Preferred precipitation of ordered a2 phase at dislocations andboundaries in near-a titanium alloys

Jun Zhang a,b,*, Dong Li b

a Department of Materials Science and Engineering, Shenyang University, Shenyang 110044, PR Chinab Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110015, PR China

Received 26 February 2001; received in revised form 4 April 2002

Abstract

The precipitation of a2 ordered phase in different aging conditions after solution treatment in b, high a�/b, or a phase field in Ti�/

Al�/Sn�/Zr�/Mo�/Si�/Nd (1#) and Ti�/Al�/Sn�/Zr�/Mo�/Si�/Nd�/Nb (2#) near-a titanium alloys was investigated. The solution-

treated microstructures consisted of either a single transformed b phase (bt), a duplex mixture of the primary a phase (ap) and

transformed b phase (bt), or a single primary a phase (ap). The precipitation characteristics of a2 phase was determined as a function

of aging temperature. When the aging temperature was high enough, the precipitation of a2 phase occurred predominately at the

boundaries and the dislocations, instead of being uniform throughout the bt matrix. The precipitation of a2 phase in ap, however,

was relatively uniform. No apparent difference in the size of a2 particles was observed. When the aging temperature decreased, the

precipitation of a2 phase became relatively uniform in bt, but the preferred precipitation and growth of a2 phase at boundaries and

dislocations was still obvious. When the temperature was sufficiently low, the precipitation of a2 phase was homogeneous at the

boundaries and dislocations as well as throughout the primary a phase and transformed b phase matrix.

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: Precipitation; Aging temperature; Titanium alloys

1. Introduction

The ordering transformation of the a2 phase based on

Ti3Al in titanium alloys has been an important problem

attracting extensive attention. Much work was carried

out and much knowledge on the a2 phase in binary Ti�/

Al alloys and near a titanium alloys has been established

[1�/4]. When the aluminum content reaches a certain

value in Ti�/Al binary alloys or a titanium alloys, the a2

phase can precipitate under suitable aging conditions.

The precipitation of a2 phase usually affects the proper-

ties of titanium alloys. As pointed by earlier researchers,

the a2 phase may result in embrittlement of titanium

alloys [5,6]. On the other hand, a2 phase also can

improve the high temperature properties of a phase

titanium alloys [7�/9].With increasing aluminum content in near-a titanium

alloys, the precipitation of a2 phase can be promoted,

and better high temperature strength and creep proper-

ties can be expected. Here, the aging treatment is a

crucial step that is directly related with the precipitation

of the a2 ordered phase. Hence, a knowledge of theprecipitation characteristic of a2 phase in near-a tita-

nium alloys is very important for controlling the

microstructure.

The purpose of the present work is to investigate the

precipitation of a2 phase in two near-a titanium alloys in

different aging conditions and to establish the micro-

structural sites for the preferred precipitation of the a2

phase.

2. Experimental material and procedure

Two near-a titanium alloys Ti�/Al�/Sn�/Zr�/Mo�/Si�/

Nd (1#) with 0.11 wt.% oxygen and Ti�/Al�/Sn�/Zr�/

Mo�/Si�/Nd�/Nb (2#) with 0.12 wt.% oxygen were

employed in this investigation. The b transformation

temperatures for these two alloys were 1035 and* Corresponding author. Fax: �/86-24-88112793

E-mail address: zhjun14@mailcity.com (J. Zhang).

Materials Science and Engineering A341 (2003) 229�/235

www.elsevier.com/locate/msea

0921-5093/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 1 - 5 0 9 3 ( 0 2 ) 0 0 2 4 0 - X

1047 8C, respectively. The following solution treat-ments and aging treatments, shown in Table 1, were

carried out.

The precipitation of a2 phase was identified by

electron diffraction and dark field microscopy. The

thin foils for electron microscopical analysis (EM-420

transmission electron microscope) were prepared by

twin-jet electro-polishing.

3. Results and discussion

3.1. The preferred precipitation of a2 phase along the

boundaries

The samples A1-a and A2-a were solution treated at

high a�/b fields and aged at relatively higher tempera-

tures, respectively. The duplex microstructures were

obtained in both A1-a and A2-a after solution treat-ments. The duplex microstructures consisted of small

volume fraction (�/25%) of equiaxed primary alpha

phase (ap) and the lamellar alpha matrix, or so-called

transformed b (bt) with some rare-earth phase dispersed

in the whole matrix. Oxygen existed in rare-earth phase.

There was almost not oxygen or neodymium in both ap

and bt. Aging treatment did not change the character-

istics of the duplex microstructure.The result of composition analysis showed that the

concentration of aluminum within ap was higher than

that within bt by �/2 at.%. The concentration of

titanium within ap was lower than that within bt by

�/2 at.%. Similar results were achieved in both 1# and

2# alloy, as shown in Table 2.

A typical TEM micrograph of the precipitation of a2

phase along the boundaries of bt in A2-a with ap�/bt

microstructure was shown in Fig. 1(a). The relative high

aging temperature of 800 8C resulted in non-uniform

precipitation of a2 phase in bt. It was very clear that the

preferred precipitation of a2 phase in bt took place along

the grain boundaries. There was rare precipitation of the

a2 phase throughout the matrix phase (bt). On the

contrary, the a2 particles homogeneously precipitated

throughout the ap parent phase, as shown in Fig. 1(b).

For the sample A1-a, the a2 particles in ap were

readily observed after aging at a higher temperature

(780 8C), as shown in Fig. 1(c). It was, however, very

difficult to find a2 particles in bt under this aging

condition, except at some places of the bt boundaries.

The results of secondary aging at lower temperatures,

730 8C for 1# alloy (A1-b) and 750 8C for 2# alloy

(A2-b), respectively, showed that the precipitation of a2

phase became somewhat more uniform throughout the

bt parent phases with decreasing temperature, as shown

in Fig. 2(a) and (b). The a2 particles precipitated in a

dispersed manner throughout the bt under the secondary

aging conditions. It was obvious that the a2 particles

that precipitated along bt boundaries in the first aging

were much larger than those precipitated in bt only

during the secondary aging. Comparing the Fig. 2(b)

with Fig. 1(a), it can be seen that the a2 particles

precipitating along boundaries in the first aging grew

into a larger size during the secondary aging. On the

other hand, the precipitation of a2 phase in ap parent

phases was predominantly characterized by growth into

larger size.

For the specimens solution-treated in the b phase field

and only aging-treated at single temperature, 730 8C for

1# alloy (A1-c) and 750 8C for 2# alloy (A2-c), the

difference in the size of a2 particles precipitated along

the bt boundaries and in the bt matrix was minor, as

shown in Fig. 2(c) and (d). This observation suggests

that the decrease of aging temperature promotes the

tendency of homogeneous precipitation of a2 phase.

This tendency of homogeneous precipitation was more

evident in 2# alloy (A2-c) than in 1# alloy (A1-c) due to

the larger aluminum concentration in 2# alloy. In

Table 1

Solution and aging treatments selected in the present study

Alloy Composition (wt.%) Solution Aging Sample number

Ti Al Sn Zr Mo Si Nd Nb Temperature/time/cooling Temperature/time/cooling

1# Bal 6.3 4.8 2.0 1.0 0.34 0.90 �/ 1025 8C/0.5 h WQ 780 8C/6 h WQ A1-a

780 8C/6 h WQ�730 8C/10 h WQ A1-b

1040 8C/0.5 h WQ 730 8C/10 h WQ A1-c

950 8C/0.5 h WQ 780 8C/6 h WQ A1-d

780 8C/6 h WQ�730 8C/10 h WQ A1-e

1025 8C/0.5 h WQ 650 8C/20 h WQ A1-f

2# Bal 6.8 4.8 2.0 0.5 0.34 0.90 0.70 1035 8C/0.5 h WQ 800 8C/6.5 h WQ A2-a

800 8C/6.5 h WQ�750 8C/10 h WQ A2-b

1050 8C/0.5 h WQ 750 8C/10 h WQ A2-c

1030 8C/0.5 h WQ 650 8C/50 h WQ A2-d

J. Zhang, D. Li / Materials Science and Engineering A341 (2003) 229�/235230

addition, the a2 particles in bt parent phase in Fig. 2(c)

and (d) showed a more dense distribution and larger size

compared with those precipitating in ordinary sites in

Fig. 2(a) and (b), respectively. This could be attributed

to the preferred precipitation of a2 particles along the

boundaries and dislocations in first aging suppressing

the growth of the a2 particles formed during the

secondary aging in the bt matrix.

Table 2

Compositions of ap, bt and rare-earth phase within 1# and 2# alloys

Alloy Phases Composition (at.%)

Ti Al Sn Zr Mo Si Nd Nb O

1# ap 83.42 12.71 1.70 0.72 0.22 1.23 �/ �/

bt 85.17 10.50 1.99 0.80 0.44 1.01 �/ �/

Rare-earth phase 2.68 0.83 10.77 �/ �/ �/ 12.81 �/ 72.91

2# ap 82.93 13.18 1.93 0.66 0.15 0.83 �/ 0.29 �/

bt 84.09 11.16 1.97 1.26 0.32 0.67 �/ 0.47 �/

Rare-earth phase 2.09 1.64 10.91 �/ �/ 2.46 7.99 �/ 74.91

Fig. 1. The precipitation of a2 phase during aging at higher temperature. (a) bt in A2-a, 1035 8C/0.5 h WQ�/800 8C/6.5 h WQ. (b) ap in A2-a,

1035 8C/0.5 h WQ�/800 8C/6.5 h WQ. (c) ap in A1-a, 1025 8C/0.5 h WQ�/780 8C/6 h WQ. (d) Super lattice diffraction spots from a2 phase

precipitated in ap of A1-a. Indices in rectangle belong to ap, and those with arrow belong to a2 phase.

J. Zhang, D. Li / Materials Science and Engineering A341 (2003) 229�/235 231

Because of the composition deviation of the two

alloys in present investigation from the stoichiometric

atomic ratio of Ti3Al (a2), the local atomic rearrange-

ment or atomic exchange between neighboring sub-

lattice sites would be not sufficient to make the a2

ordered phase precipitate. The precipitation of a2 phase

would proceed a process of nucleation and growth

controlled by Long-range diffusion.

For the ap�/bt duplex microstructure, the precipita-

tion of a2 phase was characterized by homogeneous

nucleation and growth in ap, but it exhibited hetero-

geneous nucleation and growth in bt. Since aluminum is

an a stabilizer, it partitioned preferentially to ap in

duplex microstructure and the aluminum content in ap

was larger than that in bt for each alloy in the present

study. The critical transformation temperature of a2 in

ap was higher than that in bt compared with the binary

Ti�/Al phase diagram. The aging temperatures of 780

and 800 8C for 1# alloy and for 2# alloy, respectively,

were near to the critical transformation temperature of

a2 in bt, but lower than that in ap. Because of much

smaller driving force, the nucleation of a2 in bt was

much more difficult than that in ap. The boundaries

could supply the relatively high driving force and alsoaccelerate the diffusion of aluminum atoms so that the

nucleation and growth of a2 phase at bt boundaries was

preferred to that in bt.

3.2. The preferred precipitation of a2 phase along

dislocations

The precipitation of a2 phase in bt (for the specimens

A1-b and A2-b) showed clearly the characteristic of

preferred precipitation at dislocations, as shown in Fig.

Fig. 2. The precipitation and growth of a2 phase in aging at lower temperature. (a) 1025 8C/0.5 h WQ�/780 8C/6 h WQ�/730 8C/10 h WQ, in bt of

A1-b. (b) 1035 8C/0.5 h WQ�/800 8C/6.5 h WQ�/750 8C/10 h WQ, in bt of A2-b. (c) 1040 8C/0.5 h WQ�/730 8C/10 h WQ, in bt of A1-c. (d)

1050 8C/0.5 h WQ�/750 8C/10 h WQ, in bt of A2-c.

J. Zhang, D. Li / Materials Science and Engineering A341 (2003) 229�/235232

3(a) and (c). As described above, the precipitation of a2

was a process of nucleation and growth controlled by

diffusion. Like the boundaries, the dislocations could

offer preferred sites for nucleation and growth than

other places in bt for the nucleation of a2 precipitates

when aging at higher temperature. Moreover, atom

diffusion could also be accelerated at dislocations.

Therefore, the dislocations provided a preferred site

for the precipitation of the a2 phase.

The precipitation of a2 phase at dislocations in ap in 1#

alloy solution treated in a phase field (for the specimens

A1-d and A1-e) is shown in Fig. 3(e) and (f). Here, no

obvious difference in size of a2 particles could be

observed. The density of a2 particles at dislocations,

however, was higher than that at other places in ap

matrix. The direction of growth of a2 particles seemed to

be normal to the length of the dislocations.It is worth noting that the precipitation of a2 particles

at dislocations appeared to be more pronounced in the

bt (Fig. 3(a) and (c)) than in ap (Fig. 3(e) and (f)). Alloy

composition, particularly the concentration of alumi-

num, and aging temperature appeared to play an

important role. For the same aluminum concentration,

the decrease of aging temperature resulted in a trend

from heterogeneous nucleation to homogeneous nuclea-

tion. For the same aging temperature, the homogeneous

nucleation tendency of a2 phase was strengthened with

increasing aluminum concentration.

Fig. 3. Precedent precipitation of a2 phase at dislocations. (a) Dark field image of a2 in bt of A1-b, 1025 8C/0.5 h WQ�/780 8C/6 h WQ�/730 8C/

10 h WQ. (b) Bright field image of a2 in bt of A1-b. (c) Dark field image of a2 in bt of A2-b, 1035 8C/0.5 h WQ�/800 8C/6.5 h WQ�/780 8C/10 h

WQ. (d) Bright field image of a2 in bt of A2-b. (e) 950 8C/0.5 h WQ�/780 8C/6 h WQ, in ap of A1-d. (f) 950 8C/0.5 h WQ�/780 8C/6 h WQ�/

730 8C/10 h WQ, in ap of A1-e.

J. Zhang, D. Li / Materials Science and Engineering A341 (2003) 229�/235 233

3.3. Uniform precipitation of a2 phase

When the aging temperature was decreased enough,

the uniform precipitation of a2 phase occurred. In the

present investigation, aging at 650 8C, either for 1#

alloy (A1-f) or for 2# alloy (A2-d), resulted in the

homogeneous precipitation of a2 phase in ap or bt

despite the presence of the boundaries or the disloca-

tions, as shown in Fig. 4.Because the aging temperature (650 8C) was suffi-

ciently low, the larger driving force for the nucleation of

a2 phase promoted homogeneous nucleation and pre-

cipitation of a2 phase took place throughout the whole

matrix. The boundaries and dislocations were no longer

strongly preferred sites.

3.4. General discussion

Based on the above results and discussion, it was clear

that the precipitation of a2 ordered phase in near-atitanium alloys occurred in the following three ways.

First, the precipitation of a2 phase only took place at

boundaries and dislocations when the sample was aged

at high temperature, for example, 780 8C for 1# alloy

and 800 8C for 2# alloy. Second, the precipitation and

growth of a2 phase was preferred at boundaries and

dislocations but also took place in the matrix when

aging was carried out at a moderate temperature, for

example, 730 8C for 1# alloy and 750 8C for 2# alloy.

Third, the uniform or homogeneous precipitation and

growth of a2 phase took place throughout the matrix

Fig. 4. Uniform precipitation of a2 phase during aging at relatively low temperature. (a) 1025 8C/0.5 h WQ�/650 8C/20 h WQ, in bt of A1-f. (b)

1025 8C/0.5 h WQ�/650 8C/20 h WQ, in ap of A1-f. (c) 1030 8C/0.5 h WQ�/650 8C/50 h WQ, in bt of A2-d. (d) 1030 8C/0.5 h WQ�/650 8C/50

h WQ, in ap of A2-d.

J. Zhang, D. Li / Materials Science and Engineering A341 (2003) 229�/235234

when aging occurred at a relatively low temperature, for

example, 650 8C for both of the alloys. Moreover, the

transformation temperature of a2 phase rises with

increasing aluminum content in titanium alloys. Foreach state of the precipitation of a2 phase, the aging

temperature could be increased with the increase of

aluminum content.

Because of the relatively low aluminum content in the

present near a titanium alloys (compared with the

stoichiometric composition of Ti3Al), it can be expected

that the precipitation of a2 phase was accompanied by

local composition change in parent phase. It is likelythat Long-range diffusion of atoms is necessary and that

the precipitation of a2 ordered phase is a process of

nucleation and growth controlled by diffusion.

Lutjering and Weissmann investigated the precipita-

tion characteristic of a2 phase in Ti�/Al binary alloys [1].

They reported that the temperature range for the

precipitation of a2 phase could be divided into three

regions in the same way as that presented above, basedon the nucleation feature of a2 phase. In comparison to

their result, the present investigation exhibited similar

features of the precipitation behavior of the a2 phase.

But the corresponding characteristic temperature re-

gions of the three precipitation ways of a2 phase in

present alloys were raised for the same aluminum

concentration (Table 2).

For the ap�/bt duplex microstructure (obtained bysolution treatment at higher a�/b field), the effects of

aging temperatures on the precipitation features of a2

phase in ap and bt were obvious. Aging could be carried

out within a larger temperature range for the uniform

precipitation of a2 phase in ap. Only if the aging

temperature was low enough, could homogeneous pre-

cipitation and growth of a2 phase in both bt and ap

occur. Unlike the results of a2 phase precipitation in Ti�/

6Al�/4Sn�/4Zr�/0.7Nb�/0.5Mo�/0.4Si alloy with duplex

microstructure reported by Cope and Hill [10] or by

Ramachandra, Singh and Sarma [11], the present

investigation demonstrated that the precipitation of a2

phase occurred more easily in ap than in bt at higher

aging temperature.

The precipitation characteristic of a2 phase at bound-

aries and dislocations and the precipitation difference inap and bt suggested that suitable aging temperature,

aging time and one or two-step aging treatment can be

selected easily to control effectively the precipitation of

a2 phase.

4. Conclusion

(1) The precipitation of a2 ordered phase in near-atitanium alloys was a process of nucleation and growth,

controlled by diffusion as well as thermodynamics.

(2) The preferred precipitation of a2 phase at bound-

aries and dislocations occurred at high aging tempera-

tures.

(3) Homogeneous precipitation of a2 phase took place

when aging temperature was relatively low. The fine a2

particles would disperse uniformly throughout the ap�/

bt matrix.

(4) The difference in aluminum concentration in ap

and bt in duplex microstructure influenced precipitation

characteristic of a2 phase in ap and bt.

(5) The preferred precipitation tendency of a2 phase at

boundaries and dislocations was weakened with increas-

ing aluminum concentration and/or with decreasingaging temperature.

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J. Zhang, D. Li / Materials Science and Engineering A341 (2003) 229�/235 235