Co-Al-Re三元系在1100 °C和1300 °C等温截面的相平衡-有色金属在线

简介概要

Co-Al-Re三元系在1100 °C和1300 °C等温截面的相平衡

来源期刊：中国有色金属学报(英文版)2021年第6期

论文作者：李玲玲张锦彬陈悦超杨水源王翠萍刘兴军

文章页码：1740 - 1747

关键词：Co-Al-Re；相平衡；等温截面

Key words：Co-Al-Re; phase equilibria; isothermal section

摘要：利用电子探针成分分析(EPMA)和X射线衍射分析(XRD)建立Co-Al-Re三元系的1100 °C和1300 °C等温截面相图。实验结果表明：在1100 °C等温截面相图中存在7个三相区，而1300 °C等温截面相图中存在5个三相区；随着温度从1100 °C上升到1300 °C，αCo、εRe和CoAl的固溶度略微增加；Co在AlRe₂、Al₁₁Re₄和Al₄Re相中的固溶度极小，<1.5%(摩尔分数)；等温截面相图中均未发现三元化合物。

Abstract: Isothermal sections of the Co-Al-Re ternary system at 1100 and 1300 °C were determined experimentally by electron probe microanalysis and X-ray diffraction. The results show that there are seven three-phase regions in the 1100 °C isothermal section and five three-phase regions in the 1300 °C isothermal section. The solid solubilities of αCo, εRe and CoAl increase a little with temperature increasing from 1100 to 1300 °C. The solubility of Co in compounds AlRe₂, Al₁₁Re₄ and Al₄Re is negligible, <1.5 at.%. And no ternary compounds are found.

Trans. Nonferrous Met. Soc. China 31(2021) 1740-1747

Phase equilibria in Co-Al-Re ternary system at 1100 and 1300 °C

Ling-ling LI¹, Jin-bin ZHANG¹, Yue-chao CHEN¹, Shui-yuan YANG¹, Cui-ping WANG¹, Xing-jun LIU^1,2,3

1. Fujian Provincial Key Laboratory of Materials Genome, College of Materials, Xiamen University, Xiamen 361005, China;

2. State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China;

3. School of Materials Science and Engineering, Harbin Institute of Technology, Shenzhen 518055, China

Received 15 July 2020; accepted 18 February 2021

Key words: Co-Al-Re; phase equilibria; isothermal section

1 Introduction

The phase equilibria of the Co-Al-Re ternary system are fundamentally important to understand and develop Co- and Ni-based superalloys.

Recently, L1₂-ordered phases have been observed in some Co-Al-X (X=W, V, etc) systems and have exhibited good performances [1-3]. Both Al and Co are important constituent elements of Ni-based superalloys. Thus, the Co-Al system plays an important role in Co- and Ni-based superalloys. Rhenium (Re) is an element commonly used in fourth- and fifth-generation Ni-based superalloys [4-8]. Elemental Re not only improves the strength and creep resistance of Co-based superalloys but also refines its morphology and decreases γ/γ′ lattice parameter misfits [9,10]. Therefore, in order to develop Ni- and Co-based superalloys and understand the relationships between their microstructures and properties, knowledge of the phase equilibria occurring in the Co-Al-Re system is required. However, experimental information and phase diagrams of this ternary system have not been established.

In this study, experimental investigation of the phase equilibria occurring in the Co-Al-Re ternary system at 1100 and 1300 °C was conducted. The experimental data will help to establish thermo- dynamic databases and inform the design and development of Co- and Ni-based superalloys.

Phase equilibria in three constituent sub-binary systems of Co-Al [11-14], Co-Re [15-18], and Al-Re [19-24], have been comprehensively investigated and critically assessed in the literatures. The crystal information existing for the stable phases in the Co-Al-Re ternary system is listed in Table 1.

The Co-Al system was first mentioned in Ref. [11], and then STEIN et al [12] investigated the melting behaviour and homogeneity range of the B2 phase. Later, PRIPUTEN et al [13] studied the Co₄Al₁₃ family of phases and reported that it has six phases: CoAl₃, Y1-Co₄Al₁₃, Y2-Co₄Al₁₃, M-Co₄Al₁₃, O-Co₄Al₁₃ and O′-Co₄Al₁₃. They considered that Y2-Co₄Al₁₃ phase is metastable and that a transition reaction might exist between O-Co₄Al₁₃ and O′-Co₄Al₁₃. To simplify the phase diagram, some researchers [14] treat the Co₄Al₁₃ family as a single stoichiometric phase named Co₄Al₁₃. This simplification is also adopted in the present study.

Table 1 Crystal structures of stable solid phases in Co-Al-Re ternary systems

The Co-Re binary system was first reported by ELLIOT [15] in 1965. Then, PREDEL [16] investigated the martensitic character of the phase transformation. Experiments determined that the Co-Re system is simple without an intermetallic phase. Recently, LIU et al [17] and GUO et al [18] thermodynamically evaluated the Co-Re system and their findings were consistent with the experimental data. The newly-assessed Co-Re phase diagram of GUO et al [18] is applied in the present study, as shown in Fig. 1.

For Al-Re system, HUANG and CHANG [19] were the first to publish a thermodynamic diagram based on the experimental data of SAVITSKII et al [20], ELLIOTT [15], SHRINK [21] and SCHUSTER [22]. Then, CORNISH and WITCOMB [23], using metallographic methods and X-ray diffraction, confirmed the existence of the intermediate phases of Al₁₂Re, Al₆Re, Al₄Re and Al₁₁Re₄. SCHUSTER et al [24] investigated the Al-Re binary system and confirmed that there are seven intermetallic phases: Al₁₂Re, Al₆Re, Al_33-xRe₈, Al₄Re, Al₁₁Re₄, AlRe and AlRe₂. They also described the crystal structure and lattice parameters for all these phases. Here, the results of SCHUSTER et al [24] are adopted.

Fig. 1 Binary phase diagrams constituting Co-Al-Re ternary system [14,18,24]

2 Experimental

Usually, phase relationships are studied by the equilibrated alloy [25,26] and diffusion couple methods [27]. Here, the phase relationships of the Co-Al-Re ternary system at 1100 and 1300 °C are studied using equilibrated alloy. Pure metals of cobalt (99.9 wt.%), aluminium (99.9 wt.%) and rhenium (99.9 wt.%) were used as raw materials to prepare ingots by arc melting under a high-purity argon atmosphere. The mass of each sample was about 15 g. To achieve homogeneity, the ingots were remelted at least four times and their mass losses were <0.5 wt.%.

Then, the specimens were cut into plate shapes and put into quartz capsules that were filled with argon gas. In addition, pure Ti filings were added to the quartz capsules to prevent oxidation, and samples containing a liquid phase were wrapped in a pure Ta slice to prevent contact reaction with quartz. Specimens were annealed at 1100 and 1300 °C for various durations ranging from 1 h to 60 d and then quenched in ice water.

Backscatter electron (BSE) imaging and the equilibrium composition of each phase were determined by an electron probe micro analyser (EPMA; JXA-8100R, JEOL, Japan) with an accelerating voltage of 20.0 kV and a probe current of 1.0×10^-8 A. X-ray diffraction (XRD) was performed for phase identification using Cu K_α radiation at 40 kV and 40 mA. To ensure the accuracy of the data, pure elements were used as standards to calibrate the measurements. The final values of the phase compositions are the averages of seven measurements.

3 Results and discussion

About 14 samples were used to determine the phase boundaries for isothermal sections. Figures 2 and 3 show typical BSE images of Co-Al-Re ternary alloys annealed at 1100 and 1300 °C, respectively. The corresponding XRD patterns are presented in Fig. 4. Tables 2 and 3 present the equilibrium compositions of the Co-Al-Re alloys quenched from 1100 and 1300 °C, respectively, as determined by EPMA.

Fig. 2 Typical BSE images obtained from Co-Al-Re ternary alloys annealed at 1100 °C

Fig. 3 Typical BSE images obtained from Co-Al-Re ternary alloys annealed at 1300 °C

Fig. 4 X-ray diffraction patterns obtained from alloys

Table 2 Equilibrium compositions of Co-Al-Re ternary alloys determined at 1100 °С

Table 3 Equilibrium compositions of Co-Al-Re ternary alloys at 1300 °С

3.1 Isothermal section at 1100 °C

As shown in Fig. 2(a), a three-phase microstructure of αCo (grey), ε(Co,Re) (white), and CoAl (black) is found in Co₆₁Al₁₉Re₂₀ alloy annealed at 1100 °C for 60 d. The corresponding XRD pattern is presented in Fig. 4(a). When the Co₃₀Al₄₀Re₃₀ alloy is annealed at 1100 °C for 60 d, there are three different phases, including CoAl (black), AlRe₂ (grey), and ε(Co,Re) (white), as shown in Fig. 2(b). Similarly, a three-phase microstructure is found in Co₂₂Al₇₀Re₈ alloy (Fig. 2(c)). Figure 4(b) shows the corresponding XRD pattern of the Co₂₂Al₇₀Re₈ alloy. Three phases of Al₁₁Re₄ (white), Co₂Al₅ (grey), and CoAl (black) are clearly distinguishable in Fig. 4(b). In Fig. 2(d), a two-phase microstructure can be observed in the Co₁Al₈₂Re₁₇ alloy annealed at 1100 °C for 1 d, where the liquid phase (L) is dispersed in the matrix of solid-phase Al₄Re (white). From magnified image of the liquid phase inserted in Fig. 2(d), both the grey and dark regions are solidified from the liquid phase. According to the morphology, the grey region may be a dendritic structure of the solid-phase Al (FCC). Thus, the chemical composition of the liquid phase is determined from the average value of the dark region.

Based on the experimental data, an isothermal section of the Co-Al-Re ternary system at 1100 °C is constructed (Fig. 5(a)).There are five experimentally-determined three-phase regions: [ε(Co,Re)+αCo+CoAl], [ε(Co,Re)+CoAl+AlRe₂], [CoAl+AlRe₂+Al₁₁Re₄], [CoAl+Al₁₁Re₄+Co₂Al₅] and [Al₁₁Re₄+Al₄Re+L]. In the Al-rich corner, due to a Co₄Al₁₃-family phase existing in the temperature range of 1093-1153 °C [13], the phase relationships are very complicated. Here, to simplify the phase relationships, the Co₄Al₁₃-family phase is treated as a single phase and other two three-phase regions of [Co₂Al₅+Co₄Al₁₃+L] and [Co₂Al₅+Al₁₁Re₄+L] are deduced.

On the Al-Re side, the solubilities of Co in the compounds AlRe₂, Al₁₁Re₄ and Al₄Re are negligibly small, all <1.5 at.%, as shown in Fig. 4(a).

3.2 Isothermal section at 1300 °C

As shown in Fig. 3(a), two different phases are found in the Co₃₈Al₃₂Re₃₀ alloy annealed at 1300 °C. Combined with the XRD analysis in Fig. 4(c), the white phase is identified as ε(Co,Re), while the black phase is CoAl. When the Co₃₀Al₅₀Re₂₀ alloy is annealed at 1300 °C (Fig. 3(b)), three different phases, Co₁₁Re₄ (grey), AlRe₂ (white), and CoAl (black), are identified in combination with the XRD analysis shown in Fig. 4(d). In Fig. 3(c), two solid phases of Al₁₁Re₄ (white) and CoAl (dark grey), and a liquid phase (L, light grey) occur in the Co₂₂Al₇₀Re₈ alloy annealed at 1300 °C for 1 h. Likewise, a three-phase microstructure consisting of L (black), Al₄Re (dark grey) and Al₁₁Re₄ (light grey) can be seen in the Co₂Al₇₆Re₂₂ alloy, as shown in Fig. 3(d).

Then, the isothermal section of Co-Al-Re ternary system at 1300 °C is established (Fig. 5(b)). There are five three-phase regions: [ε(Co,Re) + αCo+CoAl], [ε(Co,Re)+CoAl+AlRe₂], [CoAl+ AlRe₂+Al₁₁Re₄], [Co₁₁Re₄+CoAl+L] and [Co₁₁Re₄+ Al₄Re+L]. Compared with the isothermal section obtained at 1100 °C, there is an obvious difference in the Al-rich corner at 1300 °C: the Co₄Al₁₃-family phases decompose and the liquid phase appears. Other solid solubilities of αCo, ε(Co,Re) and CoAl increase a little from 1100 to 1300 °C. The solubilities of Co in the compounds AlRe₂, Al₁₁Re₄ and Al₄Re are still very small and negligible.

Fig. 5 Experimentally-determined isothermal sections of Co-Al-Re system

4 Conclusions

(1) Five three-phase regions are determined and two three-phase regions are deduced at 1100 °C, while five three-phase regions are determined in the 1300 °C isothermal section.

(2) The solid solubilities of αCo, ε(Co,Re) and CoAl increase a little from 1100 to 1300 °C.

(3) The solubility of Co in compounds AlRe₂, Al₁₁Re₄ and Al₄Re is very low and negligible at both 1100 and 1300 °C.

(4) No ternary compounds are found.

Acknowledgments

The authors are grateful for the financial supports from the National Natural Science Foundation of China (No. 51831007), and the National Key Research & Development Program of China (No. 2017YFB0702901).

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