The Pennsylvania State University. The Graduate School THE EFFECT OF HYDROGEN ON THE DEFORMATION BEHAVIOR OF ZIRCALOY-4.

Transcription

1 The Pennsylvania State University The Graduate School Department of Mechanical and Nuclear Engineering THE EFFECT OF HYDROGEN ON THE DEFORMATION BEHAVIOR OF ZIRCALOY-4 A Thesis in Nuclear Engineering by Michelle E. Flanagan Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science December 2008

2 The thesis of Michelle E. Flanagan was reviewed and approved* by the following: Arthur T. Motta Professor of Nuclear Engineering and Materials Science and Engineering Thesis Co-Advisor Donald A. Koss Professor Emeritus of Materials Science and Engineering Thesis Co-Advisor Jack S. Brenizer J. Lee Everett Professor of Mechanical and Nuclear Engineering Program Chair of Nuclear Engineering *Signatures are on file in the Graduate School ii

3 ABSTRACT Zirconium alloys are used in Light Water Reactors (LWR) as fuel cladding, serving as the first barrier to fission product release by physically separating the uraniumoxide fuel pellets and the primary coolant. Although the deformation behavior of zirconium alloys as a function of temperature and the strong influence of hydrogen on the fracture behavior of zirconium alloys have been fairly well established, relatively little is known of the effects of hydrogen (and the resulting hydrides) on the deformation behavior of zirconium alloys over a range of temperatures. The potential presence of such effects is especially important to high burnup fuel where hydrogen uptake within the cladding may be substantial. Deformation behavior is one of the critical inputs for fuel modeling codes, and therefore a mechanistic understanding of deformation behavior is critical for reactor safety. The purpose of this study is to quantify the effect of hydrogen on deformation behavior of cold worked and stress-relieved Zircaloy-4 plate material as a function of specimen orientation (i.e., stress axis aligned with the rolling direction, the transverse direction, or normal-to-the plate surface direction), temperature (25, 300, and 400 C), and strain rate (from 10-4 /s to 10-2 /s). Testing for this study was performed primarily in uniaxial compression because, in contrast with tensile testing, compression testing permits the observation of uniform deformation to higher strains so that a more accurate assessment of changes in deformation modeling parameters for strength, strain-hardening and strain-rate hardening with temperature and hydrogen content can be made. iii

4 A comparison of tensile and compressive stress-strain behavior of the Zircaloy-4 plate used in this study showed slightly higher but similar yield stresses and identical strain-hardening. The plate material exhibits plastic anisotropy similar to cladding tube behavior, such that through-thickness deformation is quite difficult. Thus, deformation behavior of the Zircaloy-4 plate in compression should be relevant to in-service loading of Zircaloy-4 cladding in tension. A plasticity analysis of the Zircaloy-4 plate using the Hill yield criterion also supports the notion that plastic deformation under throughthickness compression is equivalent to equal-biaxial tension, which is a strain path of particular interest for cladding behavior in reactivity-initiated accidents, where pelletcladding interactions and fission gas expansion can be significant. Only minor changes in deformation behavior as a function of hydrogen content were observed in this study, and these changes appear to be related to the annealing time experienced during the hydriding procedure. For samples which were annealed for 70 hours, there was no significant change in either yield stress or strain-hardening with increased hydrogen. However, for samples annealed for less than 20 hours during hydriding, there was a small decrease in yield stress, a significant increase in strainhardening in the rolling and normal directions and a more gradual transition from elastic to plastic behavior with increasing hydrogen content. In all cases, there was no significant effect of hydrogen on the strain-rate hardening or the plastic anisotropy of the plate material. The results of this study suggest that the small effects of hydrogen on the deformation behavior of Zircaloy-4 are caused by dislocations which form upon hydride precipitation to accommodate the difference in specific volume of hydride precipitates iv

5 and the Zircaloy-4 matrix, rather than by the hydride precipitates themselves. Thus, the observed changes in deformation behavior are sensitive to the annealing time experienced during hydrogen charging as a result of the associated recovery and recovery process, such that the effect of hydrogen on deformation behavior was diminished by long ( 70 hrs) annealing times. v

6 TABLE OF CONTENTS LIST OF FIGURES...vii LIST OF TABLES...x ACKNOWLEDGEMENTS...xi Chapter 1 - Introduction Zirconium in the Nuclear Industry Hydride Formation in Zirconium Alloy Cladding High Strain-Rate Transients Previous Studies on Mechanical Behavior of Zircaloy Statement of Purpose...6 Chapter 2 - Experimental Procedure Characterization of As-Received Material Hydriding Procedures Hydriding Procedure #1 (280, 400, and 420 wt ppm H) Hydriding Procedure #2 (163, 333, and 620 wt ppm H) Mechanical Testing...18 Chapter 3 - Results and Discussion Deformation Behavior and Texture of Plate Material and its Relationship to Cladding Material Hydride Microstructures Effect of Hydrogen on Yield Behavior Effect of Hydrogen on Strain Hardening Behavior Effect of Hydrogen on Strain-Rate Hardening Behavior...58 Chapter 4 - Conclusions...61 Bibliography...64 vi

7 LIST OF FIGURES Figure 1-1: Hydrogen Absorption in the Oxidation Reaction...2 Figure 2-1: Optical micrograph showing the grain structure of the as-received plate...9 Figure 2-2: A schematic showing the nomenclature for test specimen orientation within the Zircaloy-4 plate examined in the present study and how it correlates to orientations within cladding tube...10 Figure 2-3: Thermal and hydrogen environment cycle diagram for hydriding Procedure # Figure 2-4: Thermal and hydrogen environment cycle diagram for one cycle of hydriding Procedure # Figure 2-5: Light microscope micrograph of hydrided material with (a) 420 wt ppm +/- 50 wt ppm using Procedure #1 and (b) 620 wt ppm +/- 50 wt ppm using Procedure # Figure 2-6: Cubic compression samples (a) before and (b) after compression testing to 10% strain...18 Figure 2-7: (a) Instron model 4206 fitted with compression rams and clam shell furnace. (b) Clam shell furnace opened to reveal steel platens and compression sample within...19 Figure 2-8: Typical curve fits (dotted line) for (a) Ln σ Ln ε p data (solid line) used to determine the strain-hardening exponent and (b) power law fit (dotted line) for the stress-strain data (solid line) indicating the same strainhardening exponent...22 Figure 2-9: True stress as a function of true strain measured during data collection for strain-rate hardening Figure 3-1: The true stress-true strain response of hydrided (333 wt ppm H) and unhydrided Zircaloy-4 plate as determined in tension and compression for specimens oriented along the transverse direction of the plate; see Figure Figure 3-2: Basal pole figures for (a) Zircaloy-4 plate material, normal direction [22] and (b) Zircaloy-4 cladding tubing, radial direction [30] vii

8 Figure 3-3: (a) Identification of preferred slip system in zirconium hcp crystals and (b) a schematic of hcp crystallographic orientation relative to plate directions based on the measured pole figure...32 Figure 3-4: The effect of hydrogen on yield stress anisotropy as defined by the ratio of the yield stress in the rolling orientation to that in either the normalto-plate-surface or transverse orientations...36 Figure 3-5: Micrographs of polished and etched samples of plate material with (a) 280 wt ppm, (b) 400 wt ppm, (c) 420 wt ppm, and (d) 620 wt ppm, to reveal hydride orientation and distribution...37 Figure 3-6: Micrograph of polished and etched sample of high-burn up (67 GWd/t) cladding material with 800 wt ppm hydrogen with typical hydride orientation and distribution [34] Figure 3-7: Micrographs of (a) sample hydrided with 420 wt ppm H and (b) a sample hydrided to 620 wt ppm H...40 Figure 3-8: Yield stress (0.2% strain) as a function of hydrogen content...41 Figure 3-9: The stress-strain behavior of as-received and wt ppm hydrided material (produced by hydriding procedure one) in the (a) rolling direction, (c) transverse direction, and the (e) normal direction and the stressstrain behavior of as-received and 620 wt ppm hydrided material (produced by hydriding procedure two) in the (b) rolling direction, (d) transverse direction, and (f) normal direction. The 0.02% and 1% yield limits are indicated...42 Figure 3-10: The normalized yield stress difference parameter (YD) as a function of hydrogen content Figure 3-11: The normalized yield difference parameter as a function of annealing time...46 Figure 3-12: The strain-hardening exponent as a function of hydrogen content at (a) room temperature (b) 300 C, and (c) 400 C for Zircaloy-4 plate...52 Figure 3-13: The strain-hardening exponent as a function of temperature for asreceived and hydrided Zircaloy Figure 3-14: Solubility limit of hydrogen in α-zirconium as a function of temperature [3]...55 Figure 3-15: The strain-hardening exponent as a function of precipitated hydrogen content at 400 C...56 viii

9 Figure 3-16: Increase in strain-hardening from as received material as a function of annealing time for samples tested at room temperature...58 Figure 3-17: The strain-rate hardening exponent as a function of (a) hydrogen content at room temperature and (b) temperature for as-received Zircaloy-4 plate...60 ix

10 LIST OF TABLES Table 1-1: Chemical Composition of Zircaloy Table 2-1: Chemical Composition of Zircaloy-4 Plate Material...10 Table 3-1: Mechanical Behavior Properties of Zircaloy-4 CWSR plate examined in this study as compared to Zircaloy-4 cladding tube...27 Table 3-2: Experimental Plastic Anisotropy Strain Ratios of Zircaloy x

11 ACKNOWLEDGEMENTS I would like to thank multiple people for their guidance, assistance and contribution during this study. To begin, I would like to thank my advisors Dr. Motta and Dr. Koss for their guidance and insight during this study, and for their commitment to ensuring a valuable and comprehensive learning experience. Next I would like to thank those in the fuels team at NRC, particularly Ralph Meyer, for their role in shaping this excellent and fulfilling learning experience and for their encouragement over my entire time with NRC. In addition, I d like to thank Patrick Raynaud for the time he spent throughout the last two years teaching me to operate equipment, sharing his technical expertise as well as his practical advice. I am also grateful to Rob Daum for his assistance in hydrogen analysis and also for sharing his technical expertise. I d also like to thank Jean-Luc Béchade for his assistance in generating pole figures and performing texture analysis. I am grateful to my parents for giving me so many opportunities in life, without which I would not be where I am today. Finally, I would like to thank Jeremy for his support, encouragement and patience throughout the last two years, particularly during the writing of my thesis. xi

12 Chapter 1 - Introduction 1.1 Zirconium in the Nuclear Industry Zirconium alloys are used in Light Water Reactors (LWR) as fuel cladding, serving as the first barrier to fission product release by physically separating the uraniumoxide fuel pellets and the primary coolant. Zirconium alloys were chosen for this application primarily due to their low neutron cross section, which allows the reactor to operate with high efficiency. Zirconium alloys also exhibit good corrosion resistance, adequate mechanical strength, and are relatively resistant to radiation effects such as creep and growth, while exhibiting little swelling [1]. Zircaloy-4 is the zirconium alloy that has been most widely used in Pressurized Water Reactors (PWR) throughout the years. Table 1-1 presents the chemical composition of Zircaloy-4 as specified by ASTM standard B351 [2]. Zircaloy-4 consists of a hexagonal closed packed (hcp) zirconium matrix containing tin and oxygen in solid solution with a fine dispersion of intermetallic precipitates of the form Zr(Cr,Fe) 2 and Zr 2 (Ni,Fe) [1]. Zircaloy-4 cladding tubing is produced by extrusion, which results in strong anisotropic mechanical behavior. Table 1-1: Chemical Composition of Zircaloy-4 Element Zr Sn Fe Cr O Weight % Balance

13 1.2 Hydride Formation in Zirconium Alloy Cladding The inner wall surface of the fuel cladding is in contact with uranium-oxide fuel and fission products at 400 C, while the outer wall surface of the fuel cladding is in contact with coolant water at approximately 350 C. This reactor environment is extremely challenging, resulting in corrosion oxidation at the outer surface of the cladding and the consequent absorption of hydrogen (produced in the corrosion reaction) into the cladding. Although the oxide layer acts as a barrier to hydrogen ingress, approximately 15% of the hydrogen produced in the oxidation reaction is absorbed by the cladding [1], resulting in end-of-life hydrogen contents as high as 800 wt ppm in high-burn up cladding (compared to as-fabricated hydrogen contents of approximately 40 wt ppm H). When the hydrogen content exceeds the solubility limit in zirconium (approximately 15 wt ppm at 200 C and 200 wt ppm at 400 C [3]) it precipitates as zirconium-hydride precipitates (usually ZrH 1.66 ) [1]. Therefore, at normal operating temperatures, much of the hydrogen present in the cladding can exist as hydride precipitates. The schematic below, Figure 1-1, illustrates the oxidation reaction and hydrogen absorption. Zr + 2H 2 O ZrO 2 +4H 85% 15% Coolant (H 2 O) ZrO 2 ZrH y UO 2 Fuel Figure 1-1: Hydrogen Absorption in the Oxidation Reaction 2

14 The mechanical behavior of zirconium hydrides is very different from that of the zirconium matrix. Extensive research has confirmed that the accumulation of hydrides in cladding material results in ductility loss and can alter mechanical behavior [4-14]. The exact effect of hydrides on the ductility and mechanical behavior of cladding material depends on the orientation and distribution of the hydride precipitates, which is a function of the cladding texture, stress state and temperature [15]. The texture (preferred distribution of grains of a particular crystallographic orientation) of zirconium alloys results from the forming and extrusion of cladding material. The resulting texture causes most of the hydrides to be oriented in the circumferential direction of the cladding (as indicated in Figure 1-1) during in-service loading [16]. The temperature gradient, resulting from the heated inner cladding surface and the water-cooled outer cladding surface, leads to preferential precipitation in the cooler outer surface of the cladding, forming a dense hydride rim. Because the presence of hydrogen in the form of hydride platelets can have deleterious effects on the fracture and failure behavior of zirconium alloy cladding, the amount, orientation and distribution of hydrogen in zirconium cladding is a key factor in predicting cladding response to transient loading. Fortunately, the preferred orientation (hydride platelets in the circumferential cladding direction) during in-service loading is much more benign than radially oriented hydrides in terms of cladding failure in the event of a circumferential strain, such as seen during in a Reactivity Initiated Accident [16]. 3

15 1.3 High Strain-Rate Transients The Reactivity Initiated Accident (RIA) is a potentially high consequence, lowprobability, design basis accident, postulated for the licensing of LWRs. During an RIA, the ejection (in a PWR) or drop (in a Boiling Water Reactor) of a control rod causes an instantaneous power increase, resulting in a near instantaneous fuel temperature increase. This temperature increase causes the fuel pellet to expand and fission gases to be released. The fuel expansion loads the cladding through pellet-cladding mechanical interaction and the release of fission gases provides additional loading. This loading combination creates a strain path thought to be between equal-biaxial strain and plane strain in the circumferential direction, and the deformation occurs at a moderately high strain rate [17]. The deformation and fracture behavior of cladding under transient loading conditions determines if the strain experienced leads to cladding failure. Cladding failure can be defined either as fuel rod failure or loss of fuel coolability by the safety criteria established by the United States Nuclear Regulatory Commission (NRC) [18]. Fuel rod failure is defined by a breach in the cladding wall, resulting in a release of radioactivity to the reactor coolant. Cladding should remain intact (unfailed) during normal operation, and the release of radioactivity from rods with failed cladding after accidents must be accounted for in dose calculations. Loss of fuel coolability is defined by a deformation and fracture response of cladding that alters rod-bundle geometry significantly enough to obstruct coolant channels, preventing post-accident core cooling. 4

16 Loss of fuel coolability is unacceptable in any accident scenario, and fuel cladding must be designed to prevent loss of fuel coolability. Since hydrogen content, and therefore hydride precipitation, increases with fuel burn-up, the effects of high hydrogen concentrations on cladding deformation and fracture behavior became increasingly more important as the industry pushes for higher burn-up operation. Understanding the deformation and fracture behavior of highly hydrided Zirconium alloy cladding is necessary to characterize cladding failure and to ensure safe operation during postulated design-basis accidents even at high burn-up cladding conditions. 1.4 Previous Studies on Mechanical Behavior of Zircaloy-4 The influence of temperature on the deformation and failure behavior of zirconium alloys (and Zircaloy-4 cladding in particular) has been the subject of several investigations [5-8, 19]. As a result, the yield stress, strain hardening, and strain-rate hardening responses of these alloys have been fairly well established and serve as a basis for current computational codes. Similarly, numerous studies have identified the strong influence of hydrogen and hydrides on the fracture behavior of zirconium-based alloys in general, and Zircaloy cladding in particular [4-14, 20]. Previous studies at Penn State on the mechanical behavior of hydrided Zircaloy-4 have focused on fracture and ductility. Specifically, these studies have investigated failure under fracture mechanics conditions of hydride microstructures relevant to 5

17 hydride rims and hydride blisters [10, 21], and have measured the ductility of hydrided Zircaloy in axial and ring tension tests [7, 9]. In summary, the deformation behavior of zirconium alloys as a function of temperature has been fairly well established, and the influence of hydrogen and hydrides on the fracture behavior of zirconium alloys has been the subject of multiple studies [4-14, 19, 20]. In contrast, relatively little is known of the effect of hydrogen on the deformation behavior of these plastically anisotropic alloys over a range of temperatures. Deformation behavior is a crucial input for fuel modeling codes, and precedes fracture; therefore a mechanistic understanding of deformation behavior is critical for reactor safety. Particularly, understanding the influence of hydrogen is increasingly necessary, as fuel is utilized to higher burnups. 1.5 Statement of Purpose The purpose of this investigation is to examine the uniaxial deformation behavior of Zircaloy-4 as a function of hydrogen level (~45 to ~600 wt ppm), temperature (25, 300, and 400 C), and strain rate (10-4 /s to 10-2 /s). Strongly textured Zircaloy-4 plate is used as a model material to allow tests to be performed as a function of orientation of deformation axis (rolling direction, transverse direction, and normal-to-plate surface direction) and to obtain data on the influence of hydrogen content on the modeling parameters including the yield strength, strain hardening, and strain-rate hardening, as well as the plastic anisotropy of Zircaloy-4. 6

18 Plastic deformation of cladding has been modeled in the material properties database, MATPRO [19], according to the following constitutive relationship: σ & ε Kε 10 n = p 3 m (1-1) where the true stress (σ) is proportional by K, the strength coefficient, to the true plastic strain (ε p ) raised to a strain-hardening exponent, n, and the normalized strain rate (ε& /10-3 ) raised to a strain-rate hardening exponent, m. The goal of this investigation is to quantify the influence of temperature and hydrogen concentration on the parameters K, n, and m. Compression testing is used in this work instead of the more common tensile testing. In contrast with tensile testing, compression testing permits the observation of uniform deformation behavior to high strains, enabling a more accurate assessment of changes in deformation modeling parameters K, n, and m with temperature and hydrogen content. The results of this research will be used to assess the models for cladding deformation behavior in existing codes. The study is intended to furnish data on the influence of hydrogen content, temperature and strain rate, such that more robust and accurate temperature-dependent constitutive relationships can be developed for hydrided cladding tube materials. 7

19 Chapter 2 - Experimental Procedure This chapter describes the experimental methods, including the specifications of the Zircaloy-4 plate material, the procedures used to hydride the as-received material, and the mechanical testing procedures used in this study. 2.1 Characterization of As-Received Material The material used in this study was 4.5 mm thick Zircaloy-4 plate obtained from Teledyne Wah-Chang. The plate material was hot rolled, and then 30% cold rolled and stress-relief annealed at 500 C for one hour in a vacuum furnace at 10-6 Torr. This processing procedure resulted in a microstructure consisting of slightly elongated grains measuring about 15 µm in the plate rolling direction and about 10 µm in the plate normal direction, as seen in Figure 2-1 (The micrograph shown in Figure 2-1 was obtained using a light microscope with polarized light, after polishing and swab etching the surface for seconds with an acid solution of 45% H 2 0, 45% nitric acid (70%) and 10% hydrofluoric acid (52%)). Metallography showed no change in grain size and grain shape after stress relief, indicating no recrystallization had taken place during the one hour stress relief. Comparing the grain structure of the plate to that of typical thin-wall Zircaloy-4 cladding tube, the cladding shows significantly greater grain elongations in the equivalent orientations, with dimensions of about µm in the axial direction and 2-5 8

20 µm in the radial direction (see Figure 2-2 for corresponding reference directions for plate and cladding material). N 20µm T R Figure 2-1: Optical micrograph showing the grain structure of the as-received plate As is the case for cold worked and stress relieved Zircaloy-4 cladding tube, the plate exhibits a strong crystallographic texture. Thus, the deformation behavior and microstructural characteristics are likely to be anisotropic with respect to plate orientation and consequently testing for this study was performed in each direction of the as-received plate material, rolling (R), transverse (T) and normal-to-plate-surface (N). When correlated with cladding tube directions, these orientations correspond to the axial, circumferential and radial cladding directions, respectively, as shown in Figure

21 Model Material (N) Normal (N) Transverse (T) (T) (R) Rolling (R) Cladding Material Axial Radial (through thickness) Circumferential Rolling Axial Transverse Circumferential Normal Radial Figure 2-2: A schematic showing the nomenclature for test specimen orientation within the Zircaloy-4 plate examined in the present study and how it correlates to orientations within cladding tube. The chemical composition was measured by Luvak Inc. (Boylston, MA) using hot-vacuum extraction. The results are shown in Table 2-1. Table 2-1: Chemical Composition of Zircaloy-4 Plate Material Element Weight % Zirconium 97.9 Tin 1.52 Iron 0.23 Oxygen Chromium 0.11 Carbon Silicon Hydrogen Molybdenum Niobium <

22 X-Ray Diffraction (XRD) was used to generate direct pole figures to reveal the orientation dependence of the intensity distribution of the basal poles of the hexagonal close packed (hcp) grains in the plate and to calculate the associated Kearns Factors. XRD measurements were completed at Saclay Laboratory of the Commission of Atomic Energy (CEA) in France courtesy of J. L. Béchade. Calculated pole figures were generated based on five diffraction angles, chosen to identify the (00.2), (10.0), (10.1), (10.2), (11.0) planes using CuKα radiation. The specimen tilt (angle between the plate normal and the diffraction vector) was varied between 0 and 75 [22]. These crystallographic textures help to assess the plastic anisotropy of the plate. The Kearns factor is a parameter that quantifies basal pole intensity and is calculated for a given reference direction [15, 23]. The Kearns factor is defined as the resolved fraction of basal poles along a particular macroscopic direction. For example, for the normal direction of the model material, the Kearns factor (denoted f N ) would be given by: π / 2 2 f N = I Φ sin Φ cos ΦdΦ (2-1) 0 where I Φ is the average basal pole density at an angle Φ from the reference direction (in this case the normal direction). The value of the Kearns factor can vary between 0 (indicating perfect alignment of basal poles perpendicular to the reference direction), and 1 (indicating perfect alignment of basal poles parallel to the reference direction). When basal poles are randomly distributed in all directions, the Kearns factors f N, f R, and f T equal 1/3 for the three reference directions [23]. The texture measurement results for the plate material are presented in Chapter 3. 11

23 2.2 Hydriding Procedures The as-received material was analyzed by hot-vacuum extraction to determine the initial hydrogen concentration of approximately 45 wt ppm. The objective of this investigation is to examine the effects of temperature, strain-rate and hydrogen concentration of the deformation behavior of Zircaloy-4 for uniform hydrogen contents of wt ppm hydrogen, which is the upper range of hydrogen content in highburn up cladding. Therefore, gas-charging was used to hydride material according to the following procedures. In air, zirconium alloys form a zirconium oxide layer on their surface, which significantly inhibits hydrogen absorption. To facilitate the absorption of hydrogen, the native oxide was removed by chemical etching using a mixture of 1 part hydrofluoric acid, 10 parts nitric acid, 10 parts H 2 O. Within 15 minutes of this cleaning procedure, the specimen was inserted into a high vacuum system (10-6 Torr) and subsequently a thin layer of nickel was vapor deposited uniformly onto the sample, using a Semicore Evaporator at the Penn State University National Science Foundation (NSF) National Nanotechnology Infrastructure Network (NNIN) facility, to protect the surface from oxidation and to allow hydrogen uptake. Two gas-charging procedures were utilized to achieve varying levels of hydrogen concentration in the nickel-coated material, as described in the following sections. 12

24 2.2.1 Hydriding Procedure #1 (280, 400, and 420 wt ppm H) For the first procedure, the Zircaloy-4 plate material was gas-charged by cycling the specimens in a 12.5% hydrogen-87.5% argon gas mixture at 450 C for 1 hr followed by cooling in vacuum, but outside the furnace, to 200 C for an additional 0.5 hr. A measured volume of the hydrogen-argon gas mixture was introduced into the furnace at the beginning of each cycle. The volume of hydrogen introduced varied in order to obtain a range of hydrogen concentrations. After the one hour exposure to the hydrogenargon mixture, the system was pumped to vacuum levels for the remaining 30 minute cool to 200 C. The maximum cooling rate between cycles for this procedure is estimated to be approximately C/min. The total charging process involved 5 cycles, and the specimens were furnace cooled (maximum cooling rate approximately 2 C/min) from 450 C to room temperature after the last cycle. Figure 2-3 is a schematic roughly illustrating the thermal and hydrogen cycles in Procedure #1. Temperature (C) Hydrogen Environment Vacuum Environment Time (hrs) Figure 2-3: Thermal and hydrogen environment cycle diagram for hydriding Procedure #1. 13

25 Three separate sections of the plate were hydrided using this procedure. If a solid hydride rim formed at the surface as described above, this layer was removed by mechanical grinding before measuring hydrogen content. An effort was made to predict specimen hydrogen content from the pressure drop observed in the closed system over the one hour hydrogen gas charge. The pressure drop during the hydrogen cycle was used to determine the moles of hydrogen absorbed by the Zircaloy-4 plate, and the mass of each plate was used to estimate the weight parts per million of hydrogen absorbed during the hydrogen cycle. It was verified that the furnace was indeed a closed system by ensuring no pressure drop was observed over the same time, at the same temperature. During many cycles, pressure drops of approximately 10% were observed (12.5% pressure drop would correlate to full hydrogen absorption, since the remainder of the gas was inert argon). However, calculations using pressure drop measurements predicted hydrogen concentrations 2-3 times higher than the bulk specimen concentration (after any rim was removed). Due to the large hydrogen concentrations near the plate surfaces, particularly near the rough-cut plate edges, the significant pressure drops seen during hydriding did not correlate well to bulk hydrogen content. The hydrogen content of each section was measured by Luvak Inc (Boylston, MA) using hot vacuum extraction analysis which showed that the hydrogen contents in the bulk of the plate for the three sections were 280, 400, and 420 wt ppm after hydrogen charging. This hydrogen charging procedure resulted in the formation of a uniform distribution of hydrides in the bulk of the plate (see Figure 2-5a). As in cold worked and stress relieved Zircaloy cladding, the hydrides are elongated and aligned parallel to the plate surface. 14

26 The first procedure was developed based on gas-charging procedures described in multiple publications for which hydrogen contents reached above 1000 wt ppm [20, 25-28]. However, the highest uniform concentration achieved in the bulk of the plate using the procedure described above was 420 wt ppm, most likely due to the thickness of the plate material (approximately 4.5mm). Utilizing this technique, thick rims ( 200µm) of solid zirconium hydride were observed at the plate surface, which appeared to grow thicker with further gas-charging and to inhibit hydrogen charging within the bulk of the plate. Procedure one was utilized to gas-charge samples tested in this study with hydrogen concentrations of 280, 400 and 420 wt ppm. A second gas-charging procedure was developed utilizing longer cycles and slow heating and cooling, in an attempt to prevent the formation of solid hydride rims near the plate surface Hydriding Procedure #2 (163, 333, and 620 wt ppm H) For the second gas-charging procedure, the cycle lengths were increased from 1 hour to 8-10 hours. During this time, the plate material was exposed to the same 12.5% hydrogen-87.5% argon gas mixture environment at 450 C for up to 3 hours, and then remained at 450 C under vacuum for the remainder of the 8-10 hour cycle. The increased cycle time was intended to allow hydrogen absorbed during each cycle to migrate uniformly throughout the material, preventing the formation of solid hydride rims on the plate surface. The heating and cooling procedure between cycles was also modified for the second gas-charging procedure. To begin each cycle, the material was placed in a cool furnace, after which the furnace was powered on, such that the material 15

27 heated with the natural heating rate of the furnace (maximum approximately 30 C/min). Following each cycle, the furnace was turned off while the sample remained in the heated zone, such that the material cooled with the natural cooling rate of the furnace (maximum approximately 2 C/min). Figure 2-4 is a schematic roughly illustrating the temperature and hydrogen exposure in one cycle of Procedure #2 (this cycle was repeated two to five times depending upon the desired final hydrogen concentration). 500 Temperature (C) Hydrogen Environment Vacuum Environment Time (hrs) Figure 2-4: Thermal and hydrogen environment cycle diagram for one cycle of hydriding Procedure #2. Three sections of plate material were charged using this procedure and metallography indicated that no rim had formed at the plate surface in either case. The hydrogen content of the three sections hydrided with this procedure were measured by hot vacuum extraction analysis which showed that the hydrogen content in the bulk of each of three plates were 163, 333 and 620 wt ppm after hydrogen charging. Micrographs revealing the hydride microstructure of all specimens in this study were prepared by polishing and etching the surface with an acid solution of 1 part hydrofluoric acid, 10 parts nitric acid and 10 parts H 2 0. Specimens were swab etched for 16

28 10-15 seconds, followed by immersion in the acid solution for seconds. As shown in Figure 2-5, the hydride microstructure produced by hydriding Procedure #2 (Figure 2-5b) was very similar to that of the microstructure produced by hydriding Procedure #1 (Figure 2-5a). The hydrides are uniformly distributed through the thickness of the plate, and tend to be elongated and aligned parallel to the plate surface. N N T R (a) (b) Figure 2-5: Light microscope micrograph of hydrided material with (a) 420 wt ppm +/- 50 wt ppm using Procedure #1 and (b) 620 wt ppm +/- 50 wt ppm using Procedure #2. T R 17

29 2.3 Mechanical Testing Compression tests were designed to determine three material characteristics: the yield strength, strain hardening, and strain-rate hardening. Compression samples with a cubic geometry (4.5mm) were fabricated from the plate material with a one-to-one height to width ratio [29]. Uniaxial compression testing of these specimens was performed on an Instron Model 4206 in each of three plate orientations (See Figure 2-2), i.e. with the stress axis aligned along the rolling, transverse, or normal-to-the-plate-surface direction. The Instron was fitted with two steel rams, and the cubic sample was placed between two stainless steel platens. Molydisulfide grease was used to lubricate the top and bottom surfaces of the compression sample. This ensured minimal friction during deformation, as indicated by the very small degree of barreling seen in samples deformed up to 10%, as shown in Figure 2-6. High temperature tests were performed on the same Instron, fitted with a clam shell furnace. A photo taken of the Instron fitted with the steel rams and the clam furnace is shown in Figure 2-7. (a) Figure 2-6: Cubic compression samples (a) before and (b) after compression testing to 10% strain. (b) 18

30 Figure 2-7: (a) Instron model 4206 fitted with compression rams and clam shell furnace. (b) Clam shell furnace opened to reveal steel platens and compression sample within. Room temperature tension tests were also performed on the same Instron with the stress axis aligned along the rolling and transverse directions on specimens with a 4:1 length-to-width gauge ratio; these specimens had gauge widths of 7.5 mm. The initial strain rate in all tests was 10-3 /s. Tensile tests were performed in order to compare the behavior of the plate material to behavior measured in compression. The tensile tests were also used to measure the contractile strain ratios of deformation in the rolling and transverse directions. Scribe marks were made on the specimen surface faces within the gauge section, and a traveling microscope was used to measure the coordinates of each scribe mark before and after the tension test to measure strain in the gauge section parallel and perpendicular to the tensile axis. The strain across the width of the tensile sample was therefore measured directly as the strain perpendicular to the tensile axis, and the strain 19

31 through the thickness of the tensile sample was calculated as the difference between the strains measured parallel and perpendicular to the deformation axis, assuming constant volume. The load-displacement data collected using LabView software during compression tests were used to calculate the true stress (σ) and true plastic strain (ε P ). The true stress was calculated as: P L σ = (2-2) A o L o where P is the measured load, A o is the initial measured area, L o is the initial measured length of the cubic sample (or the gauge length of the tensile specimen), and L is the instantaneous length of the sample. The instantaneous length (L) was determined as the initial length minus plastic displacement, where the plastic displacement was calculated as the deviation in displacement from the linear slope of the load-displacement curve in the elastic region. These results permitted a calculation of the true plastic strain as: L ε = P ln (2-3) Lo The calculated true stress and true plastic strain data were used to determine the three mechanical properties of interest. The yield stress values were taken to be the true stress at true plastic strain values of either or 0.01, for reasons to be discussed later. The constitutive plastic deformation of Zircaloy-4 cladding has been previously modeled in the material properties database, MATPRO [24], with mechanical behavior properties K, n and m forming the constitutive relationship given in Equation

32 The load-displacement data from compression tests performed at a displacement rate of 10-3 /s was used to determine the strain hardening exponent within the plastic strain range 0.01 ε p It was found that in this strain range the specimen deformation was uniform, and the correlation coefficient of the n-value as determined from linear regression analysis of Ln(σ) Ln(ε p ) graphs was very high (average value of R 2 = 0.995). Figure 2-8a illustrates a typical curve fit for Ln(σ) Ln(ε p ) data, and a power-law fit for the stress-strain data is shown in Figure 2-8b. 21

33 6.46 Ln True Stress (MPa) Rolling Transverse Ln(σ) = *Ln(ε p ) R 2 = n Rolling = Ln(σ) = *Ln(ε p ) R 2 = n Transverse = Ln True Plastic Strain (a) 600 True Stress (MPa) σ = 754.5ε R 2 = σ = ε R 2 = RT, Transverse, 1:1 100 RT, Rolling, 1: True Plastic Strain (b) Figure 2-8: Typical curve fits (dotted line) for (a) Ln σ Ln ε p data (solid line) used to determine the strain-hardening exponent and (b) power law fit (dotted line) for the stress-strain data (solid line) indicating the same strain- hardening exponent. The m-value was determined experimentally from at least four measurements within a test during which the strain rate was changed instantaneously by factors of one 22

34 to two orders of magnitude, within the strain-rate range 10-4 /s to 10-2 /s; see Figure 2-9. The information from each strain-rate change was used to calculate m by the following equation: σ 2 ln m d lnσ σ 1 = = d ln & ε & ε 2 (2-4) ln & ε1 where the initial stress and final stress are taken at the same strain. Figure 2-9 illustrates a typical stress-strain response of a specimen to strain rate jumps, and it indicates the stress values used in calculating the strain-rate hardening exponent, m, for a given strain rate jump. Due to the limitations of the Instron used in this study, the cross head displacement rate did not truly change instantaneously. Since the equation used to calculate m requires initial and final stresses to be taken at the same strain, the final stress was extrapolated from the slope of the stress-strain response after the cross-head speed was adjusted. Values for m were calculated for each strain rate adjustment made within the single test. Values of the average and standard deviation of these calculations from the four strain-rate jumps are reported. 23

35 705 σ 1 = σ 2 = 677 True Stress (MPa) σ 1 = 617 σ 2 = 588 σ 2 = 668 σ 1 = 601 σ 2 = 625 σ 1 = /s 10-4 /s 10-2 /s 10-4 /s 10-3 /s True Plastic Strain Figure 2-9: True stress as a function of true strain measured during data collection for strain-rate hardening. 24

36 Chapter 3 - Results and Discussion This chapter presents the experimental results obtained in this study, starting with pre-testing characterization using x-ray diffraction, metallography and mechanical testing of the as-recieved material. This is followed by the results of the mechanical testing of hydrided material to determine the effect of hydrogen content on the plastic anisotropy, yield behavior, strain-hardening, and strain-rate hardening. These results are then discussed in context. 3.1 Deformation Behavior and Texture of Plate Material and its Relationship to Cladding Material Deformation behavior of metals is normally the same in tension and compression, provided that the testing procedure enables the material to deform uniformly. Figure 3-1 shows the room temperature uniform true stress-true strain responses of as received as well as hydrided Zircaloy-4 used in this study in tension and compression in the transverse direction. While the yield stress in tension is slightly lower in both cases, the strain-hardening behavior in tension and compression is identical for both as-received material and hydrided material, indicating similar deformation behavior in tension and compression. Although samples tested in compression were well lubricated in order to reduce friction during deformation, the small amount of friction experienced during compression could lead to the slightly higher compressive yield stress seen in this study. Given the similarity in strain hardening, and only minor difference in yield stress, the 25

37 deformation behavior in compression is considered comparable to in-service loading in tension True Stress (MPa) Compression (AR) Tension (AR) Compression (333 wt ppm H) Tension (333 wt ppm H) True Plastic Strain Figure 3-1: The true stress-true strain response of hydrided (333 wt ppm H) and unhydrided Zircaloy-4 plate as determined in tension and compression for specimens oriented along the transverse direction of the plate; see Figure 2-1. Although this study is conducted on plate material, the results of this study are intended to provide insight on the influence of temperature and hydrogen on the deformation behavior of Zircaloy-4 cladding. To ensure that the results of this study will be relevant to cladding tubing material, it is necessary to confirm that the deformation behavior and hydride microstructure of the plate material are similar to those of cladding material. Table 3-1 summarizes the deformation behavior properties of the Zircaloy-4 plate in each direction, and the deformation behavior properties for Zircaloy-4 cladding tube reported in previous studies at room temperature and 300 C. 26

38 Table 3-1: Mechanical Behavior Properties of Zircaloy-4 CWSR plate examined in this study as compared to Zircaloy-4 cladding tube. Temp. Material Direction σy (0.002) n m (MPa) Rolling Compression Rolling Tension Not measured Plate Material Transverse Compression Transverse Tension Not measured 25 C Normal Compression Zircaloy-4 Cladding [7] Circumferential Tension Zircaloy-4 Cladding [19] Axial Tension Rolling Compression Plate Material Transverse Compression C Zircaloy-4 Cladding [7] Normal Compression Circumferential Tension Zircaloy-4 Cladding [19] Axial Tension Table 3-1 indicates that the yield stress in the normal direction is significantly higher than the yield stress in the transverse or rolling directions. In addition, the yield stress values at room temperature are consistently higher than those at 300 C, as expected. In general, the strain-hardening exponent, n, is lower at 300 C than at room temperature, while the strain-rate hardening exponent, m, does not show a significant change. Although the yield stress and strength coefficient for cladding material are higher than those for the plate, the strain hardening and strain-rate hardening of the model 27

39 material compare well to data published by Link for Zircaloy-4 cladding material [7]. The values for strain hardening given in MATPRO are significantly higher than the measured strain hardening for the plate material, but this difference with MATPRO is consistent with previous results from this group [7, 9]. For the purposes of this study, to determine the relative influence of temperature and hydrogen concentration on the deformation behavior, it can be concluded that the mechanical properties of the model material are sufficiently similar to those of cladding that the results of this study will be informative to cladding material. The degree to which the anisotropic mechanical behavior of the plate will be similar to cladding tube can be investigated by quantifying the crystalographic texture and plastic anisotropy. A texture analysis was performed by J. L. Béchade of CEA, using X-Ray Diffraction (XRD) to calculate Kearns factors and generate pole figures. In addition, the plastic anisotropy strain ratios, R and P were determined from tensile testing. Based on experimental measurements of the plate [22], a calculated normal pole figure was generated and is shown below in Figure 3-2a. A typical basal pole figure for Zircaloy-4 cladding material is shown in Figure 3-2b. 28

40 Rolling Transverse (a) (b) Figure 3-2: Basal pole figures for (a) Zircaloy-4 plate material, normal direction [22] and (b) Zircaloy-4 cladding tubing, radial direction [30]. 29

41 There are noticeable differences between the basal pole figures of plate and cladding tube. First, the plate basal pole figure exhibits two intensity peaks that are both within 15 of the plate normal direction; both peaks are slightly inclined from the plate normal along a plane parallel to the rolling direction. In addition, the plate material basal pole figure shows two peaks angled 50 from the normal direction, along a plane parallel to the transverse direction, with an intensity approximately half that of the peaks aligned in the rolling direction. In contrast, a typical texture of CWSR Zircaloy-4 cladding tube shows basal pole maxima located approximately from the radial direction, along a plane parallel to the transverse direction. In Figure 3-2, the maximum peak intensity of basal poles is 3.5 times random in the plate material, as compared to 5.5 times random in cladding material. Therefore, it can be said that typical cladding material has stronger texture than the plate material. However, it is possible that the plate material may have stronger plastic anisotropy due to the concentrated alignment of basal poles with the plate normal direction. Comparing the location and intensity of the basal peaks in the 4.5 mm plate material to those in typical thin-walled cladding material, the texture of the plate material can be described as less developed. It is likely that the additional forming processes used in cladding manufacturing result in the increased alignment of basal poles in the transverse-normal plane as the material is thinned and formed into cladding. Differences between the textures of the plate and cladding can be quantified by measurement of the Kearns factors. As stated in the Experimental Procedures section, the Kearns factors for the normal, rolling and transverse directions were calculated as f N = 0.49, f R = 0.19, f T = 0.32, respectively for the plate material used in this study. Typical 30

42 cladding material exhibit Kearns factors around f N = 0.66, f R = 0.06, f T = 0.28 [19]. These results indicate that both materials are strongly textured, although cladding shows a slightly stronger texture. Due to the limited availability of slip systems in hexagonal-close-packed (hcp) crystals, the presence of crystallographic texture results in anisotropic plastic deformation (with respect to the orientation of the stress axis). The primary slip system for deformation in zirconium hcp crystals is { 1010} Figure 3-3b is a schematic to illustrate the orientation of basal poles relative to the plate directions. As the pole figure in Figure 3-2a presents, for the Zircaloy-4 plate a high intensity of basal poles aligned within 15 of the plate normal direction is observed in the plane parallel to the rolling direction. This preferred grain orientation leads to difficulty in activating throughthickness slip deformation since the resolved shear stress is small along the easy slip directions, which are confined to the prism planes. 31

43 [ 2110 ] ( 0110 ) (a) N R T (b) Figure 3-3: (a) Identification of preferred slip system in zirconium hcp crystals and (b) a schematic of hcp crystallographic orientation relative to plate directions based on the measured pole figure. The plastic anisotropy strain ratios R and P are also commonly used to characterize plastic deformation anisotropy in sheet or plate material. For uniaxial tension tests with tension applied in the rolling direction: R ε ε w transverse = = (3-1) t ε ε normal And for uniaxial tension tests with tension applied in the transverse direction: P ε ε w rolling = = (3-2) t ε ε normal 32

44 where ε w and ε t are the width and thickness strains, respectively. A plastic anisotropy strain ratio equal to one indicates isotropic plasticity, while ratios greater than one indicate that through thickness slip is more difficult than slip across the width. The plate material was tested at room temperature in uniaxial tension in both the rolling and transverse directions to determine the R and P values as reported in Table 3-2. Table 3-2: Experimental Plastic Anisotropy Strain Ratios of Zircaloy-4 Plate R = 5.4 Plate P = 4.2 Cladding [7] (hoop tension) P = 2.3 Previous tests at Penn State performed on thin-walled Zircaloy-4 cladding tubes reported values in the range of P = 2.3 [7] for hoop tension tests. Table 3-2 thus indicates a P- value of the plate material significantly higher than for typical CWSR Zircaloy-4 cladding material. (Values as high as 6.3 have been reported for textured Zircaloy-2 sheet [31]). Due to the high values of R and P, the plate material should be expected to exhibit stronger plastic anisotropy than cladding. The high values for R and P measured for the plate material indicate the difficulty in activating through-thickness slip deformation. The yielding behavior of material with difficult through-thickness deformation is usually quantified in terms of Hill s quadratic yield criterion [32]. This criterion can be expressed in terms of the plastic anisotropy strains ratios R and P if the material exhibits planar isotropy, as does the plate material in 33

45 this study (see Figure 3-4). The values of R and P reported in this section were determined from tensile tests under conditions of uniform strain for samples loaded in the rolling and transverse directions, respectively. However, in the case where the material exhibits symmetry in tension and compression deformation (as demonstrated in Figure 3-1), the plastic anisotropy strain ratio can also be obtained from compression testing by determining the accommodation of plastic strains in the two orientations transverse to the compression axis. Specifically, the transverse strain ratio under through-thickness compression was determined by the accommodation of plastic strain in the rolling ( ε R ) and transverse ( ε T ) directions from room temperature tests and measured such that ε R / ε T 0.7. Based on the Hill yield criterion and values of R and P, it is possible to predict the ratio of the normal stresses required to induce yielding in equal-biaxial tension, which is a strain path of interest in reactivity-initiated accidents where cladding-pellet interactions can be significant. If σ eq is the Hill equivalent stress, then the ratio of the normal stresses required to yield the material in equal-biaxial tension σ EBT is given by the following relationship [32, 33]: σ σ EBT eq P(1 + R) = P+ R 1/2 (3-3) In the present case, σ eq = σ y, the uniaxial yield stress along either the rolling or transverse orientations since the material exhibits planar isotropy. In addition, the Hill formulation indicates that the strain-path associated with equal-biaxial tension is throughthickness compression of the plate with a strain-path of ε R / ε T = P/R, where ε R and ε T are the respective minor strains in the plate rolling and transverse directions. 34

46 Relating the above analysis to the present study, the compressive deformation behavior in the normal-to-plate-surface orientation can be related to deformation behavior in equal-biaxial tension (Recall that equal-biaxial tensile deformation in Figure 2-2 must occur by through-thickness deformation in the normal (N) direction). Thus, the compressive equivalent of equal-biaxial tension should also be characterized by a strain path of ε R / ε T = P/R. Given the P-values and R-values in Table 3-2, deformation in the plate normal orientation should thus be associated with a transverse strain ratio of dε 1 /dε 2 = P/R 0.8. This predicted value is close to the experimentally observed value reported above of ε R / ε T = 0.7 for compression in the plate normal orientation. As such, the strain-path behavior supports the equivalence of plastic deformation under a plate-normal compression mode to equal biaxial tension. In addition, using Equation 3-3 and the R- and P-values at room temperature, it is possible to estimate the ratio of yield stresses in the normal orientation to that in either the rolling or transverse orientations. Those data predict: σ σ EBT eq σ N σ N = ( σ ) ( σ ) y R y T 1.7 for room for temperature behavior. As shown in Figure 3-4, this predicted estimate agrees well with the stress ratio experimentally observed at room temperature where σ N /σ R σ N /σ T 1.8. Thus, these results also support the equivalence of through-thickness compression to equal biaxial tension. Significantly, this ratio is roughly independent of either temperature or hydrogen content, implying that the plastic anisotropy is preserved to at least 400 C (also observed in Zircaloy-4 cladding tube by Delobelle [5]), and is not affected by the presence of hydrides. 35

47 Ratio of Yield Stess C; σ N / σ R 25C; σ T / σ R 400C; σ N / σ R 400C; σ T / σ R Hydrogen Content (wt. ppm) Figure 3-4: The effect of hydrogen on yield stress anisotropy as defined by the ratio of the yield stress in the rolling orientation to that in either the normal-to-platesurface or transverse orientations. 3.2 Hydride Microstructures As mentioned previously, the orientation and distribution of hydride precipitates in Zircaloy-4 is a function of the material texture. Due to the differences between the plate and cladding textures, the hydride microstructure will be compared here. Micrographs of polished and etched sections of the hydrided plate are shown in Figure 3-5. Typical hydride microstructures for cladding material with 800 wt ppm hydrogen are shown in Figure

48 N N 100µm 100µm R T T R (a) (b) N N 100µm R T 100µm T R (c) (d) Figure 3-5: Micrographs of polished and etched samples of plate material with (a) 280 wt ppm, (b) 400 wt ppm, (c) 420 wt ppm, and (d) 620 wt ppm, to reveal hydride orientation and distribution. 37

49 A C (a) Figure 3-6: Micrograph of polished and etched sample of high-burn up (67 GWd/t) cladding material with 800 wt ppm hydrogen with typical hydride orientation and distribution [34]. (b) The micrographs in Figure 3-5 do not indicate large differences in microstructure between the normal-rolling and normal-transverse planes. The hydride precipitates in both micrographs appear elongated perpendicular to the normal direction, in a fine, dense network. When comparing the images in Figure 3-5 to Figure 3-6, the hydride orientation appears to be similar between the plate material and the cladding material, despite the differences in the texture. The hydrides have precipitated predominantly in the plane perpendicular to the normal and radial directions in the plate and cladding tube material, respectively. The overall hydride distribution however, is different between the plate and cladding samples. The distribution in the plate material is uniform throughout the thickness of the plate, while a concentration gradient is observed in Figure 3-6 in the cladding tube material. The distribution gradient in the cladding material results from the thermal gradient which exists during operation, where the water cooled outer cladding 38

50 surface has a lower hydrogen solubility causing a greater precipitation of zirconium hydrides. The uniform hydride microstructure in plate material appears comparable to the dense hydride microstructure found near the outer wall of the cladding. To investigate the location of hydride precipitates from each hydriding procedure, micrographs were prepared by polishing and etching samples to reveal hydrides. Polarized light microscopy was combined with bright-field optical microscopy to show the location of hydrides relative to the grain structure. Photo editing software was used to combine images taken at the same location with polarized light microscopy and brightfield microscopy. Figure 3-7 shows micrographs of a sample hydrided with (a) 420 wt ppm H and (b) 620 wt ppm H. Both micrographs in Figure 3-7 reveal that hydrides have predominantly precipitated on the grain boundaries. 39

51 (a) Figure 3-7: Micrographs of (a) sample hydrided with 420 wt ppm H and (b) a sample hydrided to 620 wt ppm H. (b) 40