Microstructural evolution in high Nb containing TiAl alloys by reactive hot pressing
WANG Yan-hang(王衍行)1, LIN Jun-pin(林均品)1, HE Yue-hui(贺跃辉)2, WANG Yan-li(王艳丽)1, CHEN Guo-liang(陈国良)1
1. State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing,
Beijing 100083, China;
2. State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China
Received 20 April 2006; accepted 30 June 2006
Abstract: High Nb containing TiAl alloys with nominal composition of Ti-45Al-9(Nb, W, B) were fabricated at different hot pressing temperatures. The experimental results show that the microstructure of alloys hot pressed at 1 300 ℃ is inhomogeneous, for lots of particulate substances containing Ti3Al phase and Nb powders dispersed in TiAl phase matrix. At 1 350 ℃, only a few lamellar colonies form and the diffusion of Nb element is incomplete. With the increase of hot pressing temperature, the microstructure will be more homogeneous. However, borides appear at higher sintering temperature. Meanwhile, the microstructure becomes coarse at 1 500 ℃. 1 400 ℃ will be an appropriate hot pressing temperature for reaction synthesis of high Nb containing TiAl alloys consisting of fully lamellar (FL) microstructure with the colony size of 30-80 μm. Nb element dissolves into the Ti-Al matrix by diffusion. Pore nests form in situ after Nb powders diffusion.
Key words: titanium aluminides; powder metallurgy; microstructure
1 Introduction
Over the last decade, an extensive research on high Nb containing TiAl alloys has been performed owing to their good high temperature properties and low densities, and great progresses have been in many aspects, such as phase diagram, alloying, oxidation resistance as well as the microstructure-properties relationships [1, 2]. Although high Nb addition can significantly improve high temperature properties of TiAl based alloys, it enhances the difficulty of their preparation [3]. At present, high Nb containing TiAl alloys are mostly fabricated by ingot metallurgy. However, ingot metallurgy process is often encountered with macro-segregations [4]. Furthermore, the inherent low-temperature ductility of high Nb containing TiAl alloys is poor [5]. Therefore, many advanced processing routes have been attempted to improve their microstructural homogeneity and workability, such as forging and extrusion, but their microstructures are still coarse [4, 6].
The objective of the present investigation was to pursue a novel approach for receiving a fine and homogeneous microstructure to overcome the problem of poor workability. Powder metallurgy process is of special interest since high degrees of chemical homogeneities can be obtained and large-scale segregations are avoided [7]. Numerous work has been conducted on the microstructures and properties of powder metallurgy TiAl based alloys [8-10]. However, little attention has been paid to those of powder metallurgy of high Nb containing TiAl alloys. In this study, reactive hot pressing process was adopted to fabricate high Nb containing TiAl alloys, and the microstructural evolution under different hot pressing temperatures were discussed.
2 Experimental
Elemental Ti, Al, Nb, W and B powders with mean particle sizes smaller than 25 μm were mixed in air to the desired composition of Ti-45Al-9(Nb, W, B) (mole fraction, %). The powder mixture was die-pressed under a pressure of 380 MPa to green compacts of 32 mm in diameter and 12 mm in height. The compacts were then put into a graphite die and hot pressed for 1 h in an argon protective atmosphere to form sintered billets under a pressure of 25 MPa at 1 300 ℃ (TAN1), 1 350 ℃ (TAN2), 1 400 ℃ (TAN3) and 1 500 ℃ (TAN4), respectively.
The hot pressed samples were subsequently cut, polished and etched using Kroll’s solution (2HF+2HNO3+H2O (volume fraction, %)). Micro- structural observation was carried out by optical microscopy (OM), scanning electron microscopy using back scattering electron imaging (BSE) and energy dispersive spectroscopy (EDS). X-ray diffraction (XRD) was conducted to characterize the constituent phases. The lamellar colony size was determined by the intersection linear method.
3 Results
3.1 Phase analysis
XRD patterns in Fig.1 show the phase composition of samples after reactive hot pressing. It indicates that TAN1 mainly consists of TiAl phase, Ti3Al phase and remnant Nb powders, while TAN2 contains TiAl and Ti3Al phases. The diffraction peaks of element Nb disappear in XRD patterns of TAN2, however, residual Nb powders can be observed on the BSE micrograph, which is probably related with measure error of diffraction instrument. Meanwhile, TiAl and Ti3Al phases are found in TAN3 and TAN4. The diffraction peaks of TiAl and Ti3Al phases in TAN3 are relatively sharp, indicating that their degree of crystallization in TAN3 is better.
3.2 Microstructures
Fig.2 is the micrographs of TAN1. Fig.2(a) shows that TAN1 is mainly composed of two parts: particle substances and matrix. The particle substances dispersed in the matrix appear in two morphologies: islands and flakes. Fig.2(b) shows the BSE microstructure of TAN1. The compositions of the different phases in Fig.2(b) measured by EDS are listed in Table 1. EDS results indicate that the white regions (such as point A and E) are residual Nb powders, gray region (point B) locating at the edge of the white regions is Ti3Al phase dissolved Nb element, black region (point C) is TiAl phase, and the small black regions (point D) dispersed around the larger white region are pore nests formed in situ after Nb powders diffusion. The diffusion of elemental Nb is incomplete in TAN1, and pore nests form in situ after Nb powders diffusion. It can be concluded that, therefore,
Fig.1 XRD patterns of hot pressed samples: (a) TAN1; (b) TAN2; (c) TAN3; (d) TAN4
Fig.2 Micrographs of hot pressed TAN1: (a) Optical micro- structure; (b) SEM-BSE microstructure
Table 1 EDS analysis of Fig.2(b)
the microstructure of TAN1 is inhomogeneous. Residual Nb particles (islands) and Ti3Al phase (flakes) are dispersed in TiAl phase matrix.
Fig.3 is the micrographs of TAN2. Some fine lamellar colonies with the size of about 30 μm form (Fig.3(a)). Fig. 3(b) shows the BSE microstructure of TAN2. Compared with Fig. 2(b), the diffusion of Nb powders is more complete in TAN2. However, the diffusion of Nb powders is still incomplete since there are residual Nb powders (white phase) in the center of pore nest.
Fig.4 shows the microstructure of TAN3. A fully lamellar (FL) microstructure was developed out (Fig.4(a)). The lamellar colonies with a size of approximately 30-80 μm in TAN3 consist of very straight and smooth α2/γ laths. This FL microstructure is similar to that of as-cast alloy, but it is finer [11]. BSE microstructure of TAN3 is shown in Fig.4(b). It indicates that elemental Nb powders diffuse completely, and pore nests are formed in situ. The deformation and agglomeration phenomena of pores are observed in some pore nests. Furthermore, particulate and acicular borides firstly appear in the FL microstructure (Fig.4(c)).
Fig.3 Micrographs of hot pressed TAN2: (a) Optical micro- structure; (b) SEM-BSE microstructure
Fig.4 Micrographs of hot pressed TAN3: (a) Optical micro- structure; (b) SEM-BSE microstructure; (c) Boride morphology in deep-etched sample
FL microstructure is also successfully obtained in TAN4, but its lamellar colonies become coarse (Fig.5(a)). Their sizes are within 100-200 μm. Similarly, the deformation and agglomeration phenomena of pores and borides appear in TAN3, as shown in Fig.5(b).
Fig.5 Micrographs of hot pressed TAN4: (a) Optical micro- structure; (b) SEM-BSE microstructure
4 Discussion
Reactive sintering of Ti, Al elemental powders is a diffusion-controlled process including the formation of transient phases, such as TiAl3 and TiAl2 [7]. In the beginning of hot pressing, TiAl3 forms by the reaction between Ti and Al powders. After the reaction is completed, pure Ti and TiAl3 are present. During subsequent hot pressing, further reaction between Ti and TiAl3 leads to the formation of the intermetallic compounds Ti3Al, TiAl and TiAl2. In addition, Ti3Al can react with TiAl2 to form TiAl. Only Ti3Al and TiAl are present after a series of reactions. However, niobium aluminides are not formed in the reactive hot pressing. It indicates that the reaction ability between Ti and Al powders is dominant in the Ti-Al-Nb system, restraining the reaction between Nb and Al powders. Elemental Nb dissolves into the Ti-Al matrix by diffusion. Therefore, it suggests that reactive hot pressing process is feasible to fabricate high Nb containing TiAl alloys.
With the rising of reaction temperature, diffusion of elemental powders will be more sufficient, and composition of the alloys is more homogeneous. At 1 300 ℃, the microstructure is inhomogeneous, for lots of particulate substances containing Ti3Al phase link to island-like zone and Nb powders disperse in TiAl phase matrix. Moreover, the microstructure is inhomogeneous at 1 350 ℃, since only a few lamellar colonies form and the diffusion of Nb powders is still incomplete. However, at higher reaction temperature, the alloys become more ductile and dense with larger degrees of deformation and elimination of pores. Fine and homogeneous microstructure is observed at 1 400 ℃. However, the microstructure coarsening happens at 1 500 ℃, which is not favorable to control microstructure and leads to the difficulty in the grain boundary gliding and dislocation movement, thus impeding the densification process. Although borides appear in the alloy hot pressed at 1 400 ℃, the content of them is very small.
It is worth noting that the presence of the β phase in TiAl based ingot alloys with alloying additions of β stabilizers such as Nb, Cr, Mo and W, has been reported previously [4, 12]. However, β 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 present alloys. According to Ti-Al phase diagram, microstructures of Ti-45Al-(8-9)Nb ingot alloys evolve the following process during solidification: L→L+β→β→β+α→α→α+γ→lamella(α2+γ)+γ
For powder metallurgy high Nb containing TiAl alloys, microstructures are formed by solid-solid and solid-liquid reactions. Moreover, as the reaction temperature increases, the content of borides is higher. It can be concluded that 1 400 ℃ will be an appropriate hot pressing temperature to prepare high Nb containing TiAl alloys with fine lamellar microstructure.
The diffusion of elemental Nb is important in the diffusion-controlled process, for the diffusivity of element Nb is smaller than that of Al and Ti [13]. At 1 300 ℃, the diffusion of Nb powders is incomplete and inhomogeneous. During diffusion, large Nb powders are crashed into some small particles under the applied force. Pore nests form 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 applied unilateral force in the hot pressing process. The friction between powders and die wall leads to the loss of pressure in the direction of height. Furthermore, due to the mutual friction among powders, the pressure will have much more transfers at the height direction than at the radial one, which results in enduring inhomogeneous forces in the different directions. With the increase of hot pressing temperature, the diffusion of Nb powders is more sufficient. There are still very small amount of Nb powders remained in the center of pore nests in the alloy hot pressed at 1 350 ℃. However, Nb powders diffuse completely at 1 400 ℃ and 1 500 ℃. Meanwhile, compression deformations and agglomeration phenomena of pores appear in some pore nests at 1 400 ℃ and 1 500 ℃, which are also related with the endured inhomogenous force at higher hot pressing temperature. Pore nests can be eliminated efficiently through subsequent hot isostatic pressing (HIP) [14]. In addition, the process by replacing elemental Nb powders with Nb-Al alloy powders is favorable to increase the diffusion of Nb powders in high Nb containing TiAl alloys.
5 Conclusions
1) It is feasible to fabricate Ti-45Al-9(Nb, W, B) with fine lamellar colonies within 30-80 μm by reactive hot pressing process in argon atmospheres.
2) With the rising of hot pressing temperature, the microstructures become more homogeneous. However, borides appear at higher sintering temperature. Moreover, the microstructure becomes coarse at 1 500 ℃. 1 400 ℃ will be an appropriate hot pressing temperature to fabricate high Nb containing TiAl alloys.
3) Elemental Nb powders dissolve into the matrix by diffusion. Pore nests are formed in situ after Nb powders diffusion. With the increase of hot pressing temperature, diffusion of Nb powders is more sufficient, and the diffusion will be completed at 1 400 ℃ and 1 500 ℃.
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(Edited by YANG Hua)
Foundation item: Project (704008) supported by the Key Grant Project of Chinese Ministry of Education; Project (NCET-04-01017) supported by the Program from New Century Excellent Talents in University
Corresponding author: LIN Jun-pin, Professor; Tel: +86-10-62332192; E-mail: linjunpin@skl.ustb.edu.cn
