Keywords

1 Introduction

Zirconium alloys, because of their low thermal neutron absorption cross-section, excellent mechanical behavior under high temperature, and corrosion resistance, are extensively used as fuel cladding, pressure tube, fuel assembly spacer grids, and other structural materials in nuclear reactors, which are also the only material used as fuel cladding in PWRs at present. Compared with the Zr-Sn (Zr-2, Zr-4) and Zr-Nb (E110, M5) alloys developed in the last century, Zr-Sn-Nb alloys have gradually become one of the important directions in the research and development of new zirconium alloys under the requirements of high fuel consumption and long life of fuel assemblies, such as ZIRLO in the United States, E635 in Russia, M-MDA in Japan, HANA-4 in Korea and N36 in China, which have been proved to have better comprehensive properties inside and outside the reactor than Zr-4 [1].

During the preparation of zirconium alloy profiles for fuel assemblies, zirconium alloy ingots obtained by vacuum arc melting are usually subjected to β-phase heat treatment after forging, which will promote alloy element in zirconium alloy to a more homogeneous state and re-dissolve the precipitated second phase into the β-phase, followed by quenching. In the above process, the α-phase of the Close-packed hexagonal (HCP) structure transforms into the β-phase of the Body-centered cubic (BCC) structure with the increase of temperature, and the second phase particles from ingot gradually dissolve completely in the β-phase region [2]. The microstructure and properties of zirconium alloys will be affected by the different parameters of β-phase quenching treatment. It has been shown that zirconium alloys under slow cooling rate have better corrosion resistance, but different microstructure formation mechanisms of zirconium alloys with various alloy elements have been reported. Massih et al. [2] attributes the improved corrosion resistance of Zr-2 alloy to the presence of large-sized second phases with a low density resulting from quenching with a slow cooling rate. While for Nb-containing zirconium alloys, if the Nb content is between 1.0 and 5.0 wt%, the formation of β-phase will reduce the supersaturated Nb content, and the β-Nb in the sample will retard the transformation of oxide film from tetragonal Zr dioxide to a monoclinic Zr dioxide subsequently, which will stabilize the oxide film structure, and result in slowing corrosion down [3,4,5]. From this, the microstructure of zirconium alloys can be controlled by adjusting the heat treatment parameters in order to obtain better mechanical behavior and corrosion properties.

During cooling and β → α transformation of zirconium alloys, with the increase of cooling rate, performs with different microstructure patterns: lenticular α → parallel plate α → basketweave α → martensitic α [6, 7]. It has been reported that the quenched Zr-4 shows martensitic-α structure only when the cooling rate exceeds 1500 °C/s. But in fact, the existence of O will reduce the critical cooling rate of the martensite transformation and the existence of Nb will reduce the temperature of the β → α phase transformation meanwhile. The microstructures of many Zr-Sn-Nb-Fe alloys subjected to β-phase homogenization and water quenching showed martensitic-α [3, 8], where both the increase in O content and the decrease in cooling rate will increase the width of α [9].

It is obvious that a series of microstructural variations will take place with adjusting quenching parameters, which will ultimately affect the mechanical and corrosion properties of zirconium alloys. Especially for Nb-containing zirconium alloys, where both the Nb content and the size and distribution of the second-phase particles will lead to an increase or decrease in corrosion resistance, while these characteristics can be controlled by heat treatment. Most of the current studies on quenching of zirconium alloys have focused on the effect of cooling rate on β → α martensite transformation, and fewer studies have been conducted for β-phase heat treatment temperature and time in the quenching process. In this paper, domestic Zr-Sn-Nb-Fe alloy forging ingots prepared on an industrial scale are selected, to analyze parameters’ influences on the phase transformation behavior of zirconium alloys according to setting a series of β-phase heat treatment temperature and time.

2 Experimental

The nominal composition of the domestic Zr-Sn-Nb-Fe alloy selected in this paper is given in Table 1. The alloy forging billet plate specimens were obtained from 3000 kg grade ingots with the size of φ720 mm prepared by State Nuclear Baoti Zirconium Industry Company by secondary forging to form ingot billets with thickness δ60 mm. A tubular resistance furnace of model SK2-4-12 was taken to carry out the β-phase heat treatment of the alloy forging billet plate specimens and the parameters of heat treatment are shown in Table 2. Zirconium alloy was quickly transferred to water cooling whose rate is about 1000 °C/s after heat treatment. The samples of rolling direction- normal direction (RD-ND) were prepared after quenching. The Leica MeF3A optical microscope, the FEI Nova Nano SEM 400 field emission scanning electron microscope, and the FEI TECNAI G2 F20 Field emission transmission electron microscope were used to observe and analyze samples’ microstructure characteristics.

Table 1. The nominal composition of Zr-Sn-Nb-Fe alloy (wt%).
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Table 2. β-phase heat treatment parameters setting.
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3 Results and Discussion

3.1 Microstructure Analysis

Figure 1 shows the microstructure of the Zr-Sn-Nb-Fe alloy after quenching under each parameter, with the sampling position at the middle of the ingot away from the forging plane. As can be seen from the figure, due to the fast water cooling rate, the alloy almost all underwent β → α martensite transformation, in the set quenching parameters, there appears irregular small acicular martensitic formation, acicular martensitic clusters in a certain range of parallel row within the micro-zone, the shades of color in the polarized photo represents the martensitic laths of different orientation distribution, the original β grain boundary can be distinguished, where the Martensite nucleation takes place during phase transformation. In addition, martensitic nucleation can also occur in the subgrain boundaries and dislocations of the parent phase [10].

The conclusion that the β-phase heat treatment temperature has little effect on the morphology of martensitic after phase transformation at the same holding time can be confirmed preliminarily. This is in agreement with the results given by Kim et al. [3]. That the β-phase heat treatment temperature doesn’t affect the corrosion resistance of zirconium alloys. However, when the β-phase heat treatment temperature was constant, the post-phase transformation microstructure showed some differences with the extension of the holding time. At the temperature of 1050  °C, the acicular martensite has a tendency to increase in size with the extension of the holding time from 0.25 h to 0.5 h and even 1 h (Fig. 1b, c, d), but the discrepancy is not obvious, while when the holding time is extended to 2 h, in addition to the acicular martensite, thick lens-like microstructure distributed vertically to each other appear in the crystal whose length (Fig. 1e, Fig. 2a). This discrepancy also shows regional variability, thick lenticular microstructure only exists in the middle of the ingot away from the forging plane. While in the region near the forging plane, Only a few plates with a similar orientation at the grain boundary of the parent phase appear (Fig. 2b), which is the result of parallel plates’ nucleation in the grain boundary and growth into the crystal. This kind of microstructure will show lower ductility in mechanical properties.

Further observation of the SEM photographs of the zirconium alloy ingot in the original forged state and after different quenching processes (Fig. 3) showed that the original forged zirconium alloy plates were organized as coarse plate bundles with different dislocations and plate widths of about 4 μm, with the skeleton structure formed by the precipitation of second-phase particles at the boundary. After water quenching, the alloying elements in the zirconium alloy were re-dissolved into the matrix under water cooling conditions and underwent martensite transformation to different degrees, and most of the areas formed acicular martensite with a dislocation difference of about 60° between the laths and similar dimensions, representing the usual direction of martensite growth during the β → α transformation, which is similar to the water quenched organization of the Zr-Sn-Nb-Fe alloy characterized by CHAI et al. [9]. There are also a few coarser long plate-shaped martensites.

Fig. 1.
figure 1

RD-ND surface metallographic microstructure of Zr-Sn-Nb-Fe billet sheet specimens under different quenching processes (a) 950 °C/0.5 h (b) 1050 °C/0.25 h (c) 1050 °C/0.5 h (d) 1050 °C/1 h (e) 1050 °C/2 h (f) 1150 °C/0.25 h (g) 1150 °C/1 h

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Fig. 2.
figure 2

Zr-Sn-Nb-Fe ingot billet sheet specimens RD-ND surface metallographic microstructure after 1050 °C/2 h heat treatment (a) away from the forging plane (b) near the forging plane

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Under the heat treatment conditions set in this study, the change in heat treatment temperature hardly affected the dimensions of acicular martensite and plate-shaped martensite. Regarding the effect of holding time on the β → α phase transformation, a comparison of SEM photographs of samples observed under 1050 °C/2 h heat treatment conditions in Fig. 1e and Fig. 2a reveals that the coarse lenticular microstructure exhibits closely spaced clusters of parallel laths, and although this microstructure has the structural characteristics of basket-weave widmanstatten (Fig. 2a, c, Fig. 3f), this kind of widmanstatten mostly appears in the case of zirconium alloys with the low cooling rate after β-phase heat treatment, such as furnace cooling. Okvist et al. [11] suggested that the basket-weave widmanstatten is formed by the quenching and cooling process in which the β-phase provides a large number of nucleation sites for the α-phase (precipitated second-phase particles, etc.), and the symmetry of the BCC structure β-Zr makes the α plates grow simultaneously on multiple inertial surfaces and truncate each other, but the second phase is mostly redissolved into the β-phase during the β-phase heat treatment, and then the α plates grow from the grain boundary. In addition, the ZrC particles in the Zr-C alloy can also induce the nucleation of α plates. At a lower cooling rate, the second phase will form to provide nucleation points for the basket-weave widmanstatten due to the diffusion of alloy elements. However, no second phase particle are found in Fig. 2f, so the phase transformation is judged to be still carried out in a tangential mechanism, which proved the non-diffusivity of phase transformation and the existence of martensitic microstructure. It can be seen that the width of martensitic laths will increase significantly when the holding time is extended to a certain value under a certain temperature, which is consistent with the phenomenon observed in M5 zirconium alloy by Xu [12]. O.T. WOO et al. [6] found that an increase in oxygen content leads to an increase in the width of lath as well under certain conditions of cooling rate. Because of the condition of non-vacuum heat treatment in this experiment, the oxygen content will increase with the extension of holding time, the starting temperature of martensite transformation increase, which expands the β + α region, and the time for oxygen diffusion from β grains to α grains increases, a series of changes mentioned above will enhance the diffusion-controlled phase transformation mechanism to some extent, resulting in the increase of martensite width.

Fig. 3.
figure 3

RD-ND surface SEM images of Zr-Sn-Nb-Fe billet sheet specimens under different quenching processes (a) as forged (b) 950 °C/0.5 h (c) 1050 °C/0.25 h (d) 1050 °C/0.5 h (e) 1050 °C/1 h (f) 1050 °C/2 h (g) 1150 °C/0.25 h (h) 1150 °C/1 h

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In summary, the forged domestic Zr-Sn-Nb-Fe alloy under the β-phase heat treatment and subsequently water quenching, its microstructure transformed from the coarse β plates to some acicular and a few plate-shaped martensites, whose orientations have a certain relationship with parent phase, the latter is more coarse and long. Under the set quenching condition, the heat treatment temperature basically hardly affect the martensitic characteristics, but under the heat treatment condition of 1050 °C, with the extension of the holding time from 0.25 h to 2 h, part of the martensite width increased significantly, and a cluster laths appeared in parallel, a possible reason is that under the non-vacuum heat treatment conditions, the oxygen content increases with the extension of the holding time probably, more promote the growth of the nucleated laths rather than the nucleation of new laths, and finally lead to martensite Widening.

3.2 Component and Structure Analysis

The bright-field images of the samples under transmission electron microscopy for the four parameters 1, 2, 5, and 6 are shown in Fig. 4. A large number of martensitic laths with different widths were found in the matrix of the zirconium alloy ingot plate specimens under four different quenching conditions, most of them were around several hundred nanometers, and there were obvious twinning and parallel dislocation lines in the tissue, the existence of a large number of dislocations provided nucleation sites for the transformation and promoted the formation of acicular martensite, the twinning was related to the starting temperature of martensite transformation, and when this temperature was reduced to a certain value, the twinning opens. The diffraction spots were calibrated for the black stripes present at the boundaries of the martensitic laths. It showed that most of them are β-Zr with BCC, which satisfies the burgers orientation relationship with the α-Zr matrix, while the existence of other diffraction spots indicated that the alloy contains a few ω-Zr with HCP. The α → ω phase transformation generally occurs under high pressure. It has been reported that the above transformation can occur in zirconium alloys with high Nb content or high pressure during quenching or aging treatment [13, 14]. And as the enrichment area of oxygen, the ω-phase also proves the hypothesis in Sect. 3.1 that the oxygen content in zirconium alloy increases under 1050 °C/2 h quenching condition. It indicates that even in zirconium alloys with low Nb content and no high pressure, quenching with high cooling rate will also promote the process of α → ω transformation and increase the thermal stability of ω-Zr phase. No second-phase particle was detected in transmission electron microscopy, which also proves that the phase transformation under the water-cooled quenching condition is of the tangential type due to the high cooling rate.

Fig. 4.
figure 4

RD-ND surface TEM images of Zr-Sn-Nb-Fe billet sheet specimens under different quenching processes (a) (b) 950 °C/0.5 h (c) (d) 1050 °C/0.25 h (e) (f) 1050 °C/2 h (g) (h) 1150 °C/0.25 h

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The alloys were further scanned by surface scanning energy spectroscopy mode, as shown in Fig. 5. The quantitative analysis of the alloy composition at the martensitic laths boundary was shown in Table 3. Different degrees of Fe partial gather were found at the martensitic lath boundaries for each condition, which had the highest value at 1050 °C/2 h quenching condition, this phenomenon is consistent with the highest Fe content of more than 5 wt% at the same parameter in the quantitative elemental analysis. In addition to Fe elements, the alloying elements Sn and Nb are diffusely distributed in the alloy.

Usually, the second phase particles are re-dissolved into the zirconium alloy matrix during the β-phase heat treatment stage to form a supersaturated solid solution, and the shear martensite transformation process doesn’t have the diffusion phenomenon, while the phenomena of Fe partial gather and its increase with the extension of the holding time mentioned above are inconsistent with it. It proved that the actual quenching process is accompanied by a weak diffusion phase transformation, which also explains the existence of parallel-plate of widmanstatten in Fig. 2b. Thus, when the β-phase heat treatment temperature is certain, the extension of the holding time enhances the diffusion-controlled phase transformation mechanism, and this diffusion process transports the solute elements in the alloy into the β-Zr that is not transformed timely between the α martensitic laths due to cooling after high-temperature and makes the diffusion of alloying element Fe to the α lath boundaries more significant. But the weak diffusion process is not sufficient to transport Sn and Nb with larger atomic radius, which leads to the discrepancy in the degree of segregation of the three alloying elements. The segregation of the Fe element and the gradually increasing content of oxygen in the process will lead to an increase in the martensitic laths width, while the increase in the heat treatment temperature had essentially no effect on the martensitic phase transformation.

Fig. 5.
figure 5

Signal spectra of Fe elements at the martensite boundary of Zr-Sn-Nb-Fe alloys under different quenching processes (a) 950 °C/0.5 h (b) 1050 °C/0.25 h (c) 1050 °C/2 h (d) 1150 °C/0.25 h

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Table 3. Content of alloying elements at the martensite boundary (wt%) 1–950 °C/0.5 h; 2–1050 °C/0.25 h; 5–1050 °C/2 h; 6–1150 °C/0.25 h
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Comprehensive analysis of the above TEM bright-field images, diffraction spots and elemental signal spectra shows that the water-cooled quenched zirconium alloy undergoes β → α incomplete martensitic phase transformation after β-phase heat treatment, and both β-Zr and ω-Zr exist in the matrix of the transformed zirconium alloy, but the content is small, which proves that there is some elemental diffusion at the same time of the phase transformation process. As a result of diffusion, alloying elements such as Fe, Sn, and Nb enter the martensitic slat boundary, where Fe elements undergo obvious segregation at the boundary, and the degree of segregation increases in a certain range with the extension of holding time, which is also one of the reasons for the increase of martensitic laths width.

4 Conclusions

The results of studying the microstructure of Zr-Sn-Nb-Fe alloy under different β-phase heat treatment and following water quenching showed:

  1. (1)

    Zr-Sn-Nb-Fe alloy in the forged state has a coarse plate microstructure, while the microstructure of water-cooled quenching after β-phase heat treatment is mostly acicular martensite and a few long plate-shaped martensite with a width of several hundred nanometers; the β-phase heat treatment temperature has little effect on the microstructure of alloy, but at a certain temperature, the extension of holding time will increase the martensitic lath width, and this phenomenon only occurs far from the forging plane of the ingot.

  2. (2)

    Both BCC structure of β-Zr and HCP structure of ω-Zr exists in α-Zr matrix after β → α phase transformation under Water-cooled quenching condition. At the time all the alloy elements Fe, Sn, and Nb are solidly dissolved in the matrix, with Sn and Nb diffusely distributed and Fe gathered at the martensitic boundaries. β-phase heat treatment temperature has little effect on the composition and the structure, but the degree of segregation at a certain temperature increases with the extension of the holding time.

  3. (3)

    The domestic Zr-Sn-Nb-Fe alloy undergoes shear-controlled martensitic phase transformation during β-phase heat treatment and water-cooled quenching, but the extension of the holding time will enhance the diffusion-controlled phase transformation by increasing the O content in the alloy, and the diffusion process transports the alloy elements to the martensitic boundary and leads to the partial gathering of Fe element with small atomic radius.