Intermetallic γ-tial based alloys have received tremendous

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1 Influence of columnar grain boundary on fracture behavior of as-cast fully lamellar Ti-46Al-0.5W-0.5Si alloy Chen Hui 1,2, *Su Yanqing 1, Luo Liangshun 1, Li Xinzhong 1, Liu Jiangping 1, Guo Jingjie 1, and Fu Hengzhi 1 (1. School of Material Science and Engineering, Harbin Institute of Technology, Harbin , China; 2. College of Electromechanical Engineering, Northeast Forestry University, Harbin , China) Abstract: The fracture behavior of fully lamellar γ-tial alloys depends on the angle between the lamellar orientation and loading axis, but the role of the presentation of grain boundary cannot be ignored. To investigate the influence of the grain boundary on the initiation and propagation of cracks, the tensile test of the alloy was conducted at room temperature with loading axis parallel and perpendicular to the lamellar orientation, respectively. The cracks adjacent to the fracture zone of the tensile specimens have been investigated to analyze the fracture behavior. Results show that the grain boundary has dual influences on the fracture behavior. When the loading axis is parallel to the lamellar orientation, cracks are preferentially initiated at and propagate along the grain boundaries. When the loading axis is perpendicular to the lamellar orientation, the grain boundaries can prevent the propagation of cracks from running across. Additionally, serrated-shape grain boundaries have a better inhibiting effect on the propagation of cracks than planar boundaries. Key words: γ-tial alloy; full lamellar; grain boundary; fracture behavior CLC numbers: TG Document code: A Article ID: (2012) Intermetallic γ-tial based alloys have received tremendous attention due to their low density, high temperature strength, good creep and good oxidation resistance. These alloys have been considered as attractive candidate materials for high temperature and high-performance structural applications in the aerospace and automotive industries [1-6]. Four typical microstructures of γ-tial based alloys are fully lamellar, near lamellar, near γ phases, and duplex. Of those microstructures, the fully lamellar microstructure consisting of γ-phase (TiAl) and α 2 - phase (Ti 3 Al) displays a good combination of room temperature toughness and elevated temperature strength [7, 8]. Recent research has shown that γ-tial-based alloys with aligned lamellar orientation have a good combination of strength and ductility over a wide range of temperature [9]. Recently, a great number of investigations on mechanical properties and fracture mechanism of γ-tial alloys have been *Su Yanqing Male, Ph.D, Professor. His research interests mainly focus on metal solidification technology and new materials. He has undertaken 20 investigation projects, published more than 100 SCI articles and holds 10 patents by now. He was selected as one of the New Century Excellent Talents of the Ministry of Education of China in 2005 and honored the Science and Technology Advancement Award (the first prize) by Heilongjiang Province in suyq@hit.edu.cn The first author Chen Hui is one of the doctoral candidates supervised by him. chenhui_hit@163.com Received: ; Accepted: carried out and a series of results have been achieved. According to the literature [10-12], the lamellar interface is the weakest link in the near fully lamellar γ-tial alloys. Their properties are largely dependent on the angle between the lamellar orientation and loading axis. The aforementioned research mainly focused on the angle between the loading axis and the lamellar orientation, but, the influence of the presentation of the grain boundary on the mechanical properties was neglected. In the present work, the fully lamellar Ti-46Al-0.5W-0.5Si alloy, being composed of columnar grains, has been prepared. Dedicated experiments on the fracture behavior of this alloy have been performed, and the influence of the columnar grain boundary on the fracture behavior has been discussed. 1 Experimental procedure The γ-tial based alloy with a nominal composition of Ti- 46Al-0.5W-0.5Si (at.%) was prepared by using titanium sponge, high purity aluminum (99.99wt.%), silicon (99.9wt.%) and Al-W(55.92wt.%) master alloy. The alloy was melted in a water cooled copper crucible vacuum induction skull melting (ISM) furnace which has a capacity of 30 kg. The molten alloy was poured into a metal mould and cast into a cylindrical ingot with a diameter of Φ112 mm and a height of 220 mm. The macrostructure of the alloy consists of columnar grains with a colony width of 100 to 1,000 µm growing from the surface towards the central part of the ingot; most of the columnar grains are nearly in the same orientation as those in directionally 64

2 February 2012 Research & Development solidified crystals. The schematic illustration is shown in Fig. 1. Within the columnar grains, the microstructure consists of fully lamellar α 2 and γ phases, as can be seen in Fig. 1, the lamellar orientation is approximately perpendicular to the growth direction of the columnar grains. Growth direction Fig. 1: Schematic illustration of macrostructure of as-cast ingot; Microstructure of as-cast ingot Two types of tensile specimens were cut from the as-cast ingot, perpendicular to (Type I specimen) and parallel to (Type II specimen) the axial direction of the longitudinal section of the ingot, as shown in Figs. 2 and. During the tensile tests, for Type I specimen the loading axis was parallel to the growth direction of the columnar grains, and for Type II specimen, the loading axis was perpendicular to the growth direction of the columnar grains. Since the lamellar orientation in this alloy is approximately perpendicular to the growth direction of the Fig. 2: Cutting of tensile specimens: Fractured tensile specimen of Type I is perpendicular to the axial direction of the longitudinal section of the ingot; Fractured tensile specimen of Type II is parallel to the axial direction of the longitudinal section of the ingot columnar grains, the loading axis was perpendicular to lamellar orientation for Type I specimen; and the loading axis was parallel to lamellar orientation for Type II specimen. All specimens were polished before testing. The tensile tests were conducted at room temperature using an INSTRON 5500R testing machine. The strain rate was s -1. In order to analyze the initiation and propagation of tensile cracks, attention was focused on the region adjacent to the fracture zone in the tensile specimens, where micro-crack observations were carried out by means of backscatter electron (BSE) analysis. 2 Results 2.1 Loading axis perpendicular to lamellar orientation Figure 3 shows the crack propagation of Type I tensile specimen, in which the loading axis is perpendicular to the (c) (d) Fig. 3: BSE micrographs showing crack propagation of Type I tensile sample, (d) is the magnification of the rectangular region in (c) 65

3 lamellar orientation. Cracks are preferentially initiated at the α 2 /γ lamellar interface, where there are segregations or clusters of Ti 5 Si 3. Then, the cracks propagate preferentially along the lamellar interfaces, and finally reach the grain boundaries. The propagation of a crack initiated in Grain 1 is restrained at the grain boundary, as can be seen in Fig. 3. In this case, the grain boundary can offer resistance to crack propagation. Figure 3 shows that cracks can also go through the grain boundary and change direction along the certain lamellar interfaces with the adjacent grain, with the experiment proceeding. Comparing Figs. 3 and, the grain boundary in Fig. 3 is planar, and the grain boundary in Fig. 3 exhibits a serrated-shape. This indicates that the serrated-shape boundary exhibits a better effect than planar boundaries on restraining the propagation of the crack. As shown in Figs. 3(c) and (d), several new micro-cracks may also nucleate at the grain boundary and advance forward into the adjacent grain. It is observed that multiple microcracks preferentially nucleate at the grain boundary, and form bridge ligaments that will subsequently rupture with accompanying plastic deformation [13]. The micro-cracks will connect with each other and propagate further by translamellar cracks. In the middle of Fig. 3(d), letter A to D represents cracks. Cracks C and D can be observed just alongside the cracks A and B, respectively, which propagate along the lamellar interface. They tend to connect together in a trans-lamellar mode. 2.2 Loading axis parallel to lamellar orientation Figure 4 shows the crack propagation of Type II tensile specimens, in which the loading axis is parallel to the lamellar orientation. Micro-cracks may initiate at the clusters of Ti 5 Si 3 in the columnar grain, as shown in Fig. 4, several microcracks nucleate and link together. However, in a trans-lamellar mode, cracks propagate with difficulty along the α 2 /γ interface because of the unfavorable orientation between tensile stress and lamellar orientation [14, 15]. Micro-cracks are more likely to initiate at the columnar grain boundaries, from γ particles or the clusters of Ti 5 Si 3, as shown in Fig. 4. Therefore, the columnar grain boundary can be considered as the location where micro-cracks initiate and propagate along. (c) (d) Fig. 4: BSE micrographs showing crack propagation of Type II tensile sample The grain boundary has a serrated-shape in Fig. 4(c) and a planar-shape in Fig. 4(d). Figure 4(c) shows that several microcracks tend to link up together along the grain boundaries if the applied load increases. The crack along the grain boundary consists of many micro-cracks of various sizes and shapes. Straight cracks propagating along a planar grain boundary are shown in Fig. 4(d). Comparing the morphology of these two grain boundaries, the serrated-shape boundary exhibits a better effect than the planar boundary in restraining the propagation of the crack. 3 Discussion Figure 5 shows the process of crack initiation and propagation in Type I sample. For this type of specimen with lamellar planes perpendicular to the loading axis, cracks easily initiate 66

4 February 2012 Research & Development Fig. 5: Schematic illustration of the crack initiation and propagation in Type I sample under increasing loads from to (d) with loading axis perpendicular to the lamellar orientation and propagate along the lamellar interface, which is considered as the weakest juncture, as shown in Grain 1 of Fig. 5. The columnar grain boundaries can offer resistance to crack growth. When larger tensile stress is applied, micro-cracks can grow along the lamellar interface in the adjacent Grain 2 of Fig. 5, in which lamellar interfaces have an angle to the direction of propagation. The influence of morphology of the grain boundary on fracture behavior of γ-tial alloys has received attention. When an advancing crack gets to a planar grain boundary, it will cross the boundary easily and propagate along certain lamellar interfaces of the adjacent grain, as shown in the grain boundary of Grain 1 and Grain 2 in Fig. 5. When an advancing crack meets a grain boundary with a serrated shape, a well-interlocked morphology, it will be restrained in front of the boundary. If increasing tensile stress, a new microcrack will nucleate in the adjacent columnar grain, as shown in Grain 3 of Fig. 5(c). This micro-crack and the main crack are unconnected, resulting in a zone called share ligament [16]. The crack deflection and the non-coplanar micro-cracks ahead of the crack tip will lead to an increasing fracture resistance. The internal stresses make micro-cracks nucleate and connect together along the boundary of Grain 2 and Grain 3 as shown in Fig. 5(d). The share ligaments in the serreted-shape grain boundary are torn during crack propagating, which require larger tensile stress than a planar one. Figure 6 illustrates the process of crack initiation and propagation of Type II specimen. For this type of specimen with lamellar planes parallel to the loading axis, cracks initiate and propagate in a trans-lamellar mode. They preferentially initiate at the grain boundaries, which can be considered as the weaker juncture. The micro-crack can propagate along the grain boundary when larger tensile stress is applied. Fig. 6: Schematic illustration of the crack initiation and propagation in a Type II sample under increasing loads from to (c) with the loading axis parallel to the lamellar orientation If there is a planar grain boundary in front of the tip of a crack, it is easy for cracks to get along it, as shown at the grain boundary of Grain 1 and Grain 2 in Fig. 6. If there is a grain boundary with a serrated shape or particles, it is difficult for cracks to cross this boundary, as shown at the grain boundary of Grain 2 and Grain 3 in Fig. 6. Many micro-cracks will connect together under increasing tensile stress. In summary, either for Type I or Type II specimen, the resistance provided by a serrated-shape grain boundary is higher than that of a planar grain boundary. 4 Conclusions Tensile tests on as-cast fully lamellar γ-based Ti-46Al-0.5W- 0.5Si alloy have been performed at room temperature, with the 67

5 loading axis parallel and perpendicular to the growth direction of the columnar grain, respectively. (1) The as-cast microstructure of fully lamellar γ-tial based Ti-46Al-0.5W-0.5Si alloy consists of aligned columnar grains, which composed of α 2 /γ lamellar. The lamellar in the ingot has a similar orientation, which is perpendicular to the growth direction of the columnar grain. (2) When the loading axis is perpendicular to the lamellar orientation, cracks easily initiate and propagate along the lamellar interface, which is considered as the weakest juncture. The columnar grain boundaries can offer resistance to crack growth. Through changing direction or connecting with several micro-cracks in a trans-lamellar mode, cracks can grow along the lamellar interface in the adjacent grain with the increase of applied load. (3) When the loading axis is parallel to the lamellar orientation, the stress concentration will make micro-cracks nucleate at the grain boundaries, which are considered as the weaker juncture. There is a zone of plastic deformation in front of the crack tips; with the increase of applied load, microcracks propagate along the grain boundary. (4) No matter the loading axis is perpendicular or parallel to the lamellar orientation, the serrated-shape grain boundaries have a better effect than planar boundaries on preventing the cracks from running across the grain boundaries. References [1] Yamaguchi M, Inui H, and Ito K. High-temperature structural inter-metallics. Acta Materialia, 2000, 48(1): [2] Nan Hai, Huang Dong, Li Zhenxi, and Zhao Jiaqi. Research on investment casting of TiAl alloy agitator treated by HIP and HT. China Foundry, 2007, 4(2): [3] Kuang J P, Harding R A, and Campbell J. Microstructures and properties of investment castings of γ-titanium aluminide. Materials Science and Engineering A, 2002, 329: [4] Chen Yanfei, Xiao Shulong, Tian Jing, et al. Improvement in collapsibility of ZrO 2 ceramic mould for investment casting of TiAl alloys. China Foundry, 2011, 8(1): [5] Lu X, He X B, Zhang B, et al. High-temperature oxidation behavior of TiAl-based alloys fabricated by spark plasma sintering. Journal of Alloys and Compounds, 2009, 478: [6] Lu M, Barrett J R, and Kelly T J. Investment casting of gamma titanium-aluminides for aircraft engine applications. In: Proceedings of the Third International Symposium on Structural Inter-metallics (ISSI-3). TMS, 2001: [7] Yang Fei, Kong Fantao, Chen Yuyong, et al. Effect of V and Nb additions on microstructure, properties, and deformability of Ti- 45Al-9 (V, Nb, Y) alloy. China Foundry, 2010, 7(4): [8] Chen Y Y, Xiao S L, and Kong F T. Microstructure and interface reaction of investment casting TiAl alloys. Transactions of Nonferrous Metals Society of China, 2006, 16: [9] Johnson D R, Inui H, and Yamaguchi M. Directional solidification and microstructural control of the TiAl/Ti 3 Al lamellar microstructure in TiAl-Si alloys. Acta Materialia, 1996, 44: [10] Pus Z J, Wua K H, Shib J, and Zoub D. Effect of notches and microstructure on the fracture toughness of Ti-based alloys. Materials Science and Engineering A, 1995, 192/193: [11] Cao R, Li L, and Chen J H. Study on compression deformation, damage and fracture behavior of TiAl alloys: Part II. Fracture behavior. Materials Science and Engineering A, 2010, 527: [12] Cao R, Li L, Chen J H, and Zhang J. Study on compression deformation, damage and fracture behavior of TiAl alloys: Part I. Deformation and damage behavior. Materials Science and Engineering A, 2010, 527: [13] Chen J H, Cao R, and Wang G Z. Study on notch fracture of TiAl alloys at room temperature. Metallurgical and Materials Transactions A, 2004, 35A: [14] Wang P, Bhate N, Chan K S, and Kumar K S. Colony boundary resistance to crack propagation in lamellar Ti-46Al. Acta Materialia, 2003, 51: [15] Chen J H, Cao R, Zhang J, and Wang G Z. Fracture behavior of notched specimens of TiAl alloys. Mater Sci., 2007, 42: [16] Arata J J M, Kumar K S, Curtin W A, and Needleman A. Crack growth across colony boundaries in binary lamellar TiAl. Materials Science and Engineering A, 2002, : The work was financially supported by the National Natural Science Foundation of China ( , ), the National Basic Research Program of China (2011CB610406), the China Postdoctoral Science Foundation ( , ), the Fundamental Research Funds for the Central Universities (HIT. BRET ), and the Scientific and Technological Project in Heilongjiang Province (GZ09A206). 68