Effect of anisotropy on microstructure and high temperature stress rupture properties of Ni3Al base single crystal alloy
KONG Zhi-gang(孔志刚), HAN Ya-fang(韩雅芳), LI Shu-suo(李树索)
School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics,
Beijing 100083, China
Received 28 July 2006; accepted 15 September 2006
Abstract: The effect of anisotropy on microstructure and high temperature stress rupture property of Ni3Al base single crystal alloy was investigated. The single crystal specimens were produced by screw selection crystal method. The microstructures were examined by OM, SEM, TEM and X-ray EDS techniques. The stress rupture tests were carried out in air by constant load creep machines under 1 100 ℃ and 130 MPa. The experimental results show that the dendrites preferential orientation deviates certain angles to heat flow orientation, and the secondary arms occur for different crystallographic orientations. The single crystal alloy with different orientations shows obvious anisotropy during tensile stress rupture tests under 1 100 ℃ and 130 MPa. The <111> orientation specimen has the best stress rupture life of 211 h. The high ductility at 1 100 ℃ of the <001> orientation specimen may be attributed to the most multiple equivalent slip systems.
Key words: Ni3Al; anisotropy; single crystal superalloy; stress rupture property
1 Introduction
In order to meet the demand of higher operating temperature of advanced gas turbine hotparts, such as blades and vanes, a directionally solidified (DS) Ni3Al-base alloy IC6 was developed for advanced jet-engine blades and vanes operated in the temperature range of 1 050-1 150 ℃ [1-5]. Alloy IC6 has some merits, such as low density (7.9 g/cm3), low cost, high melting temperature (1 315 ℃) and high mechanical properties [6]. Alloy IC6 has been successfully used for turbine vanes of aero-engines[7-10]. In order to improve the high temperature creep resistance to meet the requirement of turbine blade material with the operating temperatures up to 1 100 ℃, the single crystal technique for alloy IC6 has been recently developed. Previous research works show that the mechanical behavior of nickel-base single crystal superalloys is related to their orientation[11-14]. However, the stress rupture properties of the Ni3Al base superalloys with the different orientations have been rarely researched. In this work, the effect of anisotropy on the microstructure and mechanical properties of Ni3Al base single crystal alloy is reported.
2 Experimental
The material used for the present study was a B free Ni3Al base alloy IC6, with a nominal composition of Ni-7.6Al-14Mo(mass fraction, %). The single crystal specimens were produced by screw selection crystal method with 4 mm/min drawing rate. The temperature of the mould was kept in the range of 1 540-1 560 ℃.
The microstructure and crack propagation behavior were examined in a scanning electron microscope (SEM). The backscattered electron (BSE) imaging and energy dispersive spectroscopy (EDS) were used to analyze the morphology and composition of the phases. The stress rupture tests were carried out in air by constant load creep machines under the conditions of 1 100 ℃ and 130 MPa with the specimens size of d5 mm×25 mm.
3 Results and discussion
3.1 Microstructural characterization
The dendrite structure of the alloy with the <001>, <011> and <111> orientations is shown in Fig.1. It can be seen that the second arms show typical cross-hatched structures and the first arms of the <001> orientation specimen arrange along the direction of axis, as shown in Figs.1(a) and (b). The dendrite shape and arrangement of the <011> and <111> orientation specimens are quite different from those of the <111> orientation specimen. Since the dendrites of the <011> orientation specimen have two equivalent preferential orientations. The primary dendrites can cross and grow, as shown in Figs.1(c) and (d). Figs.1(e) and (f) show that the three equivalent preferential orientations of the <111> orientation specimen have the same growth conditions and the dendrites can grow into complex dendrites structures[15]. For the <011> and <111> orientation specimens, when the dendrites preferential orientation deviates certain angles to heat flow orientation, the growth speed of secondary arms face to solid-liquid interface will be faster than that of the secondary arms back to the interface. With the increment of solidification time, the secondary arms face to solid-liquid interface grow continuously, which is disadvantageous to the growth of the back to solid-liquid interface and makes secondary arms of alloy show obvious asymmetry[16].
The primary dendrites spacing of three orientations has been determined as 0.41, 0.422 and 0.428 mm, respectively, for the <001>, <011>, <111> orientations. It has been found that the dendrites preferential orientation is consistent with the heat flow orientation for the <001> orientation specimen and the primary dendrites spacing depends on the temperature gradient and dendrites growth velocity. For the <011> and <111> orientation specimens, the dendrites preferential orientation deviates certain angles to heat flow orientation, and the primary dendrites spacing depends on the angles besides the temperature gradient and dendrites growth velocity.
Fig.1 Typical dendrite structures in different sections of alloy for three orientations: (a) Transverse section of <001> orientation; (b) Longitudinal section of <001> orientation; (c) Transverse section of <011> orientation; (d) Longitudinal section of <011> orientation; (e) Transverse section of <111> orientation; (f) Longitudinal section of <111> orientation
3.2 Stress rupture properties
The stress rupture lives of the alloy with different orientations under the testing conditions of 1 100 ℃ and 130 MPa are listed in Table 1. The results show that the <111> oriented specimen has the best stress rupture property and its tensile stress rupture life is in the range of 63 to 211 h, while the stress rupture lives of the <001> and <011> orientation specimens are less than 40 h. However the <001> orientation specimen shows better ductility compared with that of the <011> and <111> orientation specimens.
Table 1 Tensile stress rupture life with different conditions under 1 100 ℃ and 130 MPa
The different stress rupture properties may be attributed to the different numbers of most multiple equivalent slip systems. The γ′-Ni3Al phase has fcc structure and the activated octahedral {111}<110> slip systems of the <001>, <011> and <111> orientation specimens are 8, 4 and 6, respectively under 1100°C. The <001> and <011> orientation specimens have the same Schmid factor, 0.41, while that of the <111> orientation specimens is 0.27[17]. Compared with the other two orientation specimens, the <111> orientation specimen has the lowest shear stress and medium drag stress, which may be the main reason of high tensile stress rupture life.
Fig.2 shows the optical microscope image of the <111> orientation specimen creep ruptured under 1 100 ℃ and 130 MPa for 12 690 min. It can be seen that the micro-cracks appear in the interdendritic area of the fracture and link each other under stress, then combine to form a macro-crack as shown in Figs.2(a) and (b). The amount of cracks decrease and the width of cracks becomes smaller along the direction of main stress axis, as shown in Fig.2(c).
The SEM secondary electron images of the fracture surface of different orientation alloys are presented in Fig.3. The fracture surfaces of the <001> orientation specimen have many small size and homogeneously distributed dimples, as shown in Fig.3(a). The necking at fracture surface occurs and the reduction of area reaches 31.73%. Fig.3(b) shows that the dimples decrease and the creep cavities increase for the <011> orientation specimen. The bearing area at place occurring necking decreases quickly and results in fracture finally. For the <111> orientation specimen, less creep cavities form and the dendrites microstructures are observed obviously, and the cracks are arrested by the dendrites, as shown in Fig.3(c), which may be attributed to the complex dendrites structures.
Fig.2 Microcracks of <111> orientation specimen of alloy stress ruptured under 1 100 ℃ and 130 MPa for 12 690 min: (a), (b) Near fracture surface; (c) 20 mm away from fracture surface
The subgrain boundaries are usually observed in single crystal alloy, as a result of inhomogeneous distribution of composition and the small misorientation between dendrites[18]. It is found that the subgrain boundaries are parallel to the axis in the <001> orientation of present single crystal alloy, as shown in Fig.4(a), while the subgrain boundaries are more and complex and incline to the axis for the <011> and <111> orientation specimens, as shown in Figs.4(b) and (c). The existence of subgrain boundaries can reduce the tensile stress rupture life, but the effect is slighter than that of orientation [15].
Fig.3 SEM photographs of fracture surface of alloy: (a) <001> orientation; (b) <011> orientation; (c) <111> orientation
The experimental results and discussion above reveal that orientation has an effect on the stress rupture life of the alloys and the difference of stress rupture properties of different orientation may be attributed to the following factors: 1) the octahedral {111}<110> slip systems of different orientation specimens are activated;
2) the dendrites of the <111> orientation specimen can grow into complex dendrites structures, which can arrest the growth of cracks; 3) the existence of subgrain boundaries in the <011> and <111> specimen can reduce the tensile stress rupture life.
Fig.4 Back scattered electron image (BSEI) of alloy with different orientations: (a) <001> orientation; (b) <011> orientation; (c) <111> orientation
4 Conclusions
1) The first arms of the <001> orientation specimen of the present Ni3Al base single crystal alloy arrange along the direction of axis and the secondary arms of the <011> and <111> orientation specimen show obvious asymmetry, and the primary dendrites spacing of three orientations is 0.41, 0.422 and 0.428 mm, respectively.
2) The present Ni3Al base single crystal alloy with different orientations shows tensile anisotropy during the creep tests under 1 100 ℃ and 130 MPa. The main reason may be that the numbers of most multiple equivalent slip systems and the shearing stress are quite different for three orientation specimens.
3) The dendrites of the <111> orientation specimen can grow into complex dendrites structures, which can arrest the growth of cracks. The subgrain boundaries are advantages to stress rupture properties, but the effect is slighter than that of the orientation.
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(Edited by YUAN Sai-qian)
Corresponding author: HAN Ya-fang; Tel: +86-10-82314488; E-mail: zgkong@mse.buaa.edu.cn
