2017 Water Reactor Fuel Performance Meeting September … · 2 2017 Water Reactor Fuel Performance...

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2017 Water Reactor Fuel Performance Meeting September 10 (Sun) ~ 14 (Thu), 2017

Ramada Plaza Jeju • Jeju Island, Korea

FORMATION OF A DISPERSED OXIDE LAYER ON ZIRCALOY-4 BY A LASER-BEAM-INDUCED SURFACE

TREATMENT

Yang-Il Jung,1 Jung-Hwan Park,1 Dong-Jun Park,1 Hyun-Gil Kim,1 Jae-Ho Yang,1 Yang-Hyun Koo,1 Yoon-Soo Lim2

1 Korea Atomic Energy Research Institute, 989-111 Daedeok-daero, Yuseong, Daejeon, 34012, South Korea

2 Hanbat National University, 125 Dongseo-daero, Yuseong, Daejeon, 34158, South Korea

Corresponding author: Yang-Il Jung ([email protected])

ABSTRACT: The technology to suppress high temperature deformation and rupture damage by forming an oxide dispersion

strengthened (ODS) layer on the surface of Zircaloy-4 was developed. The ODS treatment on the Zr surface layer was

successfully performed using a laser beam scanning process. The Zircaloy-4 plates and tubes were coated by Y2O3 oxide and

laser beam treated to form a dispersed oxide layer. The thickness of the dispersed oxide layer was varied 80–200 μm from the

Y2O3 coating with thickness of 10–30 μm. The tensile strength of Zircaloy-4 was increased by up to 20% with the formation of

a thin dispersed oxide layer with a thickness less than 10% of that of the Zircaloy-4 substrate. However, brittle fracture was

observed in the surface-treated samples during tensile deformation. Furthermore, multiple ODS layers were formed in

Zircaloy-4 by stacking and consolidating the surface treated samples. The lamellar structure of dispersed oxide layers was

beneficial to increase ductility of the ODS Zircaloy-4.

KEYWORDS: Zr alloy, oxide dispersion strengthened (ODS) alloy, laser surface treatment, microstructure

I. INTRODUCTION

Accident tolerant fuel claddings are being developed globally after the Fukushima accident with the demands for the

nuclear fuel having higher safety at normal operation as well as severe accident conditions.1-4 Zircaloy-4 is a traditional

zirconium-based alloy developed for application in nuclear fuel assembly components. Recently, an oxide dispersion

strengthened (ODS) zirconium was proposed to increase the strength of the Zr-based alloy up to high temperatures.3,5,6 Oxide

particles in ODS alloys are thermally stable and resistant to neutron irradiation; thus, oxide dispersion strengthening

treatment of Zircaloy-4 is promising for developing fuel cladding materials with enhanced accident tolerance. Yttrium oxide

(Y2O3) is a typical material used in ODS alloys. ODS alloys are generally manufactured through mechanical alloying7 of the

source metal with oxide powders and applying consolidation processes such as hot isostatic pressing, hot extrusion, and spark

plasma sintering. Homogeneous dispersion of oxide particles during the manufacturing process is a key issue. In this study,

an ODS layer was formed in the Zircaloy-4 surface region by a laser beam treatment. Surface treatment is advantageous for

uniform distribution of oxide particles and control of their volume fractions. Moreover, the surface treatment process is

directly applicable to final products such as tubes, strips, and sheets. Y2O3-coated Zircaloy-4 plate and tube samples were

scanned by a laser beam. A dispersed oxide layer was formed by penetration of Y2O3 particles into Zircaloy-4. The formation

of the ODS layer depending on processing parameters was investigated. The effect of oxide dispersion strengthening

treatment on the mechanical strength was evaluated by tensile tests at RT and an elevated temperature of 380 °C.

II. EXPERIMENTAL PROCEDURE

Zircaloy-4 (Zr-1.5Sn-0.2Fe-0.1Cr, wt.%) alloy sheets with a thickness of 2 mm and tubes with an outer diameter of 9.5

mm and a wall thickness of 0.57 mm were used as a substrate. The Zircaloy-4 sheets initially exhibited recrystallized

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microstructure with an average grain size of ~7 μm. In the case of Zircaloy-4 tubes, the as-received microstructure was a

cold-worked and stress-relieved. The sheets and tubes were cleaned by alcohol and acetone to remove stains and organic

contamination on the surface, and then dried in an oven at 80 °C. Y2O3 was coated on the prepared Zircaloy-4 surface. To

form a paste, Y2O3 powder (99.9%, 1 μm, Alfa Aesar, USA) was placed in an agate bowl, and mixed with distilled water

containing 10 wt.% poly vinyl alcohol (PVA) as a binder. The amount of PVA was 3 wt.% of the Y2O3 content. In the case of

a dip coating solution, the Y2O3 mixture was further diluted with water to a solute concentration of 33.5 g/ml. The solution

was mixed for 24 h using zirconia balls. The Y2O3 paste or dip-solution was wet-coated on the Zircaloy-4 sheet or tube, and

then dried in a vacuum oven at 80 °C for 20 min. The final thickness of the Y2O3 coating was measured by an eddy-current

isoscope (MP30, Fischer, Germany). The developed Y2O3 coating was 7–55 μm in thickness depending on the coating speed

and solution dilution.

The Y2O3-coated Zircaloy-4 sheet was scanned by a continuous-wave diode laser (PF-1500F, HBL Co., Korea). The

wavelength of the emitted laser beam was 1064 nm, and the beam diameter was 230 μm. The maximum operating power was

250 W. The thickness of the ODS alloy layer was controlled by varying the laser beam power; the scan speed (which can also

be used to control the thickness) was fixed at 10 mm/s in this investigation. To prevent oxidation during laser beam scanning,

Ar gas was continuously blown on the samples’ surfaces. Laser beam lines were scanned repeatedly with an overlap distance

of 0.2–0.4 mm. The ODS Zircaloy-4 samples were cut cross-sectionally for microstructural observation using an optical

microscope (OM) and a scanning electron microscope (SEM). For a mechanical test, a small tensile specimen with a gage

length of 6 mm was electro-discharge-machined from the ODS Zircaloy-4 samples. Both sides of the Zircaloy-4 sheet were

surface treated by oxide dispersion strengthening for the tensile test. The test was performed at room temperature (RT) and

380 °C with a cross-head speed of 1 mm/min using a universal testing machine (Instron 3367, USA).

III. RESULTS AND DISCUSSION

III.A. Plate Samples of ODS Zircaloy-4

Fig. 1 shows the cross-sectional microstructures of Zircaloy-4 after surface treatment for oxide dispersion strengthening.

A recrystallized Zircaloy-4 plate was used as a substrate, and the interior of the Zircaloy-4 maintained its recrystallized

microstructure during laser beam scanning. The dispersed oxide layer was observed in the surface region at a thickness of

about 130 μm. The wavy interface resulted from the multiple laser beam scans with a fixed offset distance (0.2 mm). Below

the ODS layer, a heat-affected zone (HAZ) about 270 μm in thickness developed because of the high temperature induced by

the laser beam’s thermal energy. SEM micrographs [Fig. 1(b)] revealed the dispersion of oxide particles. The trace of the

scanned laser beam was also identified by agglomeration of oxide particles at that position. Except in the laser beam overlap

region, oxide particles were distributed homogeneously in the ODS layer.

Fig. 1. Cross-sectional microstructure of ODS Zircaloy-4 sample, showing the ODS layer at the surface and heat-affected

zone (HAZ) below, observed by an OM (left) and SEM with a higher magnification (right).

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The thickness of the ODS layer varied depending on the laser beam power [Fig. 2(a)] and the thickness of the Y2O3

coating [Fig. 2(b)]. As the laser beam power increased from 150 to 200 W, the thickness of the ODS layer increased from

115 to 200 μm for the samples coated with 20 μm of Y2O3. A fluctuation in the thickness of the ODS layer was observed

when the laser beam power was low (150–170 W). The average thickness of the samples exhibited an insignificant increase

as the laser power increased from 150 to 170 W. The thickness increase became noticeable as the laser beam was operated at

powers above 180 W. Moreover, the variation in the thickness decreased; that is homogeneity was attained at high laser beam

powers (180–200 W). For a fixed laser beam power (180 W), the formation of the ODS layer was also examined according to

the coating thickness of the Y2O3 layer. The thickness of the Y2O3 coating was varied from 7 to 55 μm. A parabolic increase

in the thickness of the developed ODS layer was observed as the Y2O3 coating was thickened. At the same time, the

fluctuation in the thickness of the ODS layer increased with increasing thickness of the Y2O3 coating as well as that of the

corresponding ODS layer. The estimated dilution ratio is also presented in Fig. 2. For a dilution ratio of Y2O3 particles in

Zircaloy-4 substrate. It is very difficult to determine the dilution ratio experimentally. One possible method is to measure the

areal fraction of Y2O3 particles using SEM/TEM images. However, the particle size is too small to be properly quantified.5

According to our previous report,5 the bimodal distribution of the particle size was obtained at 15 and 125 nm of average

values, and the areal fraction was about 20%. The data still limited/localized because of the characteristics of TEM analysis.

By the way, the volume fraction could be estimated in a simple manner by assuming all the oxide particles in the coating had

penetrated into Zircaloy-4 matrix. According to Fig. 2(a), the thickness of ODS layer was increased as the laser beam power

increased. Because the oxide coating was fixed at 20 μm, the increased thickness of the ODS layer indicates more dilute

distribution of Y2O3 particles in the ODS layer.

Fig. 2. Variation in thickness of ODS layer formed in Zircaloy-4 (left) and volume fraction of Y2O3 particles in the ODS

layer (right) depending on (a) laser-beam power and (b) thickness of Y2O3 coating.

Laser beam was scanned at 180 W on Zircaloy-4 without a Y2O3 coating to identify the HAZ by the laser beam thermal

energy. The temperature of Zircaloy-4 substrate was not measured during the laser beam processing. However, it was

confirmed that melting of zirconium was occurred at the surface region. The solidified surface morphology appeared on the

trace of the laser beam. The melting temperature of zirconium is higher than 1800°C, thus we expect the surface of Zircaloy-

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4 was heated higher than that. Fig. 3 shows the microstructure of the HAZ revealing the martensite structure. The HAZ was

formed at a thickness of ~ 260 μm. The phase transition from alpha to beta (during heating) and back to alpha (during

cooling) occurs in zirconium. Martensite is formed athermally during phase transition from beta to alpha; it is a typical

microstructure of the beta-quenched Zr-based alloys.8-10 By the way, the cooling rate is an important parameter for the

formation of martensite. As the cooling rate was decreased, the lath of the martensite becomes larger and forms

Widmanstatten (thermal) rather than martensite (athermal). In this work, the samples were cooled by an aluminum panel with

circulating coolant water as well as Ar gas blowing on to the surface. Since cooling of samples was achieved very fast, the

martensite structure was formed mainly. However, there is a possibility of the mixed formation of matrensite and

Widmanstatten at the middle of the samples underwent slow cooling.

Fig. 3. Microstructure of HAZ after laser beam scanning.

The formation of an ODS layer on the Zircaloy-4 surface was effective for increasing the mechanical strength of fresh

Zircaloy-4. Fig. 4(a) shows the stress–strain curves for the ODS samples under tensile test at RT. As a comparison, fresh

Zircaloy-4 and laser-beam-scanned Zircaloy-4 without a Y2O3 coating (LBS only) were tested under identical conditions. The

ultimate tensile strength and elongation of fresh Zircaloy-4 were about 510 MPa and 35%, respectively. When Zircaloy-4

was scanned by the laser beam, the tensile strength increased to about 630 MPa. The martensitic phase transformation during

laser processing is the origin of the strength increase of the LBS samples. The ODS samples, furthermore, exhibited a much

higher tensile strength than fresh and LBS Zircaloy-4 samples. The obtained tensile strength of the ODS Zircaloy-4 samples

was 651–695 MPa. However, the elongation decreased drastically to 10–21%. An abnormal drop in the tensile strength was

observed at the maximum load for the ODS samples. The brittle surface layer after oxide dispersion strengthening treatment

accounts for this behavior. It is suggested that the increased strength was provided by the ODS layer through dispersion

strengthening of oxide particles. However, once surface cracks propagated and led to failure of the ODS layer, the tensile

load would be applied entirely to the interior Zircaloy-4. Because the load is already higher than the ultimate tensile strength

of Zircaloy-4, a sharp decrease in the tensile strength could be obtained.

The strengthening of Zircaloy-4 by the ODS layer was very effective at elevated temperature. Fig. 4(b) shows the stress–

strain curves of the ODS samples tensile tested at 380 °C. The ultimate tensile strength and elongation of fresh Zircaloy-4

were about 210 MPa and 47%, respectively. The ODS samples exhibited a tensile strength of about 355 MPa, which is

almost 70% greater than that of fresh Zircaloy-4. In addition, the elongation did not exhibit a stress drop or a dramatic

decrease, which had been observed in the test at RT (Fig. 4(a)). The tensile elongation of the ODS samples was 33–37%. The

values are about 30% less than that of the fresh Zircaloy-4 sample; however, the increase in the tensile strength resulted in

comparable toughness between the samples with and without the ODS layer. The equivalent toughness is meaningful for the

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application of ODS Zircaloy-4 as an in-core nuclear structural material. Because the working temperature of fuel cladding

tubes is 300–400 °C, the increases in the strength and toughness would provide enhanced reliability and safety margins.

Fig. 4. Stress–strain curves of ODS Zircaloy-4 samples tensile tested at (a) room temperature and (b) 380 °C.

III.B. Tube Samples of ODS Zircaloy-4

Fig. 5 shows the cross-sectional microstructure of the Zircaloy-4 tube with a surface ODS layer. The tube was cut axially

and to observe its microstructure. The microstructure along the axial direction revealed the formation of a dispersed oxide

layer 50–140 μm in thickness in the surface region. The helical laser beam scans with a fixed offset distance of 0.4 mm

produced the wavy interface. In addition, Y2O3 particles were aligned in stripes, which are thought to be formed by

solidification of the Zr matrix after instantaneous melting.

Fig. 5. Cross-sectional microstructure of ODS Zircaloy-4 samples showing the distribution of oxide particles in the ODS

layer at the surface and the formation of the HAZ below.

The formation of an ODS layer on the Zircaloy-4 surface was effective for increasing the mechanical strength of fresh

Zircaloy-4. Fig. 6 shows the stress–strain curves for fresh, laser-beam-scanned (LBS), and surface-treated Zircaloy-4 samples

during ring tension tests at room temperature. The tensile stress was calculated by dividing the applied load by the cross-

sectional area. The ultimate tensile strength of fresh Zircaloy-4 were about 790 MPa. When Zircaloy-4 was scanned by the

laser beam, the tensile strength increased to about 840 MPa. As in the case of plate samples, the martensitic phase

transformation during laser processing is the origin of the strength increase of the LBS samples. The ODS samples,

furthermore, exhibited a much higher tensile strength than fresh and LBS Zircaloy-4 samples. The obtained tensile strength

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of the ODS Zircaloy-4 samples was about 870 MPa. On the contrary, the tensile elongation decreased dramatically, showing

an abrupt drop in the applied tensile load. The fracture morphologies of the ring tensile specimens of fresh and ODS

Zircaloy-4 were quite different, as shown in Fig. 6. Fresh Zircaloy-4 fractured in a ductile manner, where necking and shear

slip occurred. On the other hand, brittle fracture without plastic deformation was observed in the ODS Zircaloy-4.

Fig. 6. Stress–strain curves of fresh, LBS, and ODS Zircaloy-4 samples in a tensile test at room temperature. The fractured

samples after the test are also shown together.

III.B. Lamellar ODS Zircaloy-4

Multiple ODS layers were formed after HIP joining of surface ODS treated Zircaloy-4 samples. Six surface ODS treated

samples were stacked and HIP bonded. For each sample, an ODS layer of ~100 μm thick was formed on Zircaloy-4 substrate.

HIP was performed at 950 °C for 2 h under 100 MPa. After decanning, the thickness of the bonded block was 11 mm. Hot-

rolling was conducted at 500 °C for eight times with an 1 mm reduction magnitude. The thickness of the sample after hot-

rolling was 3.61 mm. Then, cold-rolling was conducted with a 0.3 mm reduction magnitude to become a sheet sample with

2.2 mm in thickness. The cross-sectional microstructure of the final product was shown in Fig. 2. ODS treated layers were

observed periodically with dark contrasts. In addition, cold-worked microstructures were observed in the Zircaloy-4 matrix.

The thickness of the ODS layers and Zircaloy-4 were 16–37 μm and 410 μm, respectively. The volume fraction of ODS

treated layer was less than 10%.

Fig. 7 shows the tensile stress to strain curves for the fabricated samples. For the tensile test, small-sized specimens with

the cross-sectional dimensions of 2 mm × 4 mm and gage-length of 6 mm were machined by a wire electro-discharge

machining. The tensile test was performed at RT and elevated temperatures of 380°C and 500°C. The tensile strength at RT

was varied from 480 to 700 MPa depending on their microstructures. The tendency of stress–strain behavior was similar at

elevated temperatures. The drastic reduction in elongation was improved in the lamellar structured ODS Zircaloy-4.

Fig. 7. Tensile stress-strain curves for the lamella ODS Zircaloy-4 samples at room temperature and elevated temperatures of

380 °C and 500 °C, respectively.

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IV. CONCLUSIONS

Increased strength of Zircaloy-4 was obtained by ODS treatment on its surface. To form the oxide dispersed layer, the

Y2O3-coated Zircaloy-4 was scanned a laser beam. The thickness of the ODS treated layer was dependent on the laser beam

power and coating thickness. The obtained tensile strength of ODS treated Zircaloy-4 samples was about 20% higher than

that of fresh Zircaloy-4. There are two hardening mechanisms for the ODS treated Zircaloy-4 samples, i.e. martensitic phase

transformation and dispersion strengthening by oxide particles. The partial formation of ODS layer with the thickness of 5–

10% to the substrate thickness induced the increase in tensile strength up to about 20% in Zircaloy-4. Formation of the ODS

layer, on the contrary, decreased material’s ductility drastically. However, the ductility reduction became insignificant as the

testing temperatures increased to 380 and 500 °C. In the case of lamellar ODS Zircaloy-4 samples, the reduction in ductility

was not profound as compared to surface ODS samples. However, more detailed investigation is required to correlate the

tensile behavior with their microstructures.

ACKNOWLEDGMENTS

This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government

(MSIP) (No. 2017M2A8A5015058).

REFERENCES

1. Y.-H. Koo, J.-H. Yang, J.-Y. Park, K.-S. Kim, H.-G. Kim, D.-J. Kim, Y.-I. Jung, and K.-W. Song, “KAERI’s

development of LWR accident-tolerant fuel,” Nucl. Technol., 186, 295-304 (2014).

2. S.J. Zinkle, K.A. Terrani, J.C. Gehin, L.J. Ott, L.L. Snead, “Accident tolerant fuels for LWRs: A perspective,” J. Nucl.

Mater., 448, 374-379 (2014).

3. H.-G. Kim, J.-H. Yang, W.-J. Kim, Y.-H. Koo, “Development status of accident-tolerant fuel for light water reactors in

Korea,” Nucl. Eng. Technol., 48, 1-15 (2016).

4. Y.-I. Jung, J.-H. Park, H.-G. Kim, D.-J. Park, J.-Y. Park, W.-J. Kim, “Effect of Ti and Si interlayer materials on the

joining of SiC ceramics,” Nucl. Eng. Technol., 48, 1009-1014 (2016).

5. H.-G. Kim, I.-H. Kim, Y.-I. Jung, D.-J. Park, J.-Y. Park, Y.-H. Koo, “Microstructure and mechanical strength of surface

ODS treated Zircaloy-4 sheet using laser beam scanning,” Nucl. Eng. Technol., 46, 521-528 (2014).

6. Y.-I. Jung, H.-G. Kim, I.-H. Kim, S.-H. Kim, J.-H. Park, D.-J. Park, J.-H. Yang, Y.-H. Koo, “Strengthening of Zircaloy-

4 using Y2O3 particles by a laser-beam-induced surface treatment process,” Mater. Des., 116, 325-330 (2017).

7. C. Suryanarayana, “Mechanical alloying and milling,” Prog. Mater. Sci., 46, 1-184 (2001).

8. W.G. Burgers, “On the process of transition of the cubic-body-centered modification into the hexagonal-close-packed

modification of zirconium,” Physica, 1, 561–586 (1934).

9. D. Ciurchea, A.V. Pop, C. Gheorghiu, I. Furtuna, M. Todica, A. Dinu, M. Roth, “Texture morphology and deformation

mechanisms in β-transformed Zircaloy-4,” J. Nucl. Mater., 231, 83–91 (1996).

10. Y.M. Oh, Y.H. Jeong, K.J. Lee, S.J. Kim, “Effect of various alloying elements on the martensitic transformation in Zr-

0.8Sn alloy,” J. Alloys Compd., 307, 318–323 (2000).

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