Morphological evolution of γ′ phase in K465 superalloy during thermal fatigue
Yang Jin-xia(杨金侠)1,2, Zheng Qi(郑 启)1, Sun Xiao-feng(孙晓峰)1,
Guan Heng-rong(管恒荣)1, Hu Zhuang-qi(胡壮麒)1
1.Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China;
2. School of Graduate, Chinese Academy of Sciences, Beijing 100039, China
Received 28 July 2006; accepted 15 September 2006
Abstract: The alternative heating/cooling cycles（thermal fatigue）of K465 superalloy were carried out. The specimens were held at 1 050 ℃ for 300 s, then quenched into 20 ℃ recycling water for 10 s as a cycle. During thermal fatigue, γ′ precipitates changed typically from cubical to irregular shape after 10 cycles, to complex configuration after 20 cycles and raft-like shape after 30 cycles. The very fine γ′ particles precipitated inter the original γ′ particles. The elastic energy dominated morphological evolution of large γ′ precipitates, and the thermal stress induced the directional growth of precipitates that minimized the total energy of the system, and the nucleation theory controlled the formation of fine γ′ precipitate. The results show that the volume fraction of γ′ precipitates is increased with the increase of heating/cooling cycles, which improves the mechanical property of this alloy.
Key words: superalloy; thermal fatigue; γ′ phase; mechanical property
1 Introduction
K465 alloy representing excellent high temperature strength[1-3] will be used for gas turbine blades operated in high temperature environment.
As the principal strengthening constituent in this alloy, the intermetallic Ni3Al precipitate, γ′ phase, is of crucial importance for the reliability of high-temperature structural materials. There were a lot of publications on the morphological instability of γ′ in superalloy. It should be mentioned that octet splitting structure[4,5] and raft[6-10] were observed in the thermal exposure, heat-treatment, creep and mechanical fatigue tests, but these structures differentiated from the above two structures of morphological instability because they were the equilibrium shapes determined by the minimization condition of total energy, whereas the dendrite and flower-like structures of morphological instability were the growth shape determined by the fast growth condition, namely enough supersaturation in g-matrix[6]. However, YANG et al[3,6] reported that the fine γ′ precipitate showed the high morphological stability during water-quenching.
Therefore knowledge about the change in γ′ phase in thermal fatigue (the alternate heating/cooling cycle) conditions including its amount, number, size, shape, distribution and chemical composition, etc, would be valuable for application of this alloy.
2 Experimental
The nominal chemical composition(mass fraction, %) of K465 alloy is listed in Table 1. The tested specimens with the solution treatment at 1 210 ℃ for
4 h and the following air cooling were machined according to the size and shape in Fig.1. The thermal fatigue cycles were carried out in a device including an electric resistance furnace, water-quenching unit and an automatic transfer system. The specimens were heated at 1 050 ℃ for 300 s in air, then were quenched in 20 ℃ recycling water for 10 s as a cycle.
The samples for scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observation were sectioned from the same location marked by A in Fig.1. The samples for SEM observation were ground, polished and etched. Thin foils for TEM observation were electropolished in a solution containing 7%(volume fraction) perchloric acid in methanol at -20 ℃. The blade-like specimens whose surfaces were machined for room temperature mechanical properties were sectioned according to the location and shape marked by B in Fig.1 after thermal fatigue.
Table 1 Chemical composition of alloy(mass fraction, %)
Fig.1 Specimen size and shape
3 Results and discussion
3.1 Morphological evolution of γ′ phase
The γ′ precipitates in the solid solution condition are well coherent with γ-matrix showing typical cuboidal form at dendritic arm (Fig.2(a)).
The samples subjected to 10 cycles show the irregular shape of γ′ precipitates instead of cuboidal form on the solid solution condition (Fig.2(b)). These original large γ′ particles suffering from heating and cooling shocks decrease in the number and increase slightly in the size in contrast to those under the solid solution condition. The very fine cuboidal γ′ precipitates appear regularly in γ-matrix (Fig.2(b)-insert). The morphology of large γ′ precipitates after 20 cycles exhibits the more complex shape and inhomogeneous distribution(Fig.2(c)), and the cuboidal fine γ′ precipitated in γ-matrix (Fig.2(c)- insert).
Compared with the samples of 20 cycles, the morphology of original γ′ precipitates after 30 cycles is similar to the raft-like shape (Fig.2(d)), and the adjacent γ′ particles meet and link together showing the raft in the morphology(Fig.2(d)-insert), and the fine γ′ precipitates are still cuboidal without any change except for the increase of the content (Table 2).
The morphological change could be understood in term of the competition between the elastic energy of single particle and interfacial energy. At the small particle size, the interfacial energy dominates the shape of the particle whereas elastic energy is dominant for the large size of the particle that would reflect the shape with minimized elastic energy. Since the elastic stress could break the symmetry of the particle shape, the symmetry-breaking shape transition could occur with increasing the particle size[10].
Fig.2 Morphologies of γ′ phase(insert-fine γ′ in γ-matrix): (a) Under solution condition; (b) At 10 cycles; (c) At 20 cycles; (d) At 30 cycles
Table 2 Volume fractions of g? phase at different cycles
According to the above theory, the growth of large γ′ precipitate could be explained. Fig.3(a) shows the extension of the corner of the cuboids. As the extension is increased in size, the elastic anisotropy of the system controls the γ′ coarsening, and thermal stress causes the large γ′ precipitates to grow along certain direction that minimizes the total energy of the system so that these large γ′ particles either aggregate together or cluster side by side showing the raft-like microstructure in the local region. However, the raft is not perfect because of misoriention of thermal stress.
At the same time, alternate water-cooling obviously increases the morphological instability so that the borders of large γ′ precipitates become irregular in the initial stage, and become complex in configuration at 20 cycles.
In this study, the large γ′ particles finally show the rafting trend that extends along certain direction (Fig.3(b)). This proves that the most important factor controlling morphological evolution of large γ′ particles is still the elastic energy. In the meantime, the efficiency of matrix dislocation captured by the γ′/γ interface is extremely high, so that dislocation network is formed (Fig.3(b)). Based on phase equilibrium law, the amount of fine γ′ particles should be increased with the reduction of large γ′ precipitates. But under the nonequilibrium state, this could be explained by nucleation theory. Under large undercooling rate, the driving force for the nucleation is expected to be very high. As a result, the nucleation rate of γ′ precipitates per unit volume is increased. In addition, the fine γ′ precipitates are completely dissolved, and the large γ′ particles are partly dissolved in heating stage (at 1 050 ℃ for 300 s every time), which provides the precondition for the nucleation of γ′ precipitate. Consequently, the larger amount of fine γ′ particles precipitate rapidly during the following cycling. Table 2 shows that the number of fine γ′ precipitates and the amount of total γ′ precipitates are increased with the increase of cycles.
3.2 Room temperature mechanical properties of cycled specimens
Table 3 shows that the strengths of the cycled specimens are enhanced, but the elongation decreases slightly with the increase of cycles before 20 cycles. The improvement of alloy strengths results from the increase of the volume fraction of γ′ precipitates and the precipitation of fine γ′ precipitates. And the large and fine γ′ precipitates produce the composite strengthening for the tested alloy, since they coordinate to the sliding of grain boundary and the uniform deformation of grains with the different sizes, which is advantageous to the improvement of mechanical properties of this alloy.
Fig.3 Extension of g? at 10 cycles(a) and g? morphology and dislocation(b)
Table 3 Room temperature tensile properties after different cycles
However, the alternate heating and cooling produce the larger thermal stress that causes the non-uniform deformation in the microstructure of this alloy, and induces the decrease of the strength and elongation after 20 cycles. Furthermore, the larger γ′ precipitates are much pronounced elongated and the amount of the fine γ′ precipitates hardly increases, but they are still in the cuboidal shape with the increase of the number of cycles after 20 cycles. The coalescence of γ′ precipitates results in the decrease of the strength and elongation, and the volume fraction of total γ′ precipitates is supersaturated at 20 cycles, thus does not play role in the mechanical property.
4 Conclusions
1) There are two morphologies of γ′ precipitates in alternate heating/cooling tests: the large γ′ and fine γ′ precipitates.
2) With the increase of cycles, the large γ′ precipitates decrease in the amount, and change from cuboidal to irregular shape to similar raft in the local region, and the fine γ′ precipitates totally remain in cuboidal shape.
3) The increase of the amount of total γ′ precipitates and precipitation of the fine γ′ precipitates improve the mechanical properties of the alloy after heating and cooling cycles.
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(Edited by YANG You-ping)
Corresponding author: YANG Jin-xia; Tel: +86-24-23971767; E-mail: jxyang@imr.ac.cn
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