Zirconium alloys are widely used in the nuclear industry because of their high strength, good corrosion resistance and low neutron absorption cross-section. However, zirconium has strong affinity for hydrogen, which may lead to hydrogen concentration build-up over time during a corrosion reaction when exposed to water. Hydrogen stays in solution at higher temperature but precipitates as zirconium hydrides at ambient temperatures. The formation of zirconium hydrides is considered to be a major cause of embrittlement, in particular as a key step in the mechanism of delayed hydride cracking. Despite the fact that zirconium hydrides have been studied for several decades, the basic nature and mechanisms of hydride formation, transformation and exact structure are not yet fully understood. In order to find the answer to some of these problems, the precipitation and dissolution of hydrides in commercial grade Zr powder were monitored in real time with high resolution synchrotron and neutron radiations, and the whole pattern crystal structure analysis, using Rietveld and Pawley refinements, were performed. For the first time all commonly reported zirconium hydride phases and complete reversible transformation between two different Zr-hydride phases were recorded with a single setup and their phase transformation type have been analyzed. In addition, the preparation route of controversial γ-zirconium hydride (ZrH), its crystal structure and formation mechanisms are also discussed.

Zirconium alloys are widely used in the nuclear industry because of their high strength, good corrosion resistance and low neutron absorption cross-section. Zirconium has a strong affinity for hydrogen, however, and if hydrogen concentration builds up, the material will gradually degrade. In one class of such hydrogen caused degradation, called hydride induced embrittlement, hydrogen chemically reacts with zirconium forming one, or several, crystal phases of zirconium hydride. These hydrides play a primary, but sometime not fully understood, role in crack initiation and propagation within these materials. Despite the fact that hydride induced embrittlement in zirconium have been studied for several decades, there are still some unresolved issues. It has been the aim of the research presented in this thesis to provide the research community with new and updated data of the hydrides themselves in order to aid further studies within the field of hydride induced embrittlement in general, and the mechanism of delayed hydride cracking in particular. To that end, the research presented here proceeded, in short, as follows: First, zirconium hydride powder, of well defined hydrogen concentration, was produced from commercial grade zirconium. This powder was subjected to heat treatment and the hydride phases were characterized both in situ and ex situ using neutron, synchrotron X-ray, and conventional laboratory X-ray based diffraction techniques. Next, most of the low-pressure zirconium hydride phases were produced under hydrogen/argon atmosphere from commercial grade zirconium powder. This process was simultaneously monitored and recorded in real time using synchrotron X-ray diffraction. These experiments have produced new data of the behavior of different hydride phases during thermal treatment and in situ hydrogenation. For the first time all commonly reported zirconium hydride phases and the complete transformation between two different hydride phases were recorded with a single experimental arrangement. The phase transformation between δ and ε zirconium hydride was recorded in detail and presented. Finally, the controversial γ zirconium hydride was observed both in situ and ex situ and the preparation route, its crystal structure, and formation mechanisms were analyzed and presented.

The formation of hydrides in zirconium alloys will cause degradation of the materials. Despite extensive study, the mechanisms and effects of hydrogen embrittlement have remained unclear. In this report, we had presented the calculation results of mean-field strain by different hydride phases in zirconium alloys, like δ-hydride and γ-hydride. The results of δ-hydride are compared with experimental data obtained by X-ray diffraction method by Bai et al.\cite{Baib} and note \cite{Ali} of Massih. The result of δ-hydride is identical with Massih but have some differences with Bai et al. We had also compared our calculated mean-field strain by γ-hydride with δ-hydride. The γ-hydride showed lower value than δ.

We report on the measurement of the formation of gamma-ZrH during in situ gaseous charging. The measurements were undertaken using high-energy synchrotron X-ray diffraction. Experimental observation shows that gamma-ZrH can form at 180 degrees C from a mixture of alpha+delta while dehydrogenating at slow cooling rates. The observation is further supported by ex situ laboratory X-ray diffraction on deuterated Zr powder that has undergone a similar heat-treatment cycle. The crystal structure of gamma-ZrH refinement agrees with the reported P4(2)/n structure found in the literature. (C) 2016 Elsevier B.V. All rights reserved.

For the first time, various hydride phases in a zirconium-hydrogen system have been prepared in a high-energy synchrotron X-ray radiation beamline and their transformation behaviour has been studied in situ. First, the formation and dissolution of hydrides in commercially pure zirconium powder were monitored in real time during hydrogenation and dehydrogenation, then whole pattern crystal structure analysis such as Rietveld and Pawley refinements were performed. All commonly reported low-pressure phases presented in the Zr-H phase diagram are obtained from a single experimental arrangement.

There is an internal, self-balanced stress known as residual stress (RS) that exists in all alloy systems without any external applied forces. Depending on the compressive or tensile nature and magnitude of the RS, it significantly affects the mechanical properties of the materials. Therefore, it is crucial to know the nature and magnitude of RS in material for safe and economical operation. In this work, we used unique, multipurpose, high energy (50-150 KeV) beamline I12-JEEP (Joint Engineering, Environment and Processing) at Diamond Light Source in UK with Energy Dispersive X-Ray Diffraction (EDXRD) setup to map up the RS states in one of the most popular titanium alloys with a code name Ti6Al4V. This type of titanium alloy is widely used in biomedical and aerospace industry because of their excellent combination of a high strength/weight ratio and good corrosion resistance. The Ti6Al4V which we were investigated are produced using electron beam melting (EBM) technique as a function of EBM processing parameters. In addition to relation between RS and processing parameters of EBM, the hydride formation versus processing parameters and as well as the effect of residual stress to the hydride precipitation in EBM built Ti6Al4V were investigated. To find out the effect of EBM processing parameters to the residual stress development, various samples produced with different beam size, scanning speed and different building thickness were investigated. From each type of the sample four specimens were prepared and three of them loaded with hydrogen in different concentrations, i.e. the hydrogen concentration of the various samples are 262, 772, 951 and 1410 ppm. Other than these alloy samples we also measured clean and hydrided original powder samples which are used for make these solid samples in our studies. After data collection, the whole pattern fitting method Rietveld and Pawely were performed with structure analysis software package Topas-Academic and GSAS. .

Deuteride phases in the zirconium-deuterium system in the temperature range 25-286 degrees C have been studied in-situ by high resolution neutron diffraction. The study primarily focused on observations of delta ->gamma transformation at 180 degrees C, and the peritectoid reaction alpha + delta <-> gamma at 255 degrees C in commetcial grade Zr powder that was deuterated to a deuterium/Zr ratio of one to one. A detailed description of the zirconium deuteride preparation route by high temperature gas loading is also described. The lattice parameters of alpha-Zr, delta-ZrDx and epsilon-ZrDx were determined by whole pattern crystal structure analysis, using Rietveld and Pawley refinements, and are in good agreement with values reported in the literature. The controversial gamma-hydride phase was observed both in-situ and ex-situ in deuterated Zr powder after a heat treatment at 286 degrees C and slow cooling. (C) 2017 Elsevier B.V. All rights reserved.

We investigate the formation and dissolution of hydrides in commercially pure zirconium powder in-situ using highenergy synchrotron X-ray radiation. Experimental results showed a continuous phase transition between the delta and epsilon zirconium hydride phases with indication of a second order phase transformation.

Zirconium alloys have a strong affinity for hydrogen which leads to hydrogen pick-up during a corrosion reaction when exposed to water. The hydrogen is readily in solution at higher temperature but precipitates as Zirconium hydrides at ambient temperatures. At least three phases are known to exist at ambient temperature depending on hydrogen concentration and quenching rate. However, some controversy exist regarding the exact nature and stability of the γ-ZrH phase, which is closely related to the δ-ZrH phase through ordering of the hydrogen on tetrahedral sites in the matrix. In this report, we give short introduction to our experimental work at FRM II in September 2010. In this work, we use high-resolution neutron powder diffraction on deuterated samples to re-determine and verify the reported structures which essentially date back to the 1960s [1].

Zirconium alloys are widely used in the nuclear industry because of their high strength, good corrosion resistance and low neutron absorption cross-section. However, zirconium has strong affinity for hydrogen which leads to hydrogen concentration build-up over time. It is well known that the formation of hydrides will degrade the material and leads to, for example, delayed hydride cracking during high burn up. Even though zirconium hydrides have been studied for several decades, there still remain some controversies regarding the formation mechanisms, exact crystal structure, and stability of various hydride phases. This study uses high resolution synchrotron radiation as a probing tool to observe the precipitation and dissolution of hydrides in highly pure zirconium powder during in-situ hydrogen charging. The experiment enabled the direct observation of the hydride formation and phase transformations. It, also, provided high quality data for crystal structure determination.

Better understanding of the formation and distribution of hydrides, elastic and plastic strains in deformed polycrystalline, multiphase materials such as digitally manufactured Ti-6Al-4V is important for structural engineering. Polycrystalline Ti-6Al4V alloy samples produced by additive manufacturing techniques called electron beam melting (EBM) have been studied at third generation, high energy synchrotron X-Ray diffraction beam line with energy dispersive X-Ray diffraction setup. The elastic strain in the as built and hydrogenated material determined by extracting unit cell parameters of the existing phases in the system by using whole pattern analysis method Rietveld and Pawley.

Residual stress/strain and microstructure used in additively manufactured material are strongly dependent on process parameter combination. With the aim to better understand and correlate process parameters used in electron beam melting (EBM) of Ti-6Al-4V with resulting phase distributions and residual stress/strains, extensive experimental work has been performed. A large number of polycrystalline Ti-6Al-4V specimens were produced with different optimized EBM process parameter combinations. These specimens were post-sequentially studied by using high-energy X-ray and neutron diffraction. In addition, visible light microscopy, scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) studies were performed and linked to the other findings. Results show that the influence of scan speed and offset focus on resulting residual strain in a fully dense sample was not significant. In contrast to some previous literature, a uniform α- and β-Ti phase distribution was found in all investigated specimens. Furthermore, no strong strain variations along the build direction with respect to the deposition were found. The magnitude of strain in α and β phase show some variations both in the build plane and along the build direction, which seemed to correlate with the size of the primary β grains. However, no relation was found between measured residual strains in α and β phase. Large primary β grains and texture appear to have a strong effect on X-ray based stress results with relatively small beam size, therefore it is suggested to use a large beam for representative bulk measurements and also to consider the prior β grain size in experimental planning, as well as for mathematical modelling.

Variation of texture in Ti-6Al-4V samples produced by three different additive manufacturing (AM) processes has been studied by neutron time-of-flight (TOF) diffraction. The investigated AM processes were electron beam melting (EBM), selective laser melting (SLM) and laser metal wire deposition (LMwD). Additionally, for the LMwD material separate measurements were done on samples from the top and bottom pieces in order to detect potential texture variations between areas close to and distant from the supporting substrate in the manufacturing process. Electron backscattered diffraction (EBSD) was also performed on material parallel and perpendicular to the build direction to characterize the microstructure. Understanding the context of texture for AM processes is of significant relevance as texture can be linked to anisotropic mechanical behavior. It was found that LMwD had the strongest texture while the two powder bed fusion (PBF) processes EBM and SLM displayed comparatively weaker texture. The texture of EBM and SLM was of the same order of magnitude. These results correlate well with previous microstructural studies. Additionally, texture variations were found in the LMwD sample, where the part closest to the substrate featured stronger texture than the corresponding top part. The crystal direction of the α phase with the strongest texture component was [112¯3].

Zirconium alloys, widely used in the nuclear industry, Fig 1, have a strong affinity for hydrogen that leads to hydrogen pick up during a corrosion reaction when exposed to water. The hydrogen is readily in solution at higher temperature but precipitates as Zirconium hydrides at ambient temperatures. At least three phases are presumed to exist at ambient temperature depending on hydrogen concentration and quenching rate. However, some controversy exist regarding the exact nature, exact structure and stability of the γ-ZrH phase, which is closely related to the δ-ZrH phase through ordering of the hydrogen on tetrahedral sites on alternating 110 planes.

The phase evolution in a zirconium–50 deuterium (Zr–50D, at.%) alloy system during thermal cycling has been investigated using in situ high-resolution neutron powder diffraction. The results showed that the peritectoid reaction α-Zr + δ-ZrD → γ-ZrD previously suggested to occur at high temperatures does not take place in the system. Slow cooling, from high temperatures (≥520 K) to room temperature at a rate of 5 K min–1, promoted the γ-hydride formation rather than fast cooling as reported earlier. In contrast to the observation that the δ-hydride present in the system remained at temperatures up to 740 K, the produced γ phase transformed to δ-hydride in the temperature range of 370 K to 559 K, with the transformation completing at approximately 559 K. It is confirmed that the formation of the γ-hydride was reproducible with slow cooling, and a diffusion-controlled sluggish δ-to γ-hydride transformation is suggested to be responsible for the favorable development of γ-hydride during slow cooling.