Trans. Nonferrous Met. Soc. China 24(2014) 1372-1378

Microstructure evolution and nitrides precipitation in in-situ Ti₂AlN/TiAl composites during isothermal aging at 900 °C

Yi-wen LIU, Rui HU, Tie-bang ZHANG, Hong-chao KOU, Jin-shan LI

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China

Received 27 May 2013; accepted 17 October 2013

Abstract: Microstructures of Ti₂AlN/TiAl composites prepared by in-situ method were characterized in in-situ and aging treatment conditions and the nitride precipitation was investigated in Ti₂AlN/TiAl composites aged at 900 °C for 24 h after being heat treated at 1400 °C for 0.5 h. The in-situ composites consist of γ+α₂ lamellar colonies, equiaxed γ grains and Ti₂AlN reinforcements. Matrix with nearly fully lamellar structure formed after solution and subsequently aging treatment. With the increase of Ti₂AlN content, the nearly fully lamellar structure becomes instable for the aged composites. According to TEM study, fine Ti₂AlN precipitates are found to distribute at the grain boundaries of lamellar colony. Needle-like Ti₃AlN precipitates arrange in line with growing axis parallel to [001] direction of the γ-TiAl matrix and another needle-like Ti₃AlN precipitates with lager size distribute at the dislocations.

Key words: TiAl composites; Ti₂AlN; in-situ synthesis; microstructure; precipitates; aging treatment

1 Introduction

γ-TiAl based alloys have attracted great attention for their high potential application at elevated temperatures due to their low density, high specific strength and oxidation resistance [1,2]. However, poor low- temperature ductility and insufficient elevated temperature strength are the main obstacles for their practical application. Many researches have been carried out to improve the shortages, especially for elevated temperature strength, which is ultimately important for their application. Alloying with the third element and microstructural modifications through thermomechanical treatments are usually considered to improve the elevated temperature strength of TiAl alloys [3-6]. Meanwhile, fabrication of TiAl-based composites with ceramic particles is also an effective way. Some investigations have shown that the properties of TiAl alloy could be obviously improved by addition of reinforcements, such as Al₂O₃, TiB₂, Ti₅Si₃, Ti₂AlC and Ti₂AlN [7-12]. Among these, Ti₂AlC and Ti₂AlN, as a group of ternary compound, exhibit metallic and ceramic properties, such as low density, high elastic modulus, easy machinability and high-temperature oxidation resistance. They are regarded as the promising candidates for reinforcement of TiAl composites [13-15].

SHU et al [9] investigated the mechanical properties of Ti₂AlC/TiAl composites. The ultimate compression strength and the true fracture strain of the composites were improved significantly. RAMASESHAN et al [16] also found that the 0.2% proof strength of Ti₂AlC/TiAl composites was higher than that of un-reinforced TiAl alloy both at ambient and elevated temperatures. As for Ti₂AlN, KAKITSUJI et al [10] and MABUCHI et al [17] reported that the strength and the fracture toughness of Ti₂AlN/TiAl composites were superior to those of unreinforced TiAl alloy. It is especially interesting that the elevated temperature strength of the composites at temperature as high as 900 °C was obviously higher than that of the unreinforced TiAl alloy, which demonstrated enormous potential for high-temperature application. Thus, investigation on Ti₂AlN/TiAl composites shows great attraction.

As elevated-temperature structural material, creep resistance is an important consideration for its practical application. To improve the creep resistance of TiAl alloy, precipitates such as nitrides, carbides and silicides are generally used to pin the dislocations or stabilize the microstructure. In the TiAl alloy containing nitrogen, some coarse Ti₂AlN and fine Ti₃AlN precipitates were observed at colony boundaries and interlamellar interfaces during aging treatment, which had a significant influence on the creep resistance for TiAl alloy [18-20]. With addition of carbon and silicon in TiAl alloy, Ti₂AlC, Ti₃AlC and Ti₅(Si, Al)₃ that were formed by thermal or mechanical treatment offered a remarkable improvement in creep behavior [21,22]. With regard to the Ti₂AlN/TiAl composites, there is a certain solubility of nitrogen in the matrix which would lead to the precipitation of nitrides during aging treatment and play an important role in creep strengthening. However, few investigations have concerned about the nitride precipitation in the Ti₂AlN/TiAl composites. Thus, the aim of this work is to investigate the microstructure evolution and ternary compound nitride precipitation of Ti₂AlN/TiAl composites during aging process.

2 Experimental

The Ti₂AlN/TiAl composites investigated in this study were prepared by an in-situ method. The elemental powders of titanium (99.9% purity, mass fraction) and aluminum (99.9%, mass fraction) and TiN powder (99%, mass fraction) were used as raw materials and mixed with compositions of 10% Ti48Al (mole fraction) -10%, 20%Ti₂AlN (volume fraction). The powder mixtures were cold-pressed into cylindrical samples and then sintered at 1200 °C for 2 h in a vacuum sintering furnace. During heating process, combustion reaction was carried out. After that, the resultant samples were prepared as 30 g buttons and remelted three times in order to promote densification and homogeneity by a non-consumable vacuum arc-melting. The buttons were then heat treated at 1400 °C for 0.5 h followed by furnace cooling. Subsequently, the heat treated samples were aged at 900 °C for 24 h.

To identify the phase content, X-ray diffraction (XRD) analysis was conducted using DX-2700 diffractometer with Cu K_α radiation. The microstructure characteristics of the composites were investigated by Olympus-PMG3 optical microscope (OM) and JSM-6700 scanning electron microscope (SEM) in back-scattered electron (BSE) mode. The morphologies of the nitrides precipitation were examined by Tecnai G2 F30 transmission electron microscope (TEM). TEM specimens were cut from the center of aged composites, then prepared by standard mechanical polishing and twin-jet electropolishing using a solution of 6% (volume fraction) perchloric acid + 34% butanol + 60% methanol at -20 °C and 25 V.

3 Results and discussion

3.1 In-situ Ti₂AlN/TiAl composites

Figure 1 shows the X-ray diffraction patterns of Ti₂AlN/TiAl composites. The result confirms that the in-situ composites consist of γ-TiAl, α₂-Ti₃Al and Ti₂AlN phases. The peak intensity of Ti₂AlN increases with the increase of Ti₂AlN content in composites from 10% to 20%.

Fig. 1 XRD patterns of in-situ Ti₂AlN/TiAl composites

The microstructures of composites with 10% and 20% Ti₂AlN are shown in Fig. 2, which are composed of colony grains, equiaxed single phase and a certain fraction of particles homogenously distributed in the matrix constructed by both structures demonstrated above. According to the results of energy-dispersive spectra (EDS) in Fig. 2, the matrix of composites consists of γ+α₂ lamellar colonies and equiaxed γ grains and the particle is identified as Ti₂AlN phase. By comparing Figs. 2 (a) and (b), it can be seen that morphology of Ti₂AlN reinforcement develops from granular type for 10% Ti₂AlN composite to rod-like type for 20% Ti₂AlN composite. The mean grain size of lamellar colonies decreases from about 62 μm to 39 μm with increase of Ti₂AlN content. The microstructure refinement can be explained by the nucleation effect of Ti₂AlN particles which may act as the nucleation sites during solidification of composites.

Fig. 2 Results of SEM-EDS analysis of in-situ Ti₂AlN/TiAl composites with (a) 10% and (b) 20% Ti₂AlN

Fig. 3 XRD patterns of Ti₂AlN/TiAl composites aged at (1400 °C, 0.5 h)+(900 °C, 24 h)

3.2 Phase and microstructure of aged Ti₂AlN/TiAl composites

Figure 3 shows the XRD patterns of Ti₂AlN/TiAl composites with heat treatment of (1400 °C, 0.5 h) +(900 °C, 24 h). The constituent phases are γ, α₂ and Ti₂AlN, the same as those of the in-situ composites. Nevertheless, compared with the XRD patterns in Fig. 1, the intensity of α₂ peaks decreases noticeably. The characteristic microstructures of the composites heat treated at 1400 °C for 0.5 h and 900 °C for 24 h are shown in Fig. 4, respectively. After solution treatment at 1400 °C for 0.5 h, a matrix with nearly fully lamellar structure is obtained and consists of colonies of α₂/γ lamellae and equiaxed γ, as shown in Figs. 4 (a) and (b), which is different from Ti48Al alloy. According to binary Ti-Al phase diagram, a fully lamellar structure will be produced by heat treatment at α phase field with furnace cooling. Addition of Ti₂AlN may lead to change of phase transformation of the TiAl matrix. Figures 4 (c) and (d) show the microstructure of the composites aged at 900 °C for 24 h after solution treatment. For 10% Ti₂AlN composite, the nearly fully lamellar structure is found to be maintained, while for 20% composite the nearly fully lamellar structure becomes instable, as shown in Fig. 4(d). In the lamellar colonies, α₂ laths become thinner and dissolve and γ laths coarsen. The equiaxed γ grains also coarsen. This indicates that the change is more noticeable with the increase of Ti₂AlN content.

In Figs. 4(c) and (d), many fine bright particles can be found to distribute at the boundary of lamellar colonies and interfaces of lamellar. In order to investigate these precipitates, TEM analyses were carried out in the following sections.

3.3 Nitrides precipitation in aged Ti₂AlN/TiAl composites

The representative TEM image of the fine precipitates in the aged composites is shown in Fig. 5. Figure 5(a) shows some fine precipitates of 0.4-1 μm located at the boundaries of colony grains. According to the electron diffraction patterns of these precipitates, as shown in Fig. 5(b), they can be identified as Ti₂AlN phase.

In addition, some fine needle-like precipitates can be found to distribute homogenously and arrange along a particular direction in the matrix of aged composites, as shown in Fig. 6(a). By electron diffraction analysis in Fig. 6(b), it is identified that the needle-like precipitates are Ti₃AlN phase and the matrix is γ. The growing axis of the needle-like precipitates parallels [001] direction of the matrix.

There is also another needle-like precipitates with larger size in the γ matrix, as shown in Fig. 7(a). Figure 7(b) shows the dark field image of this kind of precipitates. A plenty of dislocations can be found to distribute around these precipitates. Electron diffraction confirms that this kind of precipitate is also Ti₃AlN phase.

Fig. 4 SEM images of 10% Ti₂AlN/TiAl (a), 20% Ti₂AlN/TiAl (b) composites with heat treatment of 1400 °C for 0.5 h and 10% Ti₂AlN/TiAl (c), 20% Ti₂AlN/TiAl (d) composites aged at 900 °C for 24 h after solution treatment

Fig. 5 TEM image showing Ti₂AlN precipitates at boundaries of colony grains (a) and corresponding SAED pattern (b)

Fig. 6 TEM image showing Ti₃AlN precipitate (a) and corresponding SAED pattern (b)

Fig. 7 TEM images showing Ti₃AlN precipitate distributed at dislocations (a) and dark field image of precipitates (b)

Formation of the two different types of precipitates is related to crystal structure, misfit, structure of the interphase boundary between precipitate and matrix, and stress applied during aging treatment [23,24]. Generally, when the crystal structures are basically the same and the misfit is small, the precipitate takes the form of a sphere. The shape of the precipitate will change from spherical to needle-like, finally to plate-like with the increase of misfit between the precipitate and the matrix. When crystal structures of the precipitates and the matrix are different, the structure of interphase boundary becomes determining factor for the morphology of precipitates [25]. As discussed by TIAN and NEMOTO [25], the lattice misfit between TiAl matrix and Ti₃AlN has a smaller value in the [001] direction of Ti₃AlN and the growth of Ti₃AlN precipitates displays preferential orientation in [001] direction of the matrix. Thus, the Ti₃AlN precipitates show the needle-like morphology with growing axis parallel to [001] direction of the matrix.

The precipitation of these nitrides is related to nitrogen dissolved in the matrix. It is known that nitrogen atom has certain solid solubility in γ-TiAl alloys [20]. During in-situ preparation, the nitrogen atoms may decompose from TiN and dissolve into the matrix and precipitate as a form of nitrides in the matrix during aging treatment. It is also understood that nitrogen has much greater solubility in α₂ phase (about 0.3% in mole fraction) compared with γ phase. So, precipitation of nitrides in γ matrix was favored [26], as shown in Figs. 6 and 7. It indicates that α₂ would be consumed and causes the growth of γ phase during the formation of these precipitates, leaving the precipitates behind in γ matrix. The process can be deduced as the type of α₂→γ+ precipitates.

In the matrix of aged composites, it is interesting to note that some dislocation networks and walls can be found, as shown in Fig. 8. Due to the structure difference between TiAl matrix and precipitates, the stress is produced during nitrides precipitation and thus induces the generation of dislocations.

Fig. 8 TEM images showing dislocation structure in aged Ti₂AlN/TiAl composites

CHRISTOPH et al [27] investigated the deformation structure of TiAl alloys with perovskite Ti₃AlC precipitates and found that perfect and twinning partial dislocations were pinned by the precipitates. The high glide resistance provided by the precipitates is manifested by the strong bowing-out of the dislocation segment. In this work, it also can be inferred that the nitrides precipitates formed in TiAl matrix during aging treatment could act as barrier grid to inhibit the movement of dislocation in the matrix, which is beneficial to enhance the strength of the Ti₂AlN/TiAl composites.

4 Conclusions

1) The microstructure of in-situ Ti₂AlN/TiAl composites consists of γ+α₂ lamellar colonies and equiaxed γ grains and Ti₂AlN phase. With the increase of Ti₂AlN content, morphology of Ti₂AlN develops from granular type for 10 % Ti₂AlN composite to rod-like type for 20% Ti₂AlN composite and mean grain size of lamellar colonies decreases.

2) Nearly fully lamellar matrix consisted of colonies of α₂/γ lamellae and equiaxed γ was obtained by solution treatment at 1400 °C and subsequent aging treatment at 900 °C. The nearly fully lamellar structure becomes instable as Ti₂AlN content increases.

3) Two kinds of precipitates form in the aged Ti₂AlN/TiAl composite. Fine particle precipitates of Ti₂AlN of 0.4-1 μm distribute at grain boundaries of lamellar colony. Needle-like precipitates of Ti₃AlN arrange in line with growing axis paralleling to [001] direction of the γ-TiAl matrix. Another needle-like Ti₃AlN precipitates with larger size distribute at dislocations in γ matrix. Formation of nitride precipitates would happen in process of α₂→γ+precipitates.

4) Some dislocation patterns such as networks and walls were found in the matrix of aged Ti₂AlN/TiAl composites, which would be beneficial to the elevated temperature strength of Ti₂AlN/TiAl composites.

References

[1] KOTHARI K, RADHAKRISHNAN R, WERELEY N M. Advances in gamma titanium aluminides and their manufacturing techniques [J]. Progress in Aerospace Sciences, 2012, 55: 1-16.

[2] CLEMENS H, MAYER S. Design, processing, microstructure, properties, and applications of advanced intermetallic TiAl alloys [J]. Advanced Engineering Materials, 2013, 15(4): 191-215.

[3] CHEN Y Y, LI B H, KONG F T. Microstructural refinement and mechanical properties of Y-bearing TiAl alloys[J]. Journal of Alloys and Compounds, 2008, 457(1-2): 265-269.

[4] LORETTO M H, WU Z, CHU M Q, SAAGE H, HU D, ATTALLAH M M. Deformation of microstructurally refined cast Ti46Al8Nb and Ti46Al8Ta [J]. Intermetallics, 2012, 23: 1-11.

[5] VOJTECH D, POPELA T, HAMACEK J, KUTZENDORFER J. The influence of tantalum on the high temperature characteristics of lamellar gamma + alpha 2 titanium aluminide [J]. Materials Science and Engineering A, 2011, 528(29-30): 8557-8564.

[6] QIU C Z, LIU Y, HUANG L, ZHANG W, LIU B, LU B. Effect of Fe and Mo additions on microstructure and mechanical properties of TiAl intermetallics [J]. Transactions of Nonferrous Metals Society of China, 2012, 22(3): 521-527.

[7] XIANG L Y, WANG F, ZHU J F, WANG X F. Mechanical properties and microstructure of Al₂O₃/TiAl in situ composites doped with Cr₂O₃ [J]. Materials Science and Engineering A, 2011, 528(9): 3337-3341.

[8] NIU H Z, XIAO S L, KONG F T, ZHANG C J, CHEN Y Y. Microstructure characterization and mechanical properties of TiB₂/TiAl in situ composite by induction skull melting process [J]. Materials Science and Engineering A, 2012, 532: 522-527.

[9] SHU S L, QIU F, JIN S B, LU J B, JIANG Q C. Compression properties and work-hardening behavior of Ti₂AlC/TiAl composites fabricated by combustion synthesis and hot press consolidation in the Ti-Al-Nb-C system [J]. Materials & Design, 2011, 32(10): 5061-5065.

[10] KAKITSUJI A, MIYAMOTO H, MABUCHI H, TSUDA H, MORII K. Microstructure and mechanical properties of TiAl/Ti₂AlN composites prepared by combustion synthesis [J]. Materials Transactions, 2001, 42(9): 1897-1900.

[11] SHU S L, XING B, QIU F, JIN S B, JIANG Q C. Comparative study of the compression properties of TiAl matrix composites reinforced with nano-TiB₂ and nano-Ti₅Si₃ particles [J]. Materials Science and Engineering A, 2013, 560: 596-600.

[12] ZHOU Y, SUN D L, JIANG D P, HAN X L, WANG Q, WU G H. Microstructural characteristics and evolution of Ti₂AlN/TiAl composites with a network reinforcement architecture during reaction hot pressing process [J]. Materials Characterization, 2013, 80: 28-35.

[13] DUONG T, GIBBONS S, KINRA R, ARROYAVE R. Ab-initio approach to the electronic, structural, elastic, and finite-temperature thermodynamic properties of Ti₂AX (A=Al or Ga and X=C or N) [J]. Journal of Applied Physics, 2011, 110(9): 093504.

[14] RADOVIC M, BARSOUM M W. MAX phases: Bridging the gap between metals and ceramics [J]. American Ceramic Society Bulletin, 2013, 92(3): 20-27.

[15] DJEDID A, MECABIH S, ABBES O, ABBAR B. Theoretical investigations of structural, electronic and thermal properties of Ti₂AlX(X=C,N) [J]. Physica B: Condensed Matter, 2009, 404(20): 3475-3482.

[16] RAMASESHAN R, KAKITSUJI A, SESHADRI S K, NAIR N G, MABUCHI H, TSUDA H, MATSUI T, MORII K. Microstructure and some properties of TiAl-Ti₂AlC composites produced by reactive processing [J]. Intermetallics, 1999, 7(5): 571-577.

[17] MABUCHI H, TSUDA H, NAKAYAMA Y, SUKEDAI E. Processing of TiAl-Ti₂AlN composites and their compressive properties [J]. Journal of Materials Research, 1992, 7(4): 894-900.

[18] PERDRIX F, TRICHET M F, BONNENTIEN J L, CORNET M, BIGOT J. Influence of nitrogen on the microstructure and mechanical properties of Ti-48Al alloy [J]. Intermetallics, 2001, 9(2): 147-155.

[19] NAM C Y, WEE D M, WANG P, KUMAR K S. Microstructure and toughness of nitrogen-doped TiAl alloys [J]. Intermetallics, 2002, 10(2): 113-127.

[20] TANG J C, HUANG B Y, HE Y H, XIE K. Effect of nitrogen addition on microstructures ad mechanical properties of TiAl based alloys [J]. Transactions of Nonferrous Metals Society of China, 1999, 9(4): 692-695.

[21] KARADGE M, KIM Y W, GOUMA P I. Precipitation strengthening in K5-series γ-TiAl alloyed with silicon and carbon [J]. Metallurgical and Materials Transactions A, 2003, 34(10): 2129-2138.

[22] KARADGE M, KIM Y W, GOUMA P. Synergistic precipitation strengthening in TiAl alloys [J]. Applied Physics Letters, 2006, 89: 181921.

[23] TANAKA Y, SATO A, MORI T. Stress assisted nucleation of α″ precipitates in Fe-N single crystals [J]. Acta Metallurgica, 1978, 26(4): 529-540.

[24] TIEN J K, COPLEY S M. The effect of orientation and sense of applied uniaxial stress on the morphology of coherent gamma prime precipitates in stress annealed nickel-base superalloy crystals [J]. Metallurgical and Materials Transactions B, 1971, 2(2): 543-553.

[25] TIAN W H, NEMOTO M. Precipitation behavior of nitrides in L1₀-ordered TiAl [J]. Intermetallics, 2005, 13(10): 1030-1037.

[26] SHARMA G, RAMANUJAN R V, TIWARI G P. Interphase precipitation in a γ-TiAl alloy [J]. Materials Science and Engineering A, 1999, 269(1-2): 21-25.

[27] CHRISTOPH U, APPEL F, WAGNER R. Dislocation dynamics in carbon-doped titanium aluminide alloys [J]. Materials Science and Engineering A, 1997, 239-240: 39-45.

原位合成Ti₂AlN/TiAl复合材料900 °C时效过程中的组织演化及氮化物沉淀相析出

刘懿文，胡 锐，张铁邦，寇宏超，李金山

西北工业大学 凝固技术国家重点实验室，西安 710072

摘  要：对原位合成Ti₂AlN/TiAl复合材料在原位合成及时效热处理条件下的显微组织特征进行分析，并对Ti₂AlN/TiAl复合材料进行1400 °C，0.5 h固溶及900 °C，24 h时效热处理，研究其氮化物沉淀析出。结果表明，原位合成复合材料的显微组织由γ+α₂片层团、等轴γ晶粒和Ti₂AlN增强相组成。经固溶和时效处理后，获得近全片层基体结构。随着Ti₂AlN含量的增加，基体近全片层结构变得不稳定。对时效后的复合材料进行TEM研究，发现在片层团晶粒边界上分布着细小的Ti₂AlN沉淀相。在γ-TiAl基体内，针状Ti₃AlN沉淀相以其轴向平行于基体[001]方向排列，而另一种具有较大尺寸的Ti₃AlN沉淀相则在位错处沉淀析出。

关键词：TiAl复合材料；Ti₂AlN；原位合成；显微组织；沉淀相；时效处理

(Edited by Xiang-qun LI)

Foundation item: Project (2011CB605502) supported by the National Basic Research Program of China; Project (51001086) supported by the National Natural Science Foundation of China

Corresponding author: Rui HU; Tel: +86-29-88491764; Fax: +86-29-88460294; E-mail: rhu@nwpu.edu.cn

DOI: 10.1016/S1003-6326(14)63201-5