Surface modification of Zircaloy-4 substrates with nickel zirconium intermetallics

Journal of Nuclear Materials 433 (2013) 514–522

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Surface modification of Zircaloy-4 substrates with nickel zirconium intermetallics Walter G. Luscher ⇑, Edgar R. Gilbert, Stan G. Pitman, Edward F. Love Jr. Pacific Northwest National Laboratory, 902 Battelle Blvd., Richland, WA 99352, United States

h i g h l i g h t s " Oxidation performance of Zircaloy-4 tailored via NiZr intermetallic coating. " Parametric design of experiments used to optimize surface modification approach. " Microstructural evolution correlated with weight gain and hydrogen absorption.

a r t i c l e

i n f o

Article history: Received 26 January 2012 Accepted 24 May 2012 Available online 4 June 2012

a b s t r a c t Surfaces of Zircaloy-4 (Zr-4) substrates were modified with nickel–zirconium (NiZr) intermetallics to tailor oxidation performance for specialized applications. Surface modification was achieved by electroplating Zr-4 substrates with nickel (Ni) and then performing thermal treatments to fully react the Ni plating with the substrates, which resulted in a coating of NiZr intermetallics on the substrate surfaces. Both plating thickness and thermal treatment were evaluated to determine the effects of these fabrication parameters on oxidation performance and to identify an optimal surface modification process. Isothermal oxidation tests were performed on surface-modified materials at 290°, 330°, and 370 °C under a constant partial pressure of oxidant (i.e., 1 kPa D2O in dry Ar at 101 kPa) for 64 days. Test results revealed an enhanced, transient oxidation rate that decreased asymptotically toward the rate of the Zr-4 substrate. Oxidation kinetics were analyzed from isothermal weight gain data, which were correlated with microstructure, hydrogen pickup, strength, and hardness. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction In this study, components fabricated from Zircaloy-4 (Zr-4) are intended for service within target rods where a significant, but limited extent of oxidation is desired. The desire for enhanced oxidation is driven by the need to reduce the limited quantity of water vapor that will be generated during irradiation and contact the target rod internal surfaces. Since Zr-4 is designed to resist oxidation, a surface-modification approach was developed to provide a coating that is more susceptible to oxidation by water vapor and will preferentially oxidize without significantly altering the mechanical properties of the substrate. Several goals drove the development of the surface modification approach. The primary goal was to achieve an oxidation rate that was initially enhanced relative to the substrate before asymptotically approaching the slower rate of the substrate. Due to in-service mechanical loading requirements, it was critical that ⇑ Corresponding author. Tel.: +1 509 375 6828; fax: +1 509 372 6421. E-mail addresses: [email protected] (W.G. Luscher), edgar.gilbert@pnnl. gov (E.R. Gilbert), [email protected] (S.G. Pitman), efl[email protected] (E.F. Love Jr.). 0022-3115/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2012.05.039

strength degradation due to oxidation and nascent hydrogen pickup be minimized. Thicker coatings were expected to offer greater capacity, but it was also possible that they would flake or spall due to the volumetric expansion associated with oxidation. The surface-modified components in this study are intended for an application that requires structural integrity to be retained, so flaking or spallation was unacceptable. Ultimately, the study was intended to determine an optimal surface-modification process that yields adequate oxidation rate and capacity while retaining the mechanical and structural integrity of the component. The oxidation of zirconium alloys is a complex process that is dependent on the specific alloy and oxidizing environment and also changes as the oxide layer develops. Initially, a relatively dense, protective layer of tetragonal zirconia readily forms on the surface of zirconium alloys in the presence of oxygen. It has been suggested by some authors that this protective layer is stabilized by compressive stresses near the metal-oxide interface. The compressive stresses decrease as the oxide layer grows and monoclinic zirconia begins to form on the outer surface. Monoclinic zirconia is less dense and the oxide layer becomes non-protective as it reaches a thickness of 2–3 lm [1,2]. Due to this transition in oxidation behavior, discussions of zirconium oxidation are typically

W.G. Luscher et al. / Journal of Nuclear Materials 433 (2013) 514–522

separated to describe either the more rapid parabolic (or cubic) behavior of the pre-transition period or the slower, linear behavior of the post-transition period. In either regime, Zr-4 is highly resistant to oxidation by steam and water compared to other metals, which makes it desirable for many applications [2–4]. Since nickel–zirconium (NiZr) intermetallics are known to readily oxidize [5], the surface-modification approach selected for this study consisted of plating Zr-4 substrates with Ni and performing a thermal treatment to react Ni and Zr-4. Since the solubility between Ni and Zr is limited and there are eight intermetallics in the NiZr phase diagram, an intermetallic coating was expected to result on the surface of the substrate [6,7]. Results from previous studies have revealed desirable microstructures resulting from the interdiffusion of Ni and Zr-4 [8–12]. In the present study, it was expected that the NiZr coating would oxidize rapidly until the coating becomes saturated with oxide. After saturation, the oxidation rate was expected to decrease significantly and asymptotically approach the much slower rate of the Zr-4 substrate. 2. Experimental procedure All test coupons were fabricated by cutting coupons from larger sections of Zr-4 strip (approximately 0.4 mm thick). Surfacemodified test coupons were cut from Zr-4 strip that had been plated with Ni to 2.5, 5, 7.5, or 10 lm. All test coupons were rectangular except for coupons intended for strength testing, which were cut into dog-bone shaped tensile specimens. Thermal treatments used to surface modify plated coupons were intended to fully react the Ni plating with the substrate. Unreacted Ni was not left on the surface because this would impede oxidation by water vapor [13]. The surface modification thermal treatment temperature was 760 °C ± 10 °C and was performed under a vacuum of 1.3  102 Pa or better (e.g., 3.3  103 Pa). The heating (ramp up) period was less than 2 h. Optimal hold times for a given plating thickness were determined experimentally and are as follows: 1. 2. 3. 4.

For For For For

plating = 2.5 lm, time ffi 1 h plating = 5 lm, time ffi 4 h plating = 7.5 lm, time ffi 12 h plating = 10 lm, time ffi 20 h

Following thermal treatment, the surface-modified test coupons were cooled to room temperature in vacuum for over 2 h. Test coupons were not exposed to air until the temperature was below 50 °C. The identity and rolling direction of each test coupon was preserved to maintain traceability to the original strip stock. Oxidation tests were conducted in a tube furnace with three different temperature zones. The reactant (i.e., oxidizing) gas consisted of heavy water vapor (di-deuterium oxide, D2O), which was supplied to a sample hangar in the tube furnace at a flow rate of 250 sccm in an inert carrier gas (dry argon (Ar)) at a partial pressure of 1 ± 0.1 kPa. The total system pressure was kept at 101 kPa. Selection of D2O as the reactant facilitated subsequent efforts to distinguish via thermal desorption nascent deuterium pickup from pre- or post-test hydrogen pickup. The reactant gas flowed from the high temperature zone (370 °C), to the intermediate temperature zone (330 °C), to the low temperature zone (290 °C) before being exhausted through a bubbler to the atmosphere. Before flowing through the bubbler, a humidity gage was used to monitor the concentration of D2O leaving the sample hangar to ensure that the oxidant was not significantly depleted. Both non-destructive and destructive tests were performed on test coupons before and after exposure to oxidation. Non-destructive evaluations such as weight and dimensional measurements were performed on samples to develop isothermal weight gain

515

curves. Dimensional measurements were made with an optical micrometer and were used to determine changes in surface area for weight gain and nascent deuterium pickup analyses. Results of isothermal weight gain curves were used to determine the intervals at which samples were removed for destructive evaluation. Destructive evaluation included nascent deuterium pickup analyses via thermal desorption, strength via tensile tests, hardness and fracture toughness via microindentation, and microstructural evaluation via scanning electron microscopy (SEM) augmented with energy dispersive X-ray spectroscopy (EDS). Samples were removed from the furnace for destructive evaluation after 1, 25, and 64 days of exposure to obtain measurements during the transient oxidation rate, after the transient oxidation rate (as the rate approached steady-state), and at the end of the test, respectively. 3. Results and discussion 3.1. Oxidant partial pressure During the test, a flow of argon containing nominally 1 kPa D2O was supplied to the test coupons at a rate of 250 sccm. Initially, the reaction between the sample and the oxidizing gas reduced the D2O pressure from the nominal inlet value of 1 kPa to an outlet value of 0.67 kPa. By the end of the first exposure period (24 h), the rate of D2O consumption by reaction with the surface-modified coupons had diminished and the outlet D2O pressure increased to 0.88 kPa. At the start of the following test period, a stable outlet D2O pressure of 1 kPa was achieved and maintained for the remaining 63 days of exposure. Because of the relatively brief period that the D2O pressure was below the designated test pressure, the test coupons were not considered starved for reactant and the test results of this study are associated with a constant D2O pressure of 1 kPa. 3.2. Microstructure The microstructural evolution of oxidized test coupons was investigated via SEM augmented with EDS on polished samples. Results of microstructural analyses are presented in Figs. 1 through 3 and reveal that the intermetallic coating becomes saturated with oxides with increased exposure to the oxidizing environment. This observation is consistent with the expectation that the intermetallic layer will preferentially oxidize and correlates with isothermal weight gain curves and nascent D pickup curves. Micrographs presented in Fig. 1 reveal the intermetallic coating thickness on test coupons modified with 2.5, 5, 7.5, and 10 lm of Ni. Thicker Ni plating required longer hold times to fully react the Ni plating with the Zr-4 substrate. Consequently, the intermetallic coating thickness increases with Ni plating thickness and ranges from 10 to 30 lm. Results from EDS were used to determine the composition of the layers, which range from Ni21Zr8 to NiZr2. As expected, the Ni content of the phases decreases with increasing distance from the surface. The two innermost intermetallic phases have been routinely identified by EDS as NiZr and NiZr2. The thickest intermetallic coatings were observed on test coupons plated with 10 lm of Ni and occasionally exhibited cracks, as shown in Fig. 1d, which often accompany multilayer coatings due to residual stresses that arise from differences in density and thermal expansion among the different phases. There was no spallation or delamination observed during oxidation testing for any plating thickness under the specified test conditions. Although the absence of spallation or delamination is desired to maintain the location of the reactive surface and minimize debris within the target rod, the intermetallic coating is sacrificial and delamination is not expected to be deleterious to the substrate in service.

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Ni21Zr8 Ni10Zr7 NiZr NiZr2

Zr-4

Zr-4

10μm

10μm

(a)

(b)

Ni21Zr8

Ni10Zr7 NiZr

NiZr2 Zr-4

Zr-4

Ni-rich precipitates 10μm

10μm

(c)

(d)

Fig. 1. As heat treated microstructures of test coupons modified with 2.5 (a), 5 (b), 7.5 (c), and 10 (d) lm of Ni.

Micrographs presented in Figs. 2 and 3 reveal oxygen ingress in test coupons modified with 2.5 and 10 lm of Ni, respectively. The dark phase that appears to progress inward from the surface was identified as oxygen-rich by EDS and is consistent with oxygen ingress. The formation of a discontinuous scale is also observed in the micrographs of oxidized surface-modified test coupons, particularly on the test coupons plated with 10 lm of Ni. Some areas of this scale are more contiguous than others, leading to the non-uniform diffusion of oxygen into the intermetallic coating observed in Figs. 2 and 3. Comparison between oxidized surface-modified and uncoated Zr-4 test coupons (Fig. 3c) clearly illustrates that surface modification enhances oxidation. Although the intermetallic coating thickness varied significantly with the plating thickness and thermal treatment, the micrographs indicate that each of the coatings become increasingly saturated with oxygen after prolonged exposure to the oxidizing environment. After saturation, the oxygen ingress appears to stop, or at least become significantly retarded, which correlates with the self-limiting behavior observed in isothermal weight gain and nascent hydrogen absorption. 3.3. Isothermal weight gain Isothermal weight gain data were collected from uncoated Zr-4 test coupons to form a baseline for comparison with urface-modified coupons. These results are summarized in Fig. 4, where the error bars are an estimate of the total uncertainty in weight gain per unit surface area, DW/SA, and are based on the precision of the balance (±0.1 mg) and sample surface area (3.87 cm2). Although the error bars overlap in some instances, the data reveal increased weight gain with temperature, as expected.

Measured oxidation rates were determined by linear regression of each isothermal data set in Fig. 4 and are expected to be in the pre-transition regime for Zr-4 oxidation [2]. Table 1 shows the measured rates at each test temperature. Oxidation rates ranging from 1.47 to 1.99 lg/cm2/day were reported by two previous studies at 350 °C and 100 kPa [14,15]. Assuming pre-transition oxidation has a relatively weak pressure dependence, as observed for post-transition oxidation [2], this indicates that the measured values are consistent with previously determined oxidation rates. Establishing the oxidation behavior of uncoated Zr-4 substrates provides a baseline for comparing results from surface-modified test coupons. Isothermal weight gain plots are presented in Fig. 5 through Fig. 8 for each Ni plating thickness studied. The plots reveal significantly enhanced initial oxidation relative to the baseline isothermal weight gain for uncoated Zr-4 (Fig. 4). The error bars represent the standard deviation for three replicate specimens exposed to the same test conditions. After the transient period of enhanced oxidation, the weight gain rate decreases rapidly and approaches a steady state rate of oxidation. The transition between the transient and steady state regimes of the isothermal weight gain plots becomes less pronounced with increased Ni plating thickness. This is illustrated in the isothermal weight gain plots for test coupons modified with 5 lm of Ni (Fig. 6), particularly at low temperature. Smoother transitions are also observed in test coupons plated with 7.5 and 10 lm of Ni (Figs. 7 and 8). Smoother transitions result from a more gradual saturation of the intermetallic layer with oxygen, which can be attributed to lower reaction rates at lower temperatures and/or an increase in oxidation capacity. Oxidation capacity was observed to increase with Ni-plating thickness, as expected. For example, after 64 days at 370 °C the average weight gains measured from

W.G. Luscher et al. / Journal of Nuclear Materials 433 (2013) 514–522

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Oxidized Outer Surface NiZr NiZr2

10μm

(a) Oxide Ingress

NiZr NiZr2

Intermetallic Coating Saturated with Oxides

10μm

10μm

(b)

(c)

Fig. 2. Test coupons modified with 2.5 lm of Ni and oxidized for 1 day at 290° (a), 330° (b), and 370 °C (c).

samples plated with 2.5, 5, 7.5, and 10 lm of Ni were 2.30, 3.40, 4.94, and 5.97 mg/cm2, respectively. Although the intermetallic layer was observed to saturate more gradually with increased plating thickness, weight gain rates continued to decrease rapidly with increased exposure time for each Ni plating thickness and temperature. However, the final steady state weight gain rates increase slightly with increased Ni plating thickness. The steady state oxidation rate for the 2.5 lm Ni plating is 3–5 lg/cm2/day, which is similar to that of uncoated Zr-4 at 370 °C (2.4 ± 0.9 lg/cm2/day) and consistent with the desired self-limiting behavior. After 64 days of oxidation at 370 °C, the weight gain rates of test coupons plated with 5 lm of Ni decreased to approximately 15 lg/cm2/day whereas test coupons plated with 7.5 and 10 lm of Ni decreased to approximately 22 lg/cm2/day. Although these oxidation rates are higher than the Zr-4 substrate (2.4 ± 0.9 lg/cm2/day), they are significantly lower than the initial transient oxidation rate and consistent with the desired self-limiting behavior. Differences in oxidation behavior between the Ni plating thicknesses studied can be rationalized by examining the corresponding microstructures. As shown in Section 3.2, the number and thickness of intermetallic phases increases with increased Ni plating thickness. The differences in oxidation capacity are attributed to the overall thickness of the intermetallic layer, and oxidation capacity appears to be proportional to thickness. In addition to thickness, the Ni concentration of the outer intermetallic layer increases with increased plating thickness. Coupons plated with 2.5 lm Ni exhibited an outer layer consistent with NiZr whereas coupons plated with 5, 7.5, or 10 lm of Ni exhibited an outer layer consistent with Ni10Zr7 or Ni21Zr8. Such compositional differences in the outer intermetallic layer(s) are also expected to influence oxidation kinetics and influence oxidation performance.

3.4. Oxidation kinetics Isothermal weight gain data were analyzed to determine the effect of Ni plating thickness on oxidation kinetics. An attempt was made to identify the rate controlling mechanism by determining the apparent activation energy for oxidation at each plating thickness. Parabolic, cubic, and logarithmic rate equations were considered for curve-fitting the isothermal data at each Ni plating thickness. The rate equation that provided the best overall fit at each temperature was used to obtain rate constants at each temperature. An Arrhenius plot was then developed using these rate constants and the apparent activation energy was determined from the slope of the plot. Despite efforts to fit the aforementioned rate equations to the isothermal weight gain data, differences in microstructure, composition, and coating thickness prevented a precise measurement of the apparent activation energy. Regarding microstructural and compositional differences, test coupons modified with 2.5 lm of Ni exhibited a two-layer microstructure consistent with NiZr2 and NiZr whereas test coupons modified with 5, 7.5, and 10 lm of Ni exhibited a four-layer microstructure consistent with NiZr, NiZr2, Ni10Zr7, and Ni21Zr8. In addition to microstructural and compositional differences, the oxidation capacity of the intermetallic coating is proportional to the plating thickness. Thinner intermetallic coatings exhibit less oxidation capacity, saturate more quickly, and require a change in the rate controlling mechanism(s) for continued oxidation. This is illustrated by the relatively sharp transition between transient and self-limited oxidation in Fig. 5 after 9 days. This leaves relatively few data points for curve-fitting and makes it difficult to fit with any rate equation. Nevertheless, the cubic rate equation provided a reasonable fit for test coupons modified with 7.5 lm of Ni. A subsequent Arrhenius

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NiZr

NiZr

NiZr2

NiZr2

10μm

10μm

(b)

(a) Oxide Saturated Intermetallic Coating

Outer Surface of Uncoated Zr-4 Oxidized

10μm

10μm

(c)

(d)

Fig. 3. Test coupons modified with 10 lm of Ni and oxidized for 64 days at 290°(a), 330° (b), and 370 °C (c). Uncoated Zr-4 oxidized for 64 days at 370 °C (d) is shown for comparison.

0.3

7.0

370ºC 290ºC

ΔW/SA (mg/cm2)

ΔW/SA (mg/cm2)

370ºC

6.0

330ºC

0.2

0.1

330ºC 290ºC

5.0 4.0 3.0 2.0 1.0

0.0

0

10

20

30

40

50

60

70

0.0

0

10

Time (days)

20

30

40

50

60

70

Time (days)

Fig. 4. Baseline weight changes for uncoated Zr-4 test coupons. Error bars represent the uncertainty introduced by the precision of the balance (±0.1 mg) and the sample surface area (3.87 cm2).

Fig. 5. Isothermal weight gain plot for test coupons surface-modified with 2.5 lm of Ni collected at 290°, 330°, and 370 °C. Weight change per unit surface area, DW/ SA, plotted as a function of exposure time.

Table 1 Comparison between oxidation rates measured for the uncoated Zr-4 test coupons (uncertainties represent a 95% confidence interval).

(0.42 eV/atom) [17], the data scatter prevents the oxidation mechanism from being conclusively identified.

Temperature (°C)

Measured (lg/cm2/day)

290 330 370

0.4 ± 0.5 1.5 ± 0.7 2.4 ± 0.9

analysis indicated an activation energy of 38 ± 25 kJ/mol. Although the nominal activation energy is similar to literature values reported for hydrogen diffusion in Ni2Zr (0.32 eV/atom) [16] and Ni

3.5. Nascent pickup During oxidation, species other than oxygen can become incorporated in the oxidizing material. Since D2O was used as the oxidant during this study, some nascent deuterium (D) was expected to be picked up during oxidation. Significant absorption of hydrogen species can lead to hydride formation and subsequent degradation. Consequently, the nascent pickup fraction (atoms absorbed/atoms reacted) was evaluated.

W.G. Luscher et al. / Journal of Nuclear Materials 433 (2013) 514–522

7.0 370°C

ΔW/SA (mg/cm2)

6.0

330°C 290°C

5.0 4.0 3.0 2.0 1.0 0.0

0

10

20

30

40

50

60

70

Time (days) Fig. 6. Isothermal weight gain plot for test coupons surface-modified with 5 lm of Ni collected at 290°, 330°, and 370 °C. Weight change per unit surface area, DW/SA, plotted as a function of exposure time.

7.0 370°C

ΔW/SA (mg/cm2)

6.0

330°C 290°C

5.0 4.0 3.0 2.0 1.0 0.0

0

10

20

30

40

50

60

70

Time (days) Fig. 7. Isothermal weight gain plot for test coupons surface-modified with 7.5 lm of Ni collected at 290°, 330°, and 370 °C. Weight change per unit surface area, DW/ SA, plotted as a function of exposure time.

7.0 370°C

ΔW/SA (mg/cm2)

6.0

330°C 290°C

5.0 4.0

519

The thermal desorption technique was used on surface-modified test coupons that had been oxidized for either 1, 25, or 64 days at each of the three test temperatures (290°, 330°, and 370 °C). Thermal desorption was performed with a LECO hydrogen analyzer on oxidized samples. Unexposed samples were also tested to provide a baseline evaluation of as-heat treated test coupons, which permitted accurate measurement of nascent D pickup. In addition to testing unexposed, surface-modified test coupons, coupons of unexposed, uncoated Zr-4 were analyzed via thermal desorption to determine whether or not surface modification introduced significant amounts of hydrogen. The average hydrogen atom fractions (H/Zr) detected in the uncoated Zr and the 2.5 and 10 lm surface-modified coupons were 0.008, 0.008, and 0.002, respectively. This indicated that surface modification (i.e. plating and heat treatment) did not introduce a significant amount of hydrogen into the Zr-4 substrates. The LECO thermal desorption and length change data are presented in Table 2. The average nascent D pickup fractions from the dimensional and LECO analyses are 0.69 and 0.50, respectively. The larger value computed from dimensional analyses is attributed to oxidation and additional swelling of the intermetallic coating. Results obtained from the LECO thermal desorption analyses are independent of geometric measurements and are considered to be more accurate than results obtained from length change data. Nevertheless, dimensional analysis provided a fast and bounding estimate of nascent D pickup relative to the more accurate determination by thermal desorption. Results of the LECO thermal desorption analyses for surface modified test coupons plated with 2.5 and 10 lm of Ni are also presented in Figs. 9 and 10, respectively. Based on Figs. 9 and 10, it appears that nascent D absorption occurs more rapidly in the first 25 days than it does between 25 and 64 days. This trend is similar to the isothermal weight gain behavior, as expected. Although the test coupon modified with 10 lm of Ni and oxidized at 370 °C did not exhibit a decrease in D absorption rate after 25 days, this behavior can be rationalized by the microstructure. Test coupons modified with 10 lm of Ni exhibited significantly thicker intermetallic layers, which possess greater oxidation capacity than thinner intermetallic coatings. With additional time, it is expected that test coupons modified with 10 lm of Ni would also reach a slower D absorption rate.

3.0

3.6. Strength testing

2.0 1.0 0.0

0

10

20

30

40

50

60

70

Time (days) Fig. 8. Isothermal weight gain plot for test coupons surface-modified with 10 lm of Ni collected at 290°, 330°, and 370 °C. Weight change per unit surface area, DW/SA, plotted as a function of exposure time.

Two different methods were used to determine the extent of nascent D pickup in the test coupons. In the first method, the D pickup was estimated from dimensional changes measured on weight gain samples. This method is based on an isotropic relationship between swelling and hydrogen absorption in Zr that associates 0.23% elongation for every 1000 wppm of absorbed hydrogen [18]. Using a method based on dimensional analysis provided a quick estimate of nascent pickup. The second method used to determine nascent pickup was thermal desorption. This is an analytical chemistry technique that was performed on a limited number of samples.

Tensile tests were performed on dog-bone shaped, uncoated Zr4 and surface-modified test coupons to determine the effects of surface modification and subsequent oxidation on strength. Tensile tests were performed by applying a tensile load perpendicular to the rolling direction. This approach was selected because it is known that basal planes, which are oriented parallel to the rolling direction, are susceptible to hydride formation. Since hydrides are associated with embrittlement and strength degradation, testing in this configuration was believed to provide a conservative, lowerbounding estimate for strength. In service, surface-modified components will oxidize and the intermetallic coating will become saturated with oxides. As indicated previously, the ingress of nascent hydrogen is expected to reach the Zr-4 substrate and promote hydride formation. As a result, the intermetallic coating of a surface-modified component and the Zr-4 substrate may become brittle after prolonged exposure to oxidizing environments containing hydrogen. The tests coupons were subjected to an unlimited supply of oxidant during the tests whereas the anticipated service environment will expose the materials to a limited supply of oxidant. Consequently, the tensile tests performed on oxidized surface-modified test coupons in

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W.G. Luscher et al. / Journal of Nuclear Materials 433 (2013) 514–522 Table 2 Summary of deuterium analyses. The ‘‘±’’ values for DL/Lo are based on three measurements and the ‘‘±’’ values for LECO are based on the number of measurements listed. Oxidation temperature (°C)

Exposure (days)

Ni thickness (urn)

D/Zr (AL/Lo)

D/Zr (LECO)

Number of measurements

290 290 290 290 290 290 330 330 330 330 330 330 370 370 370 370 370 370

1 25 64 1 25 64 1 25 64 1 25 64 1 25 64 1 25 64

2.5 2.5 2.5 10 10 10 2.5 2.5 2.5 10 10 10 2.5 2.5 5 10 10 10

0.012 ± 0.008 0.072 ± 0.012 0.078 ± 0.006 0.018 ± 0.011 0.078 ± 0.001 0.106 ± 0.002 0.017 ± 0.014 0.076 ± 0.009 0.096 ± 0.020 0.025 ± 0.005 0.106 ± 0.008 0.127 ± 0.012 0.057 ± 0.010 0.085 ± 0.020 0.194 ± 0.045 0.045 ± 0.007 0.235 ± 0.011 0.313 ± 0.009

0.0062 ± 0.0018 0.0526 ± 0.0019 0.0536 ± 0.0045 0.0108 ± 0.0026 0.0496 ± 0.0000 0.0639 ± 0.0061 0.0242 ± 0.0032 0.0631 ± 0.0016 0.0662 ± 0.0022 0.0248 ± 0.0005 0.0789 ± 0.0027 0.0927 ± 0.0113 0.0519 ± 0.0018 0.0719 ± 0.0001 0.1092 ± 0.0033 0.0357 ± 0.0018 0.1009 ± 0.0042 0.1894 ± 0.0103

2 2 4 2 2 4 3 2 3 2 2 2 3 2 3 2 3 4

0.25 370°C 330°C 290°C

0.15 0.10 0.05 0.00

0

10

20

30

40

50

60

70

Time (days) Fig. 9. LECO thermal desorption results for deuterium content in test coupons surface-modified with 2.5 lm of Ni. Results for test coupons surface-modified with 5 lm of Ni at 370 °C are shown for comparison. Error bars represent the standard error on two to four measurements.

Ultimate Tensile Strength (MPa)

D/Zr atom ratio

5 micron at 370°C

0.20

800

Zr-4

2.5

5

7.5

10

700 600 500 400 300 200 100 0

0

1

25

64

Exposure Time (days) Fig. 11. Ultimate tensile strengths for samples surface modified with each of the four Ni plating thicknesses and the uncoated Zr-4 test coupons as a function of exposure time at 290 °C.

D/Zr atom ratio

370°C 330°C

0.20

290°C

0.15 0.10 0.05 0.00

0

10

20

30

40

50

60

70

Time (days) Fig. 10. LECO thermal desorption results for deuterium content in test coupons surface-modified with 10 lm of Ni. Error bars represent the standard error on two to four measurements.

this study are considered conservative for their particular application. Results of tensile tests performed on uncoated Zr-4 and surfacemodified test coupons following oxidation at 290°, 330°, and 370 °C are presented in Figs. 11–13, respectively. Error bars, where present, represent the standard deviation on up to three independent

Ultimate Tensile Strength (MPa)

0.25

800

Zr-4

2.5

5

7.5

10

700 600 500 400 300 200 100 0

0

1

25

64

Exposure Time (days) Fig. 12. Ultimate tensile strengths for samples surface modified with each of the four Ni plating thicknesses and the uncoated Zr-4 test coupons as a function of exposure time at 330 °C.

measurements. Based on these results, the surface-modification heat treatment enhanced the strength of the test coupons relative to the uncoated Zr-4 coupon, particularly for coatings of 5 lm or greater. Enhanced strength was maintained by all surface-modified

521

800

Zr-4

2.5

5

7.5

10

700 600 500 400 300 200 100 0

4.0

Average Hardness (GPa)

Ultimate Tensile Strength (MPa)

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3.5 3.0 2.5 2.0 1.5

0

25

64

test coupons after 1 day of oxidation at each test temperature. Enhanced strength relative to uncoated Zr-4 test coupons was maintained throughout the 64 days of exposure for test coupons modified with 2.5 lm of Ni at all test temperatures. Similar results were observed for test coupons modified with 5 lm of Ni at 290° and 370 °C. At 330 °C, the strength of test coupons modified with 5 lm of Ni had degraded below that of the uncoated Zr-4 after 25 and 64 days of exposure. Strength results for test coupons modified with 7.5 and 10 lm of Ni exhibited lower strength than uncoated Zr-4 after 25 and 64 days of exposure at all temperatures. Failure strains decreased with increased plating thickness and exposure time, as expected. The nominal unexposed, baseline strains obtained for bare Zr-4, 2.5-, and 10 lm Ni plated samples were approximately 0.35, 0.23, and 0.10, respectively. Although the total elongation of the bare Zr-4 sample did not change significantly, the 2.5 and 10 mm Ni plated samples exhibited total elongations of 0.1 and 0.01, respectively, after 64 days of oxidation. Overall, there was a trend for increased strength degradation with increased Ni plating thickness, oxidation temperature, and exposure time, as expected. Strength degradation was most severe after test coupons modified with 10 lm of Ni were oxidized for 64 days at 370 °C. However, these test coupons still exhibited an average ultimate tensile strength of 214 MPa, which is adequate for the intended application. 3.7. Hardness Hardness was measured via microindentation with a Vickers tip on polished cross sections of surface-modified and bare Zr-4 test coupons. Hardness data were obtained in both the longitudinal and transverse rolling directions of the test coupons to evaluate texture effects on hydrogen and oxygen ingress. No measurements were made in the intermetallic coating; however, five equally spaced measurements were made through the thickness of the substrate to discern hardness gradients. Results of microindentation tests were expected to reveal whether or not chemical gradients in the substrate, which can form by interdiffusion of Ni during surface modification or the ingress of oxygen and/or hydrogen during oxidation, affected the mechanical properties across the substrate. Microindentation tests did not reveal any statistically significant differences between the transverse and longitudinal rolling directions of the test coupons. In addition, there were no hardness gradients detected through the thickness of the substrate. As a result, data collected from the two surfaces were combined to provide an average hardness measurement for each test coupon.

290-10

330-2.5

370-2.5

370-10

370-Zr-4

1

25

330-10

64

Time (days)

Exposure Time (days) Fig. 13. Ultimate tensile strengths for sample surface modified with each of the four Ni plating thicknesses and the uncoated Zr-4 test coupons as a function of exposure time at 370 °C.

0

290-2.5

Fig. 14. Average hardness as a function of exposure time for selected test coupons.

Results of the hardness tests are summarized in Fig. 14, which presents the average hardness of the material as a function of exposure time to oxidation at the specified temperature. Test results are labeled by the exposure temperature followed by the thickness of the Ni plating before surface modification. For example, a sample exposed to oxidation at 370 °C that had been surface-modified with 10 lm of Ni is labeled 370-10. Although the error bars, which represent the standard deviation of ten independent measurements on each sample, overlap, there is a subtle trend within the error of the measurement for hardness to increase with increasing exposure time. This trend is consistent with hardening due to oxygen and hydrogen ingress despite the lack of hardness gradients detected within the substrates. Nevertheless, the overlap of the error bars indicates that the trend is not statistically significant. As a result, the Zr-4 substrates of the surface-modified test coupons do not appear to exhibit significant changes in hardness and appear to be insensitive to surface modification and oxidation. In addition to hardness, surface-modified samples were evaluated for fracture toughness via microindentation. Fracture toughness measurements were made by measuring the lengths of the cracks that typically emanate from the corners of the Vickers microindents under sufficient loads. It was expected that oxidation would result in reduced fracture toughness, which in turn would result in increased crack lengths. Despite increased indentation loads (up to 25 kg) and regardless of exposure time and oxidation temperature, no cracks were observed at the corners of the indents. Although this prevented a fracture toughness value from being determined, it did suggest that the fracture toughness of the substrate was not significantly affected.

4. Conclusions Results of this study have confirmed the expected oxidation behavior of Zr-4 substrates modified with NiZr intermetallic coatings. Isothermal weight gain data have revealed initially enhanced oxidation rates that asymptotically approach the slower rate of the Zr-4 substrate, thus exhibiting self-limiting oxidation. Micrographs revealed that the intermetallic coating preferentially oxidizes until it becomes saturated, which correlates with the self-limiting trend observed in isothermal weight gain data. Nascent deuterium analyses by thermal desorption indicated a pickup fraction of 0.5 and also correlated with isothermal weight gain data. Analysis of the oxidation kinetics suggested that oxidation by water vapor was controlled by hydrogen diffusion, but significant data scatter prevents the rate controlling mechanism from being conclusively identified. Regardless, the results of this study indicate that Zr-4 substrates surface-modified with NiZr intermetallics satisfy the

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need for components that exhibit enhanced, yet self-limited, oxidation that is limited to the coated surface. Evaluation of different Ni plating thicknesses and thermal treatments permitted the effects of these surface modification parameters to be determined. Thicker Ni plating required longer heat treatments to completely react with the substrate and resulted in thicker intermetallic coatings. Oxidation capacity increased with increased intermetallic thickness, which also led to increased pickup of nascent deuterium. Although cracks were observed in the intermetallic layers of test coupons modified with 10 lm of Ni, no flaking or spalling was observed, which confirms that the structural integrity of surface-modified test coupons was preserved. Mechanical testing revealed strength degradation increased with oxidation and deuterium pickup, as expected. Although mechanical testing revealed strength degradation after oxidation, it also indicated that the extent of degradation could be manipulated by selecting the appropriate parameters for surface modification. Hardness measurements performed on cross-sections of the substrate showed no significant dependence on exposure time. Additional indentation measurements were performed to evaluate fracture toughness. However, the absence of cracking suggested that the fracture toughness of the substrate was not affected by surface modification or subsequent oxidation. Overall, the results of this study indicate that surface modification of Zr-4 with NiZr intermetallics provides a viable path for fabricating components with tailored oxidation performance. The oxidation capacity and extent of mechanical degradation are proportional to the thickness of the intermetallic coating, which can be manipulated by selecting the appropriate surface modification

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