Diffusion behavior of Nb element in high Nb containing TiAl Alloys by reactive hot pressing

RARE METALS Vol. 25, No. 4, Aug 2006, p . 349

Diffusion behavior of Nb element in high Nb containing TiAl Alloys by reactive hot pressing WANG Yanhang”, LIN Junpin”, HE Yuehui2’,WANG Yanli”, and CHEN Guoliang’) 1) State.Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083.China 2) State Key Laboratory for Powder Metallurgy, Central South University, Changsha410083,China (Received 2005-07-24)

Abstract: Diffusion behavior of Nb in elemental powder metallurgy high Nb containing T i alloys was investigated. The results show that Nb element dissolves into the matrix by diffusion. Pore nests are formed in situ after Nb diffusion. With the increase in hot pressing temperature, the diffusion of Nb will be more sufficient, and the microstructure is more homogeneous. Nb element diffuses completely at 1400°C. Meanwhile, compression deformation and agglomeration phenomena of pores are observed in some pore nests. Hot isostatic pressing (HIP) txatment can only efficiently decrease but not eliminate porosity completely. Key words:powder metallurgy;TiAl-based alloy; hot pressing; diffusion; hot isostatic pressing

[This work i s j m c i a l l y supported by the Key Grant Pmject of Chinese Ministry of Education (No. 704008)and by the Progrmfrorn New Century Excellent Talents in University,China (No. NCET-04-01017).]

1. Introduction High Nb containing T i alloys have attracted much attention owing to their low density and potential application in high-temperature environments [1-4]. It has been found that Nb is the essential and effective element to improve mechanical properties, especially,high-temperature strength for T i alloys [5]. However, high Nb addition increases the dificulty in the preparation of T i alloys. At present, high Nb containing TiAl alloys are generally fabricated by ingot metallurgy, but this process is usually encountered with microsegregation [6]. Meanwhile, the inherent low-temperature ductility of high Nb containing TiAl alloys is poor [7]. Therefore, many advanced processes, such as forging, HIP, and extrusion, have been attempted to improve microstructurd homogeneity and workability, but their microstructures are still coarse [6-81. Of late, powder metallurgy technique is of special interest since high degrees of chemical homogeneiCorresponding author: LIN Junpin

ties can be obtained and large-scale segregationsare avoided [9]. Among the powder metallurgy techniques, the elemental powder metallurgy (EPM) is an attractive technique because of the economical reasons, and this has been investigated extensively by Dahms [lo], Kim [Ill as well as Hwang e? al. [12] to produce Ti-based alloys. However, little attention has been paid to the preparation of high Nb containing T i alloys with powder metallurgy. Therefore, it is interesting to fabricate by EPM the high Nb containing T i alloys with a fine and homogeneous microstructure to overcome the problem of poor workability. The diffusion of elemental Nb is important in improving densification for EPM high Nb containing T i alloys because of its low diffusivity [13]. In this article, the diffusion behavior of Nb element in EPM high Nb containing TiAl alloys is discussed.

2.

E-mail:[email protected]

Experimental Elemental Ti,Al, Nb, W, and B powders with

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mean particle sizes smaller than 25 pn were mixed in air to the desired composition of Ti-45Al-9 (Nb, W, B) (at.%). The powder mixture was die-pressed under a pressure of 380 MPa to green compacts with 32 mm in diameter and 12 mm in height. The compacts were then put into a graphite die and hot pressed to form sintered billets for 1 h in an argon protective atmosphereunder a pressure of 25 MPa at 1300°C (TANl), 1350°C (TAN2), 1400°C (TAN3), and 1500°C (TAN4), respectively. Subsequently, HIP for TAN3 was performed at 1200°C and 200 Mpa for 4 h. Microstructuralobservations were carried out by scanning electron microscopy (SEM) using back scamring electron imaging (BSE) and energy dispersive spectroscopy (EDS). X-ray diffraction (XRD) was conducted to characterize the constituent phases.

3. Results 3.1. Phase analyses

The XRD patterns in Fig. 1 show the phase composition of samples after reactive hot pressing. It indicates that TANl mainly consists of T i phase, T i f l phase, and remnant element Nb, while TAN2 is composed of TiAl and T i& phases. The diffraction peaks of element Nb disappear in TAN2. However, remnant Nb powders can be observed on the BSE micrograph, which is probably related with the measurement error of the diffraction instrument. When the hot pressing temperature increases up to 14OO"C, Nb powders diffuse completely. Therefore, the diffraction peaks of Nb element disappear in TAN3 and TAN4, where only T i and T i d phases are found. 6ooo

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Fig. 1. XRD patterns of samples hot pressed at merent temperatures: (a) TAN1; (b)TAN2; (c) TAN3; (d) TAN4.

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Fig. 2(a) is the BSE microstructure of TANl.

EDS analysis results of the phases for Fig. 2(a) are

listed in Table 1 and suggest that white regions (such as points A and E) are remnant element Nb powders, the gray region (Point B) located at the edge of the white regions is T i 4 phase dissolving Nb, the black

Wung Y.H. et ul., Diffusion behavior of Nb element in high Nb containing TiAl Alloys by.. . matrix (Point C) is TiAl phase, and the small black regions (Point D) dispersed around the larger white region are pores. Therefore, it can be deduced that the diffusion of elemental Nb is incomplete in TAN1, and pore nests are formed in situ after Nb powders

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diffusion.Furthermore, the microstructure of TAN1 is not homogeneous. The matrix is mainly T i phase, and the particulate substances are composed of Ti3AI phase linked to island-like and remnant Nb powders.

Fig. 2. BSE microstructures of the samples hot pressed at different temperatures: (a) TAN1; (b)TAN2; (c) TAN3; (d) TAN4. Table 1. EDS analyses of the phases on the Fig. 2(a) Point A

B C D E

Element content / at.% Ti Al Nb 3.089 4.633 92.278 67.977 23.552 8.472 50.869 54.862 2.194

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2.970 8.163 94.329

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The BSE microstructureof TAN2 is shown in Fig. 2(b). Compared with Fig. 2(a), the diffusion of Nb powders is more complete in TAN2. However, the diffusion of Nb powders is still incomplete since there is remnant Nb element (white phase) in the center of pore nests. It is also found that some fine lamellar colonies with a size of 30 pn are formed.

Fig. 2(c) shows the BSE microstructure of TAN3. It indicates that elemental Nb diffuses completely, and pore nests are formed in situ. The compression deformation and agglomeration phenomena of pores are observed in some pore nests. Similarly, a typical fully lamellar (FL,)microstructure containing fine lamellar colonies (a&) with a size range of 30-80 pn is formed. Furthermore, particulate and acicular borides appear first in the matrix. The diffusion behavior of elemental Nb powders in TAN4 is similar to that in TAN3. However, its FL microstructure becomes coarse (as shown in Fig. ad)).

3.3. Effect of HIP on the diffusion of Nb element The as-HIP microstructure is shown in Fig. 3. It indicates that HIP treatment can only efficiently de-

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crease but not eliminate porosity completely. The agglomeration phenomenon of pores is more intensive. However, the compression deformation of pores disappears, which is the result of applied uniform force in the HIP treatment.

Fig. 3. BSE microstructureof TAN3 after HIP.

4. Discussion The reactive sintering of Ti and Al elemental powders is a diffusion-controlledprocess including the formation of transient phases such as T i 3 a n d TiA12. During subsequent hot pressing, TL4l3phase transforms to TiAl phase via the expense of Ti particles through further diffusion, while large Ti particles that have not reacted with Al maintain the composition of TiA phase [9]. Niobium aluminides

are not formed in EPM high Nb containing TiAl alloy. Therefore, it is deduced that elemental Nb dissolves into the matrix by diffusion in reactive hot pressing process. The dissolved Nb element will improve distinctly the mechanical properties by solid solution strengthening [6]. The diffusion behavior of elemental Nb is important in the diffusion-controlled reactive sintering process since the diffusivity of Nb is smaller than b system. Fig. that of Al and Ti in the Ti-Al-Nternary 4 is the sketch map of the diffusion of Nb in hot pressed and HlF'ped samples. At 1300"C, the diffusion of Nb is incomplete and not homogeneous. Large Nb powders are crashed into some small particles under the applied force. Pore nests are formed in situ after these small particles diffuse into the matrix. Besides, some Nb powders have a very little diffusion even if their sizes are smaller. The diffusion inhomogeneity is related to the applied unilateral force in the hot pressing process. The friction between powders and die wall leads to the loss of pressure in the vertical direction. Furthermore, because of the mutual friction among powders, the pressure will have much more transfers in the vertical direction than in the radial one, which results in enduring uneven forces in the different directions.

Fig. 4. Diffusion sketch map of Nb element.

With the increase of hot pressing temperature, the diffusion of Nb is more efficient. There are only very

small Nb powders remaining in the center of pore nests in the alloy hot pressed at 1350°C. However,

Wang Y.H. ef aL, Diffusion behavior of Nb element in high Nb containing TiAl Alloys by...

Nb diffuses completely at 1400°C and 1500°C. However, the compression deformations and agglomeration phenomena of pores appear in some pore nests at 1400°C and 1500"C, which are with the result of the enduring uneven force at an elevated sintering temperature. With the diffusion of Nb element, the microstructure of the high Nb containing TiAl alloys becomes increasingly homogeneous. At 1350"C, the microstructure is inhomogeneous since only a few lamellar colonies are formed. The microstructure, which is fine and homogeneous, is observed at 1400°C. However, the microstructure coarsening happens at 1500"C, which is not favorable to control the microstructure and leads to the difficulty in the grain boundary gliding and dislocation movement, which impedes the densification process. Although borides appear in the matrix at 1400"C, the quantity is very small. It is worth noting that the presence of the p phase in TiAl-based ingot alloys with alloying additions of p stabilizers such as Nb, Cr, Mo, and W, has been reported previously [6,14-151. However, p phase cannot form in high Nb containing TiAl alloys prepared from powder metallurgy, even if Nb and W powders, especially, high Nb addition, exists in the alloys studied in this article. According to Ti-Al phase diagram, the microstructure of Ti-45Al-(8-9)Nb ingot alloys undergoes the following process during solidification: L + L + + p+ p + a + a + a + y + lamella(a2+ y) + y. For powder metallurgy high Nb containing TiAl alloys, their microstructures are formed by solid-solid and solid-liquid reactions. Moreover, as the reaction temperature increases, the content of borides is higher. HIP treatment is more efficient in compressing pores [16-171. However, in the present experimental alloy, pores cannot be eliminated completely through HIP treatment. The main reasons are the following: (1) The hot pressed sample is not encapsulated before HIP treatment to remove open pores because this process is relatively expensive, (2) Most pores in the alloy are closed pores, including pore nests formed in situ after Nb powders diffusion as well as Kirkendall pores generated from the transient

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phases through the reaction between A1 and Ti powders. The closed pores are filled with the protective gas (Ar) at a certain pressure. During HIP-compaction these pores shrink until their inner gas pressure equals the applied HIP-pressure (200 MPa in this case), (3) The applied force in the hot pressing process may be small, which leads to porous sintered billets. Therefore, it is deduced that to obtain dense alloys the compression of closed pores is of special importance. Presently, many efficient methods have been reported on other alloys, such as using finer powders and applying higher force in hot pressing [9,13,18]. In addition, the process by replacing elemental Nb powders with Nb-A1 alloy powders is favorable to increase the diffusion of Nb powders in the high Nb containing TiAl alloys. This study is underway.

5. Conclusion The diffusion behavior of Nb element in elemental powder metallurgy high Nb containing TiAl alloys was investigated. It is concluded that Nb element dissolves into the matrix by diffusion. Pore nests are formed in situ after Nb diffusion. With the increase of hot pressing temperature, the diffusion of Nb is more efficient, and the microstructure of the alloys is more homogeneous. Nb element diffuses into TiAl alloy completely at 1400°C. Meanwhile, the compression deformation and agglomeration phenomena of pores are observed in some pore nests. HIP treatment can only efficiently decrease but not eliminate porosity completely.

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