Trans. Nonferrous Met. Soc. China 26(2016) 423-430

Microstructure stability of Ti₂AlN/Ti-48Al-2Cr-2Nb composite at 900 °C

Yi-wen LIU, Rui HU, Tie-bang ZHANG

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

Received 10 March 2015; accepted 14 September 2015

Abstract: Microstructure stability of in situ synthesized Ti₂AlN/Ti-48Al-2Cr-2Nb composite during aging at 900 °C was investigated by XRD, OM and TEM, and the unreinforced Ti-48Al-2Cr-2Nb alloy was also examined for comparison. The result showed that in the TiAl alloy, α₂ lamellae thinned and were broken down, and became discontinuous with increasing aging time. The decomposition of α₂ lamella to g which was characterized by parallel decomposition and breakdown of α₂ lamellae led to the degradation of the lamellar structure. While in the composite, lamellar structure remained relatively stable even after aging at 900 °C for 100 h. No breakdown of α₂ lamellae except parallel decomposition and precipitation of fine nitride particles was observed. The better microstructural stability of the composite was mainly attributed to the precipitation of Ti₂AlN particles at the α₂/g interface which played an important role in retarding the coarsening of lamellar microstructure in the matrix of composite.

Key words: TiAl composite; Ti₂AlN; microstructure stability; lamellar structure; precipitation

1 Introduction

Intermetallic γ-TiAl-based alloys have been investigated extensively for many years and are considered for higher temperature applications in aerospace and automotive industries due to their attractive properties such as low density, high modulus and good oxidation and creep resistance up to 800 °C [1-4]. However, poor ductility at room temperature and relatively low strength at high temperature limit their application. Many efforts have been made to overcome these shortcomings, and composite technology is regarded as an effective method to improve both toughness and creep resistance [5]. Among various kinds of reinforcements, ternary nitride Ti₂AlN has been used as an attractive candidate for reinforcements in TiAl-based composites due to its metallic and ceramic properties, such as low density, high elastic modulus, easy machinability and high-temperature oxidation resistance [6,7]. Moreover, Ti₂AlN is comparable to TiAl in both density and thermal expansion resistance. Introduction of Ti₂AlN can refine the as-cast microstructure and improve the mechanical properties of TiAl-based alloy remarkably. More attractive thing is that Ti₂AlN/TiAl composites show higher elevated temperature strength than unreinforced TiAl alloy at temperature as high as 900 °C and present great potential in high temperature application [8]. In view of the long-term use at high temperature, the thermal stability of the Ti₂AlN/TiAl composites is necessary to be evaluated. However, few works have focused on this issue up to now.

It is well known that the mechanical properties of composites are associated with their matrix closely. According to the researches, fully lamellar structure consisting of γ and α₂ lamellae provides excellent balanced properties of yield strength, fracture toughness and creep resistance than its counterparts in TiAl alloys [9,10]. However, the lamellar structure would be unstable even at temperature as low as 700 °C due to the reduction of both the chemical free energy and the interfacial energy so that the strength and creep resistance are reduced inevitably [11,12]. In order to improve the microstructure stability, refractory metals such as W, Hf and light elements such as Si and C are added, which causes concentration of refractory metal elements and formation of fine silicide and carbide particles at the α₂/γ lamellar interfaces and stabilizes the lamellar structure [13-16]. The addition of N element is also known as an effective method to enhance creep resistance by nitride (Ti₂AlN, Ti₃AlN) particle hardening [17,18]. Nonetheless, the formation of such precipitates was found to induce coarsening of lamellae in fully lamellar TiAl alloy containing N and lead to microstructure degradation [19]. This was mainly attributed to deprivation of Ti during formation of nitrides and left Al rich in the matrix. In the Ti₂AlN/TiAl composites, Ti₂AlN reinforcement was designed with stoichiometry and synthesized in advance. It could be expected that the influence of formation of nitrides on microstructure of TiAl matrix in Ti₂AlN/TiAl composites would be different from that in N-containing TiAl alloy during aging at intermediate temperatures.

Therefore, in this work, the microstructure changes of Ti₂AlN/TiAl composite prepared by an in-situ method, reactive arc-melting, were investigated during long-term aging treatment. For the composite, Ti-48Al-2Cr-2Nb (mole fraction, %), the classic fully lamellar TiAl alloy, was chosen as the matrix. In order to get a better understanding of the microstructure changes, the Ti-48Al-2Cr-2Nb alloy was also studied for comparison.

2 Experimental

The Ti₂AlN/Ti-48Al-2Cr-2Nb (Ti₂AlN/TiAl) composite was prepared by in-situ method of reactive arc-melting and the Ti-48Al-2Cr-2Nb (TiAl) alloy was produced by arc melting technique. To improve the chemical homogeneity, the two ingots were remelted four times. The microstructure of as-cast TiAl alloy shows coarse columnar dendrites and Ti₂AlN/TiAl composite exhibits a fine near lamellar structure in the matrix and rod-like reinforcement particles dispersed in the matrix. After that, the as-cast materials were subjected to a solution treatment at 1400 °C for 10 min and furnace cooling to room temperature so as to obtain fully lamellar microstructure. Then, the treated samples were aged at 900 °C for 50 h and 100 h in air, respectively. For each aging step, the samples were air cooled to room temperature.

The phase analysis was carried out on an X-ray diffractometer (DX-2700) using Cu K_α radiation for an angle range of 20°-90° (2q), with a step of 0.02° (2q). The microstructural features were observed by optical microscope (OLYMPUS GX71) and the samples for optical metallographic observation were prepared by standard mechanically techniques and etched using Kroll’s reagent. Transmission electron microscopy (TEM) investigation was carried out on an FEI Tecnai F30 G² microscope operated at 200 kV. Thin foil specimens for TEM observation were first ground mechanically from both sides to about 50 μm and then thinned by standard double-jet electropolishing technique with an electrolyte consisting of 60% methanol, 35% butanol and 5% perchloric acid (volume fraction) operated at 35 V and -30 °C. For the evaluation of grain size and lamellar spacing of each sample, more than ten micrographs were analyzed and a mean linear intercept method was used to achieve the average value.

3 Results and discussion

3.1 As-annealed microstructures

Figure 1 shows the X-ray diffraction patterns of TiAl alloy and Ti₂AlN/TiAl composite solution-treated at 1400 °C for 10 min. According to the analysis results, γ-TiAl and α₂-Ti₃Al phases are identified in both materials. It should be noted that Ti₂AlN phase cannot be found in the composite. The main reasons are that, on one hand, a nominal Ti₂AlN content of 3% (volume fraction) is designed to introduce into TiAl matrix in this work, which is too little to be detected by XRD with limited precision; on the other hand, Ti₂AlN phase was found to be susceptible to decomposition above 1400 °C [20].

Fig. 1 XRD patterns of TiAl alloy (a) and Ti₂AlN/TiAl composite (b) solution-treated at 1400 °C for 10 min

The microstructures of the solution-treated materials are shown in Fig. 2. The TiAl alloy displays a coarse fully lamellar microstructure consisting of α₂+γ lamellar colonies with an average grain size of 1026 μm (Fig. 2(a)) and well interlocked grain boundaries which are caused by incursion of lamellae into the neighboring grains. By contrast, the Ti₂AlN/TiAl composite shows a refined microstructure with a grain size about 486 μm (Fig. 2(b)) and much finer and shorter lamellar incursions of the grain boundaries. The refined microstructure was inherited from the as-cast one, which was refined due to the introduction of Ti₂AlN particles. They acted as nucleation cores during melting and solidification.

3.2 Microstructure evolution during aging at 900 °C

The X-ray diffraction patterns of TiAl alloy and Ti₂AlN/TiAl composite aged at 900 °C for different time are shown in Fig. 3. Both γ and α₂ phases are detected in TiAl alloy after aging for 50 h, while only γ phase remains as time extends to 100 h (Fig. 3(a)). In the aged composite, α₂ phase is still remained even after 100 h aging, and some diffraction peaks of Ti₂AlN phase can be confirmed (Fig. 3(b)).

The microstructures of TiAl alloy aged at 900 °C for different time are presented in Fig. 4, and some noticeable changes can be observed. After aging for 50 h, the perfectly aligned lamellar structure is disturbed. Some α₂ lamellae underwent decomposition and transformed into γ phase. In consequence, thin γ lamellae laid in the remaining α₂ lamellae and fine alternating arrangement of γ and α₂ laths was formed, which is called “parallel decomposition”. The generated α₂/γ interfaces are parallel to the original interfaces (Fig. 4(a)). In addition, boundary migration is observed. In Fig. 4(b), the α₂/γ interface is bulged and the α₂ lamella is even broken up, which leads to coalescence of the γ lamellae. The interface bulging was believed to only present at the 120° rotational fault interfaces [21]. The lamellae migration caused the coarsening of the γ lamellae. In these two cases, the α₂/γ lamellar interface keeps straight basically. Nevertheless, remarkable microstructure degradation appears at the interface, as displayed in  Fig. 4(c). The interface between γ and α₂ lamellae is not straight anymore and the γ lamellae become coarse. Many dislocations are observed in the γ lamellae and at the α₂/γ interface even without applying stress. The generation of these dislocations is caused by loss of α₂ phase which transforms into γ phase. It is known that the Shockley partial associated with the transformation of α₂ interacts with the Frank partial associated with interface misorientation on the interface plane and gives rise to ordinary hard-mode dislocations within the γ matrix via reactions [22], i.e.,

                     (1)

As aging time increases up to 100 h, α₂ lamella is found to be discontinuous (Fig. 4(d)). When the α₂ lath between two γ laths of the same variant completely disappears, the two γ laths coalesce to form a wider one. As indicated in Fig. 3(a), the content of α₂ phase decreases dramatically with aging time prolonging, suggesting that α₂ phase has undergone the transformation. In comparison with Fig. 4(c), the dislocation density decreases. This might be because prolonged aging leads to annihilation of matrix dislocations or their arrangement into low energy configurations, thus reducing the net mobile density. Further observation shows that the α₂ lamella is decomposed by parallel decomposition and fine γ and α₂ lamellae are formed (Fig. 4(e)).

Fig. 2 Optical images showing microstructures of TiAl alloy (a) and Ti₂AlN/TiAl composite (b) solution-treated at 1400 °C for 10 min and furnace cooled

Fig. 3 X-ray diffraction patterns of TiAl alloy (a) and Ti₂AlN/TiAl composite (b) aged at 900 °C for different time

Fig. 4 TEM images showing parallel decomposition (a), breakdown of α₂ lamellae (b), dislocation in γ lamellae and at α₂/γ  interface (c) after aging for 50 h, and discontinuous α₂ lamellae (d) and parallel decomposition of α₂ lamellae (e) after aging for 100 h in TiAl alloy

It is well known that during aging α₂ phase will transform into γ phase due to the non-equilibrium phase constitution, which provides one of the driving forces to reach equilibrium conditions, thus results in structural changes upon exposure at elevated temperatures [10,23]. As revealed in this work, the α₂ lamella is decomposed by the phase transformation of α₂→γ during aging, and two modes are involved. One is characterized by parallel decomposition of coarse α₂ lamellae into fine γ and α₂ lamellae, which leads to the microstructure degradation such as thinning of α₂ lamellae and coarsening of neighboring lamellae. The other is characterized by breakdown of α₂ lamella occurred by nucleation of all possible orientations of γ (e.g., twinned or rotated by 120°) within the α₂ phase and by growth of the original lamellae from either side of the α₂ phase to produce regions where no α₂ phase remains [24]. Along with the decomposition processes in TiAl alloys, α₂ phase is dissolved and thus results in disappearance of α₂ lamella and coarsening of the γ lamella. The coarsening of the lamellar structure will degrade the creep properties by increasing the creep strain rates. In addition, the density of α₂/γ lamellar boundaries is reduced due to the dissolution of α₂ lamella, which leads to the reduction in the channeling stress and the increase in the mean free path of mobile dislocations, then associated increase in creep rates [12].

In the Ti₂AlN/TiAl composite, slight parallel decomposition of α₂ lamella can be observed after aging for 50 h, as shown in Fig. 5(a). Thin α₂ laths are separated by ultrathin γ laths. The white rod-like precipitates with an average size of 0.32 μm in length and 0.14 μm in width are assumed to be Ti₂AlN phase. These precipitates tend to stay at α₂/γ interfaces and their long-axis is parallel to the parent α₂ lamella, suggesting that the precipitates may nucleate at α₂/γ interfaces. Although parallel decomposition and precipitation occur, the lamellar interface is basically kept in a straight way. The aged composite exhibits finer lamellar structure with an average lamellar spacing of 0.16 μm than the aged TiAl alloy with 0.84 μm. For longer aging time of 100 h (Fig. 5(b)), the parallel decomposition of α₂ into fine α₂ and γ lamellae becomes more common, whereas more α₂ laths maintain a relative stability. It is interesting to note that the short rod-like Ti₂AlN particle precipitated at the α₂/γ interface is developed into a slender one with a size of 1.37 μm in length and 0.06 μm in width and its growth direction is parallel to the lamellar interface. Also, many nano-sized precipitates are found at the interface.

Compared with the counterpart of TiAl alloy, the composite possesses better lamellar structure stability during aging at 900 °C for different time. No breakdown of α₂ lamellae has been found except for some parallel decomposition and precipitation of Ti₂AlN. The Ti₂AlN particles are generally adjacent to α₂ lamellae, indicating that the decomposition of α₂ phase and the nucleation of the Ti₂AlN phase are related. The TEM observations indicate that secondary fine Ti₂AlN particles with nano-size dispersively precipitate at primary α₂/γ interface in the aged Ti₂AlN/TiAl composite. The process can be illustrated as α₂→γ+Ti₂AlN. Thus, two types of phase transformation get involved in the process of α₂ decomposition, α₂→γ and α₂→γ+Ti₂AlN.

Fig. 5 TEM images showing lamellar microstructures of Ti₂AlN/TiAl composite aged at 900 °C for 50 h (a) and 100 h (b)

3.3 Precipitation in aged composite

According to TEM examination, many precipitates with different morphologies are revealed in the aged Ti₂AlN/TiAl composite, as shown in Fig. 6. The rod-like precipitate forms at the α₂/γ interface after aging at  900 °C for 50 h (Fig. 6(a)). According to the electron diffraction analysis, the interfacial precipitate with an ordered hexagonal structure is identified as Ti₂AlN phase, which lies parallel to the lamellae. In addition, another nano-sized precipitate is found to distribute continuously along lamellar interfaces in the composite (Fig. 6(b)). The corresponding selected area electron diffraction pattern clearly illustrates that the nano-sized precipitate is Ti₂AlN phase with crystallographic orientation relationship of  and  . As the aging time increases to 100 h, the rod-like precipitate becomes elongated and its growth orientation keeps the same relationship with the lamellae, and the nano-sized precipitate tends to arrange in line (Fig. 6(c)). The other needle-like precipitate with low content is found to arrange along a particular direction in equiaxed γ matrix (Fig. 6(d)). This kind of precipitate prefers forming at the dislocations. By electron diffraction analysis, it is identified as Ti₃AlN phase. The growing axis of the needle-like precipitates is parallel to [001] direction of the matrix.

Fig. 6 TEM images showing Ti₂AlN precipitates at α₂/γ interfaces (a, b, c) and Ti₃AlN precipitates in γ matrix (d)

The formation and morphology of different types of precipitates are considered to be closely related to crystal structure, misfit, structure of the interphase boundary between precipitate and matrix and stress applied during aging treatment [25,26]. Ti₃AlN precipitate shows preferential orientation in [001] direction of the γ matrix due to minor lattice misfit between them in this direction [27]. Therefore, it often forms in γ matrix. As for Ti₂AlN precipitates, they prefer presenting along α₂/γ lamellar interfaces, rather than in the γ lamella. This may be associated with the decomposition of α₂ phase. During in-situ synthesis of Ti₂AlN/TiAl composite, a trace of nitrogen was inevitably dissolved into the TiAl matrix. Moreover, the in-situ Ti₂AlN reinforcements tended to be decomposed at 1400 °C and additional nitrogen was introduced. The two factors caused excess concentration of the element in the TiAl matrix, especially in α₂ phase. It is known that nitrogen has a higher solubility in α₂ phase than in γ phase [28]. During aging at 900 °C, nitrogen atom is likely to be segregated at α₂/γ interfaces because α₂ phase is supposed to approach an equilibrium condition. Thus, nitride precipitates would nucleate at the α₂/γ interfaces and subsequently grow along the interfaces. Although decomposition of α₂ phase occurs, the composite contains certain amount of α₂ phase and the α₂+γ lamellar structure is still remained even after aging for 100 h, suggesting that the dissolution of α₂ phase is retarded by the precipitation processes in the composite. This could be attributed to the fact that nitrogen atoms segregated at α₂/γ interfaces may lead to the decrease of the number of faults in the lamellar structure. The density of faults present in the lamellar structure influences the rate of mass transport associated with a unit volume of the lamellar structure [29]. Meanwhile, nitrides precipitated in lamellar region would retard initial critical processes that lead to instability.

It is known that stabilized microstructures with precipitates decorated on or parallel to the lamellar interfaces are identified to hold the promise for high temperature applications [22]. In the Ti₂AlN/TiAl composite, formation of nitride precipitates at the α₂/γ interfaces is expected to effectively block the emission and motion of dislocations at and along the lamellar interfaces and is beneficial to reducing creep rates.

4 Conclusions

1) Fully lamellar microstructure consisting of γ and α₂ phases was obtained for TiAl alloy and Ti₂AlN/TiAl composite after solution-treated at 1400 °C for 10 min. Aging at 900 °C resulted in noticeable microstructure changes for TiAl alloy. Parallel decomposition and breakdown of α₂ lamellae were introduced. As aging time increased, α₂ lamellae became discontinuous and γ coalesced to form a wider lath. The α₂ lamella was decomposed by the phase transformation from α₂ to γ.

2) The Ti₂AlN/TiAl composite displayed a relatively stable lamellar microstructure during aging process. Parallel decomposition and nitride precipitation were the two major decomposition processes of the α₂ lamellae. The former was characterized by phase transformation of α₂ to γ and became more common with increasing aging time, while the latter was represented by precipitation of rod-like or nano-sized Ti₂AlN particles at α₂/γ interface, which were elongated or arranged in line at the interface for longer aging time. The need-like Ti₃AlN particles mainly formed in the equiaxed γ matrix. The precipitation of fine Ti₂AlN particles at α₂/γ interface would retard the degradation of lamellar structure of Ti₂AlN/TiAl composite and was beneficial to microstructure stability of the composite.

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Ti₂AlN/Ti-48Al-2Cr-2Nb复合材料在900 °C的组织稳定性

刘懿文，胡 锐，张铁邦

西北工业大学 凝固技术国家重点实验室，西安 710072

摘  要：采用XRD、OM和TEM等方法对原位合成Ti₂AlN/Ti-48Al-2Cr-2Nb复合材料在900 °C时效过程中的组织稳定性进行研究，并与未增强的Ti-48Al-2Cr-2Nb合金进行对比分析。研究结果发现，在TiAl合金中，α₂片层变细并发生破碎，且随着时效时间的延长变得不连续。以α₂板条平行分解和破碎为特征的α₂片层向g 的分解导致片层结构退化。而在复合材料中，在900 °C时效100 h，片层结构保持相对稳定。除了片层平行分解和细小的氮化物沉淀外，没有发现α₂片层破碎。复合材料较好的组织稳定性主要与α₂/γ界面上的Ti₂AlN颗粒沉淀有关。Ti₂AlN相的析出对延缓复合材料基体片层组织粗化具有重要作用。

关键词：TiAl复合材料；Ti₂AlN；组织稳定性；片层结构；沉淀

(Edited by Xiang-qun LI)

Foundation item: Project (2011CB605502) supported by the National Basic Research Program of China; Project (B08040) supported by Introducing Talents of Discipline to Universities, China

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

DOI: 10.1016/S1003-6326(16)64095-5