ( 9 7 ) 0 0 0 8 5 - 9

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2. Experimental procedures The amorphous phase was obtained by mechanical alloying o f e l e m e n t a l m i x t u r e s o f aluminum(99.5%, average size 60 (m), titanium (99.5%, average size 20 (m) and silicon (99.5%, average 40 (m) powders with a composition of TixAl]0o_xSil0 (in at.%), which were denoted as T3, T4, T5 and T6 for x = 30, 40, 50, and 60 at.%, respectively. Mechanical alloying was performed in a planetary ball mill in vacuum. The weight ratio of ball to powder was 30:1. The ball and mill vial materials were made of stainless steel. The rotating rate of the vial was 720 rpm. The annealing treatments were carried out in vacuum. The powders were characterized by X-ray diffraction (XRD) with Cu K (radiation A = 0.154 nm) and electron microscope (TEM). Structure evolution of amorphous phase were also investigated by a Perkin-Elmer 7 differential thermal analysis (DTA) system. Heating was carried out in flowing purified argon gas.

3. Results X-ray diffraction (XRD) patterns of the amorphous phase produced by mechanical alloying are shown in Fig. 1. Typical broad maxima of the amorphous phase are shown in the diffraction patterns. This indicates that the amorphous phases were ob-

-r-

100 200 300 400

500 600

700 g00 900

1000

1100

1200

Temperature [ °C I Fig. 2. DTA curves of as-milled amorphous powders of (a) Ti3oAl60Siio, (b) Ti4oAlsoSijo, (c) TisoAl40Sito, and (d) Ti6o A13oSilo.

tained with mechanical alloying for all powders. A trace of nanocrystalline phase (about 10-30 nm), which are titanium solid solution and aluminum solid solution in T3 sample, and aluminum solid solution in T4, T5 and T6 samples, can also be observed. Fig. 2 shows DTA scans of all the as-milled powder mixtures using a heating rate of 20 K min- 1. There is a broad exothermic peak starting from 350°C to about 890°C for A3 and A4 powders. There are three exothermic peaks for A5 and A6: the first is a broad peak starting from 150°C to about 500°C,

+: TiAI

x

OxTi:Sis

(a)x

(b) (c)

_=

(~

20

;o

',o

9o

2 Theta

Fig. 1. XRD patterns of as-milled amorphous phase of (a) Ti30A160Silo, (b) Ti40AlsoSilo, (c) TisoAl40Sito, and (d) Tiro A130Sito.

I-

20

i

I

30

i

J

40

I

50

i

I

60

i

I

70

i

I

80

l

90

Z Theta

Fig. 3. XRD patterns of amorphous phase after DTA, (a) Ti30AlroSilo, (b) TinoA150Sito, (c) TisoAl40Sito, and (d) Ti6oA13oSilo.

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K.W. Liu et al. /Journal of Non-Crystalline Solids 215 (1997) 140-145

x

X: AI~Ti O: Tis Si3

l

x

(a>

'

X

X

I

X

=. Io

_

Ul

~

(b)

~t

_=

_= ,,

~..E×

i

20

i

~

,

30

i

i

40

~

i

,

50

i

J-~_..~.

i

i

60

,

70

i

t

80

20

90

30

40

50

60

70

80

90

2 Theta

2 Theta

Fig. 4. XRD patterns of amorphous phase in Ti30Al60Silo powders after annealing at 1000°C × 1 h (a), 800°C x 1 h (b), 600°C X 1 h (c), 350°C X 1 h (d).

Fig. 6. XRD patterns of amorphous phase in TisoAl4oSil0 powders after annealing at 850°C x 1 h (a), 750°C X 1 h (b), 650°C × 1 h (c), 540°C X 1 h (d).

the second is a comparatively sharper one starting from about 540°C to 640°C, and the third one is quite weak compared to the first two ones. The XRD results of as-milled powders after DTA are shown in Fig. 3. It can be seen from Fig. 3 that the products of as-milled amorphous phase after DTA are mainly Al3Ti, TiA1, TiA1 and Ti3Al, and Ti3Al for powder mixtures with a composition of TixAl90_xSi10, x = 30, 40, 50 and 60, respectively. The XRD results of powders after DTA show that the crystallization products strongly depend on the

chemical composition of the mixture, e.g., A1 or A1/Ti ratio, consistent with the location of the composition in the Ti-AI phase diagram. TisSi 3 is the only titanium silicide produced by crystallization. In order to study the transformation process corresponding to the observed thermal features, isothermal annealing experiments were performed for the as-milled powders. The annealing treatments were carried out in vacuum (10 -4 bar) at temperatures above and below exothermic peaks on the DTA curves. The XRD results of as-milled amorphous

) I

=;

I~

.i

I

-¢-:TiAI O: Ti5 Si3

+

+

(a)

+

o

I

fil

+: TiA1 *: Ti3Al O: Ti5 Sis

] I [

;Jlr

+ (a)

I

o

_=

-

•

r ..........

20

30

40

50

60

(c)

70

[

_,,t

gO

L

90

2 Theta

Fig. 5. XRD patterns of amorphous phase in Ti4oAlsoSi~o powders after annealing at 1000°C × 1 h (a), 800°C X 1 h (b), 600°C X 1 h (c), 350°C x 1 h (d).

20

I

30

i

I

40

n

I

50

L

I

60

L

I

70

i

I

80

i

90

2 Theta

Fig. 7. XRD patterns of amorphous phase of TiroAl3oSilo powders after annealing at 900°C x 1 h (a), 800°C X 1 h (b), 700°C X 1 h (c), 540°C x 1 h (d).

K.W. Liu et al. /Journal of Non-Crystalline Solids 215 (1997) 140-145

50.

540°C

71 : TiAI O: TisSI3

40 ¸

o []

~ 30 •~.

lO

650°C

o

20 ==

ForT5: Am. ~

540°C

For T6: Am. ~

--* TisSi 3 + Ti3AI + Am. 800°C

Fig. 8. Grain size of Ti40AlsoSi]0 amorphous powders after annealing for 1 h at different temperatures.

350°C

For T3 : Am. ---> Am. + A13Ti 600°C

A13Ti + TisSi 3 + Am.

800°C

---> A13Ti + TisSi 3,

350°C

For T4: Am. ---> Am. + TisSi 3 600°C

800°C

TiA1 + TisSi 3 + Am. TiA1 + TisSi 3,

A m . + TisSi 3

700°C

800 ' 10'00

Temperature lee}

phases with a composition of T3, T4, T5 and T6 after annealing at different temperatures for 1 h are shown in Figs. 4-7. It can be seen from Figs. 4 and 5 that a trace of AI3Ti and TisSi 3 phases for T3, and a trace of TisSi 3 phase for T4 powders begin to appear after annealing at 350°C in addition to the amorphous phase. There exist mainly TisSi 3 and small amounts of TiA1 and Ti3A1 phases for T5, and only TisSi 3 phase for T6 powders after annealing at 540°C. The amorphous phase crystallized gradually with increasing annealing temperature. The crystalline phases, after annealing at temperatures > 800°C can be mainly identified as AI3Ti and TisSi 3 phases for T4 powders, TiA1 and TisSi 3 phases for T4 powders, TiA1, Ti3A1 and TisSi 3 phases for T5 and T6 powders, respectively. In summary, the structure evolution of the asmilled amorphous powder alloys during the annealing treatments is as follows:

TiAI + TisSi 3 + Am.

--* TiAI + Ti3A1 + TisSi 3,

8

600

A m . + TisSi 3 + TiA1

750°C

D

460

143

Ti3A1 + TisSi 3 + TiA1.

Using the Scherrer formula d = 0.9A/13cos 0, where d is the average crystalline size, A the X-ray wavelength (0.154 nm for Cu Kcx),/3 is the linewidth at half maximum intensity, and 0 is the angle that exactly satisfies the Bragg law for the value of A, the grain size of all the phases can be calculated. The calculated results show that the relationship of grain size to annealing temperature is quite similar for all phases. Fig. 8 shows the relationship of grain size of T4 after annealing at different temperatures. As shown in Fig. 8, the increasing of grain size of TiAI and TisSi 3 after annealing treatments below 800°C for 1 h is not obvious, but the grain size increases sharply after treatments at 900°C for 1 h.

4. D i s c u s s i o n

As shown in Figs. 4-7, the identity of the titanium aluminides produced after annealing treatments is strongly dependent on the corresponding composition of the as-milled powders, consistent with the location of the composition in the Ti-AI phase diagram. By changing the relative amount of Ti and A1 in the elemental mixture, various kinds of titanium aluminides can be synthesized by the process. From Figs. 4-7, it can be seen that TisSi 3 is among the first intermetallic compounds produced by annealing at lower temperatures. There exist traces of TisSi 3 and AI3Ti phases in T3 powder in addition to amorphous phase after annealing at 350°C for 1 h (Fig. 4(d)). For T4, T5 and T6 powders, TisSi 3 is the major phase crystallized from the amorphous matrix after annealing at the lowest temperatures, which can be observed in Fig. 5(d), Fig. 6(d) and Fig. 7(d), respectively. The amount of TisSi 3 phase

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K.W. Liu et aL / Journal of Non-Crystalline Solids 215 (1997) 140-145

is more than that of intermetallic compounds TiA1 and Ti3AI after annealing at 540°C for T5 and T6 powders. With increased annealing temperature, the amounts of TiAI and Ti3A1 increase more rapidly than that of TisSi 3. And finally the amount of intermetallic compounds TiAI and Ti3A1 is more than that of TisSi 3 phase after annealing at 850°C to 900°C. TisSi 3 phase is the only silicide produced by the crystallization reaction. It is believed that the formation of TisSi 3 is thermodynamically favored while that of other Ti silicides are not. For example, the Gibbs free energies of formation for Ti5Si 3, TiSi, and TiSi 2 at 298 K are - 5 7 9 . 5 kJ/mol, - 1 2 9 . 7 kJ/mol, and - 1 3 3 . 9 kJ/mol, respectively [9], and there is no intermetallic compound in the AI-Si phase diagram. The formation of TisSi 3 is also in good agreement with the results reported by Manesh [8], where TisSi 3 is the only silicide produced in the simples with a composition of Ti: 50-52, Si: 0.5-3.5 (in at.%). From the XRD patterns of T3, T4, T5 and T6 powders in Fig. 4(d), Fig. 5(d), Fig. 6(d) and Fig. 7(a), we can observe the existence of TisSi 3 phase and a trace of corresponding titanium aluminides in addition to the amorphous phase, but the amount of TisSi 3 phase increases after annealing at lowest temperatures as the content of Ti in the powders increases from 30 at.% to 60 at.%. Because the lowest temperatures were chosen just at the end of the first exothermic peak and before the second one in DTA curves of T5 and T6 powders, and also because the exothermic peaks are possibly overlap at a heating rate of 20 K / m i n in DTA scan, it is regarded that the first exothermic peak in T5 and T6 cases corresponds to crystallization of amorphous phase which consequently produces TisSi 3 phase. Because TiAI and Ti3A1 are produced by crystallization of the amorphous phase after increasing annealing temperature to just after the second exothermic peaks on the DTA curves of T5 and T6 powders (which can be seen on Fig. 6(b) and Fig. 7(b)), it is suggested that the second exothermic peaks correspond to crystallization of amorphous phase producing TiAI and Ti3AI phases. The last exothermic peaks must correspond to the grain growth of all the phases, because the grains of all the phases continue to grow as the annealing temperature is increased.

Although the DTA curves of T3 and T4 are different from these of T5 and T6 powders, the structure evolution of T3 and T4 as-milled powders are quite similar to that of T5 and T6 powders at elevated temperatures, which can seen on the XRD patterns. Similarly, the structure evolution of T3 and T4, consists of crystallization of amorphous phase producing TisSi 3, A13Ti and TiA1 phases and of grain growth of all the phases. But the crystallization reaction of TisSi 3, AI3Ti and TiAI phases are mixed together in T3 and T4 cases, i.e., TisSi 3, AI3Ti and TiA1 phases are precipitated from the amorphous matrix simultaneously after annealing at low temperature through crystallization reaction, with the annealing temperature increased, the grains of previously formed TisSi 3, A13Ti and TiA1 phases will grow, so the crystallization and grain growth process are mixed together in DTA curve. Therefore it is concluded that the structure evolution of as-milled amorphous phase with increasing temperature is crystallization of amorphous phase producing Ti 5Si3, and corresponding titanium aluminides and the grain growth of all phases.

5. Conclusion Structure evolution of the as milled amorphous phase with a composition of Ti: 30-60 at.%, AI: 30-60 at.% and Si: 10 at.% during the annealing treatment is in three stages: the first stage is crystallization of a part of amorphous phase producing TisSi 3 phase, the second stage is crystallization of the remaining amorphous phase producing corresponding titanium aluminides, and the last one is grain growth of all phases. Crystallization results in formation of TiaA1, TiA1 and A13Ti according to the relative amount of elemental mixtures of Ti and A1. TisSi 3 is the only silicide produced by crystallization.

Acknowledgements This work was supported by the National Nature Science Foundation of China.

K. W. Liu et al. / Journal of Non-C~_ stalline Solids 215 (1997) 140-145

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