I. INTRODUCTION

Developing relatively clean nuclear energy is an effective way to solve the problem of energy crisis, and structural materials for nuclear reactors are crucial to their operation safety. Ni-16Mo-7Cr-4Fe alloy manufactured by Institute of Metal Research, Chinese Academy of Sciences is a nickel-based superalloy similar to Hastelloy N alloy, which was developed in 1960s for constructing a container of molten fluoride salts at Oak Ridge National Laboratory, and is known for its good performance against corrosion, oxidation, neutron irradiation and high temperature in a molten salt reactor (MSR) [1]. In China’s efforts to develop the next generation MSR, Ni-16Mo-7Cr-4Fe alloy will be used as structural material to endure the harsh environment of high temperature (1073 K or even higher) of molten fluoride salts. This alloy must be protected from the external oxidation at higher temperatures, because air oxidation can lead to a leak in the cooled tube during the service process [2, 3]. The high temperature oxidation resistance performance of Ni-16Mo-7Cr-4Fe alloy does not meet the requirement of next generation MSR. This problem is expected to be solved by rare earth microalloying modification.

Yttrium, as a rare earth element, has been applied successfully in metallurgy [4-11] and chemical and surface engineering [12-17]. Recently, yttrium has been regarded as a promising alloying element for superalloys. It improves oxidation resistance of nickel-based alloys [7], Fe-Ni-Cr ternary alloys [18], Co-based alloys [19] etc. The promotion in formation of compact layers, formation of phase transformation [6, 20], formation of less harmful phases by counteraction with other deleterious elements [7, 18, 20-22] and formation of fine dendrites [23] are thought to be beneficial effects of yttrium doping. However, to the authors’ knowledge, little work focuses on effects of yttrium in matrix on oxidation resistance of Ni-based superalloys. In this paper, synchrotron radiation X-ray fluorescence (SRXRF) is used to determine distribution and content of trace yttrium in the matrix, which may affect microstructure and oxidation resistance of Ni-16Mo-7Cr-4Fe alloy. This study may help extend the usable critical temperature of nickel-based superalloys for molten salt energy systems.

II. MATERIALS AND METHODS

Master alloys supplied by the Institute of Metal Research, Chinese Academy of Sciences (Shenyang, China), were melted in a vacuum induction furnace and different amounts of yttrium were added to obtain the Ni-16Mo-7Cr-4Fe alloys (G00, G05, G12, G21 and G43). The results of ICP-AES (inductively coupled plasma atomic emission spectroscopy) analysis are given in Table 1. The ingots were hot-rolled at 1453 K to sheets of about 2 mm thickness. They were homogenized at 1450 K for 10 min and aged at 1173 K for 20 h in KSL-1400X muffle furnace, followed by water-quenching. Each specimen (22×8×2 mm³) for testing was cut from the sheets and was grinded, polished and cleaned.

Oxidation testing was performed at 1273 K in air using a KSL-1400X muffle furnace. An ESJ180-4 electric balance, with an accuracy of 0.1 mg, was used to measure the weight change of each specimen (five specimens were tested to obtain an average) after completion of each exposure period. Microstructure of the specimens before and after oxidation test was observed using a LEO 1530VP scanning electron microscope (SEM) and a Tecnai G² F20 transmission electron microscope (TEM) with an energy dispersive spectroscopy (EDS). X-ray fluorescence (XRF) at 20.5 keV and Grazing incidence X-ray diffraction (GIXRD) at 10.0 keV were conducted at Shanghai Synchrotron Radiation Facility (SSRF).

III. RESULTS AND DISCUSSION

Figure 1 shows SEM images of the Ni-16Mo-7Cr-4Fe alloys before oxidation. Coarse particles were observed along grain boundaries. The number of coarse particles in the alloy samples increased with yttrium concentration. The SEM-EDS results of the coarse particles indicate that there are Mo-rich and Y-rich phases. The amount of Y-rich phase observed in the Ni-16Mo-7Cr-4Fe alloys containing yttrium increased with yttrium concentration. The amount of Mo-rich phase in G05 was less than that of G00, but it increased with further enhancing yttrium concentration. TEM results indicate that the two phases were M₆C and Ni₁₇Y₂, respectively.

Fig. 1.Full-screen View

SEM images of Ni-16Mo-7Cr-4Fe alloys with various yttrium concentrations.

The selected area diffraction patterns (SADPs) of M₆C and Ni₁₇Y₂ are shown in Fig. 2. M₆C is angular and has a face centered cubic structure. Ni₁₇Y₂ has a hexagonal crystal structure with a=0.8320 nm, c=0.8042 nm and a space group of P63/mmc. Figure 3 shows TEM morphology of M₆C phase and Ni₁₇Y₂ phase. The shape of Ni₁₇Y₂ phase changed from round to irregular with increasing yttrium content. The Ni₁₇Y₂ phase was circular in G05 (Fig. 3(a)). It grew and became irregular shape (Fig. 3(b)). When the content of Y reached 0.43wt.%, some multi-phase regions appeared, including M₆C and Ni₁₇Y₂ phases (Fig. 3(c)). The M₆C phase presented strip shape in G00 (Fig. 3(d)). However, in G05, it changed to discrete particle (Fig. 3(e)) and turned back to strip shape when further increasing yttrium concentration (Fig. 3(f)).

Fig. 2.Full-screen View

TEM morphology of Ni₁₇Y₂ phase (a) and the SADP (b, c); and morphology of M₆C phase (d) and the SADP of M₆C (e, f).

Fig. 3.Full-screen View

TEM morphology of Ni₁₇Y₂ phase (a) and the SADP (b, c); and morphology of M₆C phase (d) and the SADP of M₆C (e, f).

Oxidation curves of all specimens are shown in Fig. 4(a). It can be seen that adding yttrium into Ni-16Mo-7Cr-4Fe alloy improves substantially the oxidation resistance, especially G05. However, the oxidation resistance weakens gradually with increasing yttrium concentration. So, proper yttrium improves adhesion of the oxide scale to substrate significantly (Fig. 4(b)). Severe spallation of the oxide scale of alloy G05 did not appear in 300 h. However, alloy G00 exhibited the most severe spalling, followed by alloy G43, G21 and G12.

Fig. 4.Full-screen View

Weight gain (a) and mass of rupture scale (b) as function of exposure time of the alloy specimens air-exposed at 1273 K for 300 h.

The GIXRD results of oxide scale of G00 and G05 (Fig. 5) show that both the outmost layers of the oxide scales are composed of NiO (PDF 65-6920) and NiFe₂O₄ (PDF 47-1417). Figure 6 shows the cross-sectional XRF-mapping of alloys. Unfortunately, most of the outer oxide layer of G00 and G43 spalled, and the XRF mapping area (100 μm×300 μm) was so small that the outmost Ni/Fe-rich layer was not detected clearly (Fig. 6(b)). Also, the oxide scale of G05 was much thinner and more compact than that of G00 and G43 (Fig. 6). For the oxide scale on the surface of G00, there were only NiO and NiFe₂O₄ and MoNi (PDF 50-1270) until the incidence angle increased to about 10 (Fig. 5(a)). Cr₂O₃ (PDF 38-1479), few spinel oxides (NiCrMnO₄, PDF 020-0780; NiCr₂O₄, PDF 23-1272; FeCr₂O₄, PDF 24-0511) and matrix also appeared when the incidence angle increased to 10. Moreover, diffraction peaks of NiO and NiFe₂O₄ remained at this higher incidence angle. When the incidence angle increased to 15, the intensity of diffraction peak of the matrix and the spinel oxides evidently increased. However, for the surface oxide scale on alloy G05, diffraction peaks mainly corresponded to NiO and NiFe₂O₄ when the incidence angle was 0.1. When the incidence angle increased to 0.5, Cr₂O₃, YCrO₃, spinel oxides (NiCrMnO₄; NiCr₂O₄; FeCr₂O₄) and matrix also appeared. When the incidence angle increased to 0.8, the intensity of diffraction peak of the matrix and the spinel oxides evidently increased. It can be conclude that the oxide scale of G00 is much thicker than that of G05.

Fig. 5.Full-screen View

Grazing incidence X-ray diffraction profiles of the oxide scales formed on specimens oxidized at 1273 K for 300 h (a) G00; (b) G05.

Fig. 6.Full-screen View

(Color online) Cross-sectional XRF-mapping of the oxide scales for the specimens oxidized at 1273 K for 350 h (a) G00, (b) G05 and (c) G43.

The possible reasons may include the followings. One is the higher yttrium concentration in the matrix of G05 than that of G43, which can be seen by the Y mapping in Fig. 6 and the EPMA analysis in Fig. 7. With increasing yttrium contents, the volume fraction of Ni₁₇Y₂ increases (Fig. 7(a)) and the yttrium concentration in the matrix decreases (Fig. 7(b)). Consequently the internal compact YCrO₃, Cr₂O₃ and spinel oxide layer are easier to be formed in alloy G05, hence the hindering of Mo diffusion; Another possible reason is the interfaces between coarse Ni₁₇Y₂ and matrix, locating in the surface of the alloys containing excess yttrium. This may supply the diffusion channels for oxygen and the oxidizable elements especially Mo. Therefore, Mo-rich layer was not observed in the oxide scale of G05 which had the smallest amount of Ni₁₇Y₂, while the Mo-rich layer of G00 and G43 was quite thick. Thirdly, proper yttrium can enhance the strength of grain boundaries of oxides at the surface. Figure 8 shows the morphology of surface oxide scale on Ni-16Mo-7Cr-4Fe alloys after oxidation for 100 h at 1273 K. There is no crack on the surface of G05, but some intergranular cracks appeared on the surface of G00. It was suggested that proper yttrium addition might form internal oxide boundaries where vacancies could condense and thus diminish void formation [24], hence the enhanced strength of grain boundaries of oxides.

Fig. 7.Full-screen View

EPMA analysis of the yttrium in Ni-16Mo-7Cr-4Fe alloys: (a) volume fraction of Ni₁₇Y₂ phase in the alloys, (b) Y contents in the matrix.

Fig. 8.Full-screen View

Morphology of surface oxide scale on Ni-16Mo-7Cr-4Fe alloys of G00 (a) and G05 (b) after oxidation for 100 h at 1273 K.

IV. CONCLUSION

Proper yttrium improves distribution of the coarse M₆C particles. The number and size of detrimental Ni₁₇Y₂ increase with yttrium concentration and the shape of Ni₁₇Y₂ changes from round to irregular. When the concentration of yttrium increases to 0.43wt.%, some coarse Ni₁₇Y₂ chains and multi phase regions containing Ni₁₇Y₂, M₆C and γ phase appear.

The oxidation resistance of Ni-16Mo-7Cr-4Fe alloy at 1273 K is improved by proper yttrium addition. The oxide scale of alloy containing 0.05wt% yttrium is the thinnest and most compact. Proper yttrium can promote the formation of the compact YCrO₃ and Cr-O internal oxide layer, and then inhibit the diffusion of Mo. With increasing yttrium concentration, the oxidation resistance of Ni-16Mo-7Cr-4Fe alloy decreases because coarse Ni₁₇Y₂ consumes most of yttrium in the matrix and reduces the strengthening effect of yttrium.