Hydride Precipitation Research Articles

The mechanical property evolution and grain boundary accommodation during hydride transformation in commercial pure titanium

The mechanical property evolution of titanium surface during hydrogen charging was investigated by nanoindentation tests. The nanohardness increases while elastic modulus decreases due to the formation of hydride precipitation. By mean of interrupted in-situ electron backscatter diffraction (EBSD) measurements, the microstructure evolution of hydrogenated surfaces with different grain sizes was examined. The more prevalent grain-boundary accommodation in fine grain sample leads to the faster hydride transformation from metastable γ to stable δ hydride than coarse grain sample. The formation of intergranular hydride pair is regarded as one type of grain-boundary accommodation mode. Adjacent hydrides prefer to be induced at grain boundaries with both low c-axis misorientation angle θc (θc < 30o) and high θc (75o < θc < 85o). The synergistic shear of Shockley partial dislocation and the c-axis dilatation misfit of hydride transformation are the fundamental reasons for the above two types of favorable grain boundaries, respectively.

In-situ tensile deformation of Zircaloy-4 sheets with different hydrogen contents at RT and 300 °C

In this paper, the in-situ tensile deformation behaviors of Zircaloy-4 sheets with different hydrogen contents were investigated at room temperature (RT) and 300 °C. The results showed that at RT, the ultimate tensile strength (UTS) increased from 407 MPa to 508 MPa and the reduction of area (RA) decreased from 51% to 29% with the hydrogen content increased from 0 ppm to 850 ppm. When the hydrogen content reached 1300 ppm, both the UTS and RA dropped abruptly, showing an obvious ductile-brittle transition. The in-situ observations showed that for the un-hydrided samples, with the increase of tensile deformation, there first appeared slip deformation, then grain distortion and elongation, and finally crack initiation at the grain boundaries. For the hydrided samples, due to the precipitation of hydrides, the slip deformation and grain distortion were reduced, and the cracks trended to initiate around the hydrides. At 300 °C, the UTS increased from 246 MPa to 318 MPa and the RA decreased from 66% to 55.7% with the hydrogen content increased from 0 ppm to 850 ppm. When the hydrogen content reached 1300 ppm, no ductile-brittle transition was observed. The in-situ observations showed that for the un-hydrided and hydrided samples, with the increase of tensile deformation, they both exhibited good plastic deformation except more cracks initiated around the hydrides in the hydrided samples. The transmission electron microscope observations showed that at RT, with the increase of hydrogen content, the number of hydrides increased, and when the hydrogen content reached to a high level, these hydrides would intersect or collide with each other. However, at 300 °C, these hydrides were observed to be twisted or kinked after tensile deformation, which showed a good deformation ability. Based on these observations, the tensile deformation behavior of the hydrided samples was discussed. • A ductile-brittle transition occurs at hydrogen content of 850 ppm at RT. • Plastic deformation is reduced with the increase of hydrogen content at RT. • Intersect between hydrides is the reason for the ductile-brittle transition. • The ductile-brittle transition can be inhibited below 1300 ppm hydrogen at 300 °C.

A study of hydride precipitation in zirconium

Zirconium-based alloys used in nuclear reactors are susceptible to hydrogen pickup. When the hydrogen concentration in the solid solution exceeds the threshold limit (terminal solubility), a solid-state phase transformation occurs in these alloys. The zirconium hydride phase so precipitated is brittle and may influence the integrity of in-core structural components by various mechanisms, such as hydride blistering, delayed hydride cracking (DHC) etc. Precipitation which usually occurs by nucleation, growth and coarsening is typically influenced by the coherent stresses as well as the external stress field. In this work, Eshelby-type analyses are carried out to understand the mechanics and energetics associated with precipitation of zirconium hydrides. In particular, the role of elastic mismatch between the matrix and inclusion and the external stress in controlling the morphology and orientation of a hydride precipitate is analyzed. Elastic calculations lead to several interesting findings and reveal that under the influence of an external stress, a hydride precipitate has a tendency to reorientate along a certain crystallographic plane of the zirconium matrix. Incorporation of hydride plasticity, in our proposed scheme, leads to hydride reorientation at realistic magnitudes of applied stress. The strong anisotropy in interfacial energy, however, causes a hydride precipitate to remain parallel to the basal plane. Potential applications of our proposed numerical scheme to other relevant fields are also discussed.

A fast efficient multi-scale approach to modelling the development of hydride microstructures in zirconium alloys

A mechanistic understanding of hydrogen diffusion and hydride precipitation at the microscale underpins the prediction of delayed hydride cracking in zirconium alloy nuclear fuel cladding. We present a novel approach to modelling the microstructures created by hydride precipitation at loaded notches in polycrystalline Zr alloys. The model is multi-scale in that it includes the elastic dipole tensor of interstitial hydrogen in α-Zr, it treats the stress-driven diffusion of hydrogen at the meso-level (mm), it calculates the thermodynamically favourable spatial arrangement of microhydrides and their assembly into macrohydride colonies, in a textured polycrystalline sample, and it treats the full elastic field of the loaded notch and all the hydrides at a scale similar to the cladding thickness. A simplifying innovation is the representation of the elastic field of a microhydride by a dislocation dipole, where the Burgers vector is set to create the experimentally measured strain in the 〈11¯00〉 direction. The model provides a predictive framework for treating elastic anisotropy, a variety of potential nucleation sites, and different grain sizes. Simulated micrographs of hydride networks in polycrystalline samples with blunt and sharper loaded notches are compared with experimental micrographs obtained at the same scale. The simulations are extremely fast and calculations typically take around tens of seconds. This makes it possible to carry out detailed sensitivity studies with respect to several pertinent metallurgical variables, as well as conducting ensemble averaging of hydride microstructures.

Formation mechanism of hydride precipitation in commercially pure titanium

Since titanium has high affinity for hydrogen and reacts reversibly with hydrogen, the precipitation of titanium hydrides in titanium and its alloys cannot be ignored. Two most common hydride precipitates in α-Ti matrix are γ-hydride and δ-hydride, however their mechanisms for precipitation are still unclear. In the present study, we find that both γ-hydride and δ-hydride phases with different specific orientations were randomly precipitated in the as-received hot forged commercially pure Ti. In addition, a large amount of the titanium hydrides can be introduced into Ti matrix with selective precipitation by using electrochemical treatment. Cs-corrected scanning transmission electron microscopy is used to study the precipitation mechanisms of the two hydrides. It is revealed that the γ-hydride and δ-hydride precipitations are both formed through slip + shuffle mechanisms involving a unit of two layers of titanium atoms, but the difference is that the γ-hydride is formed by prismatic slip corresponding to hydrogen occupying the octahedral sites of α-Ti, while the δ-hydride is formed by basal slip corresponding to hydrogen occupying the tetrahedral sites of α-Ti.

Implementation and Validation of the Hydride Nucleation-Growth-Dissolution (HNGD) model in BISON

During the operation of a light water reactor, a fraction of the hydrogen produced by waterside corrosion is absorbed into the nuclear fuel cladding. When the hydrogen concentration reaches its solubility limit, a brittle zirconium hydride phase precipitates, leading to a loss of ductility of the cladding. To assess cladding integrity, an accurate simulation tool is needed to predict hydrogen distribution within the cladding and hydride precipitation. Recent studies have developed an improved understanding of the physical processes involved in hydrogen redistribution, and hydride precipitation and dissolution. This research led to the development of a new model, called Hydride Nucleation-Growth-Dissolution (HNGD). The present work describes the implementation of HNGD into the fuel performance code BISON, developed at Idaho National Laboratory. The main innovative feature of the HNGD model is that it accounts for hydride nucleation and growth as two distinct precipitation components, using the Johnson-Mehl-Avrami-Kolmogorov model to describe hydride growth kinetics. Each step of the model implementation into BISON was systematically verified, and simulations of experiments performed for validation, showing that the HNGD model provides improved predictions, and captures some experimentally observed physical phenomena related to hydride growth that the previous model could not.

A first principles investigation of the hydrogen-strain synergy on the formation and phase transition of hydrides in zirconium

The precipitation of hydrides phases is known to be a source of embrittlement in zirconium and its alloys. In the present work, the effects of hydrogen (H) in the bulk of Zirconium (Zr) systems has been investigated using density functional theory (DFT). A model for the nucleation of hydrides in terms of the thermodynamic and strain conditions has been determined from the examination of interstitial H configurations in hcp-Zr (α-Zr). We found that several coherent hcp structures for Zr2H have similar formation energies and structural properties, and thus are relatively easy to form. A number of Zr2H hydrides with hcp structures were found to be dynamically unstable, and were found to promote a phase transformation to the fcc lattice. Based on the hydrogen distribution, the orientation relationship and the strain conditions, the transition of Zr2H hydrides from the hexagonal to cubic phases have been determined. Our results suggest that hydrogen plays a fundamental role, since a substantial rearrangement is required to promote the hcp→fcc transformation of hydrides phases. We determined that a hcp→fcc transformation of the Zr host lattice is assisted by the lower barrier for phase transition in Zr2H compared to that of pure α-Zr. Thus, leading to the formation of the brittle δ hydride phase. The reported formation mechanisms of hydrides provide the basis to understand the precipitation of hydrides observed experimentally.

Effect of hydrogen pick-up on the fatigue behavior of the β-type Ti-12Mo-6Zr-2Fe alloy with ω-nanoprecipitation

The metastable β-type Ti-12Mo-6Zr-2Fe alloy was subjected to an assessment of its fatigue mechanical behavior. Promising tensile and fatigue properties were obtained when the alloy was solution treated and quenched, because nanocrystalline particles of ω phase were formed throughout the β matrix. When the material was subjected to a protocol of chemical etching and a subsequent surface polishing, the fatigue limit decreased significantly as a result of H pick-up, although the tensile properties were not changed. The increase of the local H concentration caused the precipitation of fine hydrides at high-energy grain boundaries. Fatigue crack initiation changed to a subsurface mode induced by the cleavage of hydride precipitates.

Hydrogen redistribution during hydride precipitation in Zr-2.5Nb pressure tubes

Hydrided furnace-cooled and water-quenched samples of Zr-2.5Nb pressure tube were studied in order to aid comprehension as regards the hydrogen redistribution that occurs during hydride precipitation in this material. Results showed a high fraction of γ-hydrides grown at hoop-grains in the quenched condition, resulting from the strong reduction in diffusion times imposed by quenching. This also lead to variations in δ-hydride distribution, δ-hydride populations and hydrogen solubility (TSS) during subsequent thermal cycling, for which an interpretation based on the memory effect of hydride precipitation was proposed. Experimental observations suggest that hydrogen redistribution during cool-down from high temperature is controlled not only by certain preferential grains, but also by the presence of misfit dislocations left by pre-existing hydride particles. A hypothesis was presented to explain the differences in precipitation solubility (TSSP) between furnace-cooled and water-quenched material, which was associated to the possible influence of misfit dislocations on the driving force for hydrogen diffusion.

Phase transformations during cooling from the βZr phase temperature domain in several hydrogen-enriched zirconium alloys studied by in situ and ex situ neutron diffraction

In hypothetical accidental conditions, zirconium-based nuclear fuel claddings can absorb high hydrogen contents (up to several thousand wppm) and be exposed to high temperatures (βZr phase temperature range) before being cooled. This paper thoroughly investigates the microstructural and microchemical evolutions that take place in such conditions. Two zirconium-based alloys and unalloyed zirconium were pre-charged with hydrogen at various contents up to 3300 wppm and heat-treated at 1000 or 850 °C. Neutron diffraction analyses were performed in situ upon slow cooling from 700 °C and at room temperature. In the materials containing 3300 wppm of hydrogen, βZr progressively transforms into αZr during slow cooling then extensively transforms into αZr and δZrH2-x hydrides precipitate via a eutectoid reaction. Thermodynamic predictions at equilibrium are in good agreement with the experimental results. However, depending on the cooling scenario and the average hydrogen content, the precipitation of γZrH hydrides, potentially metastable, is evidenced below 350 °C and a significant amount of hydrogen can remain in solid solution in αZr. These metallurgical evolutions and the evolution of the different phase lattice parameters are strongly influenced by the partitioning of oxygen and hydrogen (revealed by electron probe and elastic recoil detection microanalyses) that occurs during the βZr to αZr transformation and hydride precipitation.

Investigation of the temporal rearrangement of zirconium hydride precipitates in cladding material for the interim and final storage period

Abstract Within the scope of the pending search for a final storage site location for highly radioactive waste such as used fuel assemblies, the cladding integrity in case of a relocation of the rod from an interim storage to a final storage cask remains uncertain. In order to assess the potential reduction of the rod integrity by hydride precipitation and reorientation, a study of the long-term behaviour of hydrides in Zircaloy 2 is carried out. The focus is on simulation and analysis of the precipitation behaviour under different load conditions such as starting temperatures of 673 K, hoop stresses of up to 150 MPa and cooling rates between 4 and 1 K/h. The analytical methods comprised microscopic investigations as well as X-ray diffraction analyses and first results from imaging with X-rays.

Microstructural and crystallographic analysis of hydride reorientation in a zirconium alloy cladding tube

Understanding the hydride platelet reorientation in zirconium (Zr) alloy fuel claddings that potentially occurs during the dry storage process of spent nuclear fuel is of great technological importance for the cladding integrity. In this study, detailed microstructural and crystallographic analysis of the circumferential and reoriented radial hydrides in a Zr–Sn–Nb cladding tube has been performed. The results show that the crystallographic nature of the reoriented radial hydrides in the material remains the same as that of the circumferential hydrides. The radial hydrides remain to be the δ-phase precipitated along the {10–17} habit plane of the α-Zr matrix and follow the {0001}α//{111}δ and <11–20>α//<110>δ orientation relationship with the matrix. The local misfit strain between the matrix grains and the circumferential/radial hydrides is accommodated by the development of {01–10}α <a> type misfit dislocations in the matrix grains as well as the dislocations and twinning in the hydrides. It is confirmed that the hoop stress applied on the cladding tube during the hydride precipitation only changes the growth direction of the macroscopic hydrides from the circumferential direction to the radial direction of the tube material. A qualitative micromechanical stress analysis shows that the shear stress on the basal plane of α-Zr grains promotes the activation of {01–10}α <a> type dislocations slip in the matrix. In addition, to accommodate the c-axis stretching of the matrix unit cell due to the application of hoop stress, the precipitated hydride sub-platelets stacks tend to grow along the direction perpendicular to the direction of the applied hoop stress, leading to the eventual formation of macroscopic radial hydrides (i.e., the occurrence of the stress reorientation).

Sample container for high-resolution neutron imaging of spent nuclear fuel cladding sections.

In this work, for the first time, high-resolution neutron imaging (true spatial resolution of 13 μm) is used for irradiated nuclear fuel cladding, applying an adapted procedure for transfer, handling, and measurements of highly radioactive samples in combination with the neutron microscope detector at Paul Scherrer Institut. A sample container referred to as an active box for high-resolution neutron imaging of highly active spent nuclear fuel cladding sections was developed. Sections of unirradiated and irradiated cladding of duplex type, having a liner, with hydrogen average concentrations of 420 wppm and 450 wppm were investigated using this device. The irradiated cladding originated from a fuel rod operated for five cycles in a Swiss pressurized water reactor. The irradiated cladding sample was measured inside the active box. Long circumferential hydride accumulations were revealed together with notable hydride precipitation at the liner-substrate interface. Measurements of the unirradiated cladding in air and inside the active box delivered consistent results, confirming the applicability of the developed device for high-resolution neutron imaging.

Hydride or martensite? Identification of a new phase in cold-rolled hexagonal close-packed Ti

Two types of lamellar γ-phases are observed in cold-rolled pure α-Ti: the P-type adopting [0001]α//[001]γ and (101¯0)α//(110)γ, and the B-type adopting [21¯1¯0]α//[101]γ and (0001)α//(1¯1¯1)γ. By using high-resolution transmission electron microscopy, selected area electron diffraction and stereographic projection, the γ-phase is identified to have a face-centered tetragonal (fct) structure. In situ heating experiment further confirms the γ-phase to be hydride (TiH) precipitate in α-Ti. This work provides convincing evidences that the so-called stress-induced face-centered cubic (fcc) martensite in deformed α-Ti reported previously could be mistakenly identified. Available approaches to distinguish the fcc-Ti martensite from the fct-hydride are also discussed.

Effect of Hydrogen Precharging on Mechanical and Electrochemical Properties of Pure Titanium

The electrolytic hydrogen precharging process is used to analyze the influence of hydrogen on the microstructure, corrosion resistance, and mechanical properties of pure titanium. The results demonstrate that two types of titanium hydrides (δ‐TiHx and ϵ‐TiH2) are detected by electron backscatter diffraction (EBSD), and the volume fraction of titanium hydrides, the local strain caused by hydrogen precipitation, and the thickness of hydrides layer increase with the increase in precharging time. It is noteworthy that the formation of hydrides layer has a protective effect on general corrosion, but the local strain caused by hydrides precipitation leads to the destruction of corrosion resistance, as a result the corrosion resistance of pure titanium increases first and then decreases with the increase in hydrogen precharging time. Moreover, with the precipitation of hydrides, the fracture mode changes from ductile failure to duplex‐ and multimode failure, resulting in the decrease in ultimate tensile strength (UTS) and elongation. Therefore, the mechanical and electrochemical properties of the precharged samples first increase and then decrease with the increase in precharging time, reaching their optimum values at 10 h, and the critical precharging time is determined to be 75 h when comparing those properties with pure titanium.

A Review of Hydride Precipitates in Titanium and Zirconium Alloys: Precipitation, Dissolution and Crystallographic Orientation Relationships

This work proposes a review of recent results on the formation and dissolution of hydrides in HCP alloys (Ti and Zr alloys) correlated to the nature of crystallographic hydride phases and their ORs. The crystallographic coherence observed between the surface hydride layer and the substrate is very important for many applications as for biomaterials devices. Five particular orientation relationships (OR) were identified between titanium/zirconium hydride precipitates and the oc-Ti and a-Zr substrates. In addition, the nature of hydrides have a large implication on the ductility, the strain hardening, and the local plastic strain accommodation in the Ti alloys. Our studies using XDR, TEM and SEM-EBSD have been demonstrating that the nature of the hydride phase precipitates depends on the hydrogen content. DSC has been used to obtain the hydride dissolution and precipitation energy values at the bulk scale, whose difference can be associated to misfit dislocations. Local in-situ TEM dissolution observations show the depinning of part of misfit dislocations during dissolution process. Hydride reprecipitation is thus possible only if hydrogen is not driven away during heating by misfit dislocations depinning.

Impact of rolling conditions on the hydriding of grade 2 titanium welded tubes

In this article, we study the impact of rolling conditions on the texture of the commercially pure titanium grade 2. In a previous work, NEOTISS in collaboration with ICB laboratory, shown that the texture highly influences the precipitation of hydrides in Titanium. In order to create different textures, Titanium sheets grade 2 are cold rolled asymmetrically and symmetrically with or without lubricant. The inverse pole figures and direct pole figures obtained allow us to deduce that symmetrical cold rolling does not change the grains orientation but generates a rotation of grains along c-axis (normal to basal plan). However, asymmetrical cold rolling leads to the formation of a new crystallographic texture, which could limit the formation of the hydrides in titanium grade 2 submitted to a hydrogen-rich environment. Key words: asymmetrical rolling, symmetrical rolling, titanium, hydriding, texture

Crystallographic orientation dependence of hydride precipitation in commercial pure titanium

Ti and its alloys have a variety of applications in aerospace industry and medical implants. The formation of hydride has been used in biomedical areas and can significantly influence the mechanical performance of materials. In this work, we investigate the orientation dependence of hydride precipitation in commercially pure titanium via interrupted in-situ electron backscatter diffraction (EBSD) measurements. The results reveal that hydrides during hydrogen charging at room temperature exhibit two types of orientation relationships with α-titanium, i.e., {0001}α//{11¯1}δ<12¯10>α//<110>δ with interface plane {101¯3}α//{11¯0}δ (B-type), and {0001}α//{001}δ<12¯10>α//<110>δ with interface plane {101¯0}α//{11¯0}δ (P-type). Significant orientation dependence of hydride precipitation is observed, especially when {101¯3}, {101¯0}, {0001} or {112¯0} planes of the parent grains are parallel to the diffusion surface. The displacement gradient tensor based accommodation shows that the orientation dependence is attributed to the strain relaxation of hydride transition. Three types of hydride platelets are characterized: parallel hydride platelets (Type I), crossed hydride platelets (Type II) and clustered hydride platelets (Type III). The multiple morphologies of hydride platelets resulting from the hydride variant selection and interaction are dependent on the crystal orientation of the matrix.

A study on macroscopic fuel cladding ductile-to-brittle transition at 300°C induced by radial hydrides

ABSTRACT In this paper, to clarify the conditions for the occurrence of macroscopic hydrogen-induced ductile-to-brittle transitions (DBT) at operational temperatures, the effect of hydride precipitation on the mechanical properties of fuel cladding at 300°C was investigated. Tube burst tests were performed on the high-burnup Zr-lined Zircaloy-2 fuel cladding samples that were subjected to a hydrogen charging and hydride reorientation treatment. The precipitated hydrides had no significant effect on the cladding strength until the cladding lost its plasticity. Circumferential permanent strains at failure tended to decrease as the hydrides became longer, more densely precipitated, and more aligned to the radial direction. These results confirm that cladding ductility can be reduced significantly by hydrogen absorption even at 300°C if an extent of radial hydride formation is sufficiently high. Further, observations suggest that approximately 1% of the circumferential strain serves as a threshold for identifying the macroscopic hydrogen-induced DBT of the Zircaloy-2 cladding.

Theory model combined with XFEM of threshold stress intensity factor and critical hydride length for delay hydride cracking

In order to determine the threshold stress intensity factor and critical hydride length for delayed hydride cracking in Zr-2.5Nb pressure tube alloy, the distribution of normal stress in the plastic zone of crack tip by the developed method that combines theory calculation with extended finite element method (XFEM) was improved. The fracture process of two-phase composites containing Zr-2.5Nb and hydride precipitate was simulated by XFEM. Based on that, critical hydride length LC corresponding to the theoretical model for K1H was estimated. Meanwhile, to illustrate the validity of theoretical and numerical methods, recent theoretical models and experimental measurements were utilized to verify the results of this paper. The theoretical model of DHC was improved to estimate the critical hydride length corresponding to threshold stress intensity factor. The predicted value of critical hydride length is close to the experimental values.