Thesis (D. Eng.)--Cleveland State University, 2008. / Abstract. Title from PDF t.p. (viewed on Feb. 4, 2009). Includes bibliographical references (p. 87-93) and appendix. Available online via the OhioLINK ETD Center. Also available in print.

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Thesis (Ph. D.)--University of Washington, 1923. / Reprinted from the Journal of the American Chemical Society, v. 46, no. 6, June, 1924.

The interaction of hydrogen with metals has great environmental significance in problems ranging from the catastrophic failure of materials due to hydrogen embrittlement to safe and efficient storage of hydrogen as a metal hydride. Chromium (Cr) is widely used as an alloying agent to produce materials such as stainless steel and as an electroplated coating on materials to prevent corrosion and to minimize wear. Hydrogen which co-deposits with chromium during electroplating forms hexagonal close packed CrH or face centered cubic CrH2 which cracks the deposit. The behavior of hydrogen in Cr, especially the crystal structure modifications of metal Cr when it is exposed to hot, dense hydrogen gas is not completely understood. In equilibrium study, chromium hydride has been found of hexagonal close packed structure under 400 °C with high hydrogen pressure. Experiments at higher temperatures are limited by the equipment and technology. This dissertation describes a novel, non-equilibrium method which was used to synthesize a new chromium hydride phase. Single crystal, body centered cubic Cr thin films were prepared by vacuum evaporation. These films were exposed to high temperature (close to the melting point of Cr), high pressure hydrogen gas in a ballistic compressor. This was followed by rapid cooling (>105 ˚C/s) to room temperature. Using the transmission electron microscope (TEM), second phase particles of superhexagonal structure, which has lattice constant A=4.77Å and C/A=1.84, are found in the films. This structure has a volume per Cr atom slightly larger than that of hexagonal closed packed CrH, so that the superhexagonal structure may contain more hydrogen than the hexagonal close packed CrH. The superhexagonal particles have a definite orientation relationship with the matrix: [021][subscript sh] II [OOl][subscript b] and (212)[subscript sh] II (IIO)[subscript ]b. The superhexagonal structure is quite stable in air and at room temperature, but decomposes to body centered cubic Cr when heated by the electron beam illumination in the TEM. No such particles were observed in Cr films exposed to pure argon under similar conditions in the ballistic compressor. Positive identification of hydrogen content was obtained by high-temperature vacuum extraction in a discharge tube. After vacuum extraction, hydrogen spectrum was observed, and the intensity of electron diffraction from superhexagonal structure decreased. Using an energy dispersive spectrometer with the capability of detecting elements down to atomic number six (carbon), no changes in composition of the films were found by comparing the characteristic x-ray spectra of the same film before and after exposure to hot, dense hydrogen in the ballistic compressor. This result suggests that this non-equilibrium method may be used for other metal-hydrogen systems to obtain new structural phases that are of scientific or technological interest.

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Pulse plated nickel and nickel based superalloys have been used extensively in the Ariane 5 space launcher engines. Large structural Ariane 5 space launcher engine components such as combustion chambers with complex microstructures have usually been manufactured using electrodeposited nickel with advanced pulse plating techniques with smaller parts made of nickel based superalloys joined or welded to the structure to fabricate Ariane 5 space launcher engines. One of the major challenges in manufacturing these space launcher components using newly developed materials is a fundamental understanding of how different materials and microstructures react with hydrogen during welding which can lead to hydrogen induced cracking. The main objective of this research has been to examine and interpret the effects of microstructure on hydrogen diffusion and hydrogen embrittlement in (i) nickel based superalloy 718, (ii) established and (iii) newly developed grades of pulse plated nickel used in the Ariane 5 space launcher engine combustion chamber. Also, the effect of microstructures on hydrogen induced hot and cold cracking and weldability of three different grades of pulse plated nickel were investigated. Multiscale modelling and experimental methods have been used throughout. The effect of microstructure on hydrogen embrittlement was explored using an original multiscale numerical model (exploiting synthetic and real microstructures) and a wide range of material characterization techniques including scanning electron microscopy, 2D and 3D electron back scattering diffraction, in-situ and ex-situ hydrogen charged slow strain rate tests, thermal spectroscopy analysis and the Varestraint weldability test. This research shows that combined multiscale modelling and experimentation is required for a fundamental understanding of microstructural effects in hydrogen embrittlement in these materials. Methods to control the susceptibility to hydrogen induced hot and cold cracking and to improve the resistance to hydrogen embrittlement in aerospace materials are also suggested. This knowledge can play an important role in the development of new hydrogen embrittlement resistant materials. A novel micro/macro-scale coupled finite element method incorporating multi-scale experimental data is presented with which it is possible to perform full scale component analyses in order to investigate hydrogen embrittlement at the design stage. Finally, some preliminary and very encouraging results of grain boundary engineering based techniques to develop alloys that are resistant to hydrogen induced failure are presented. Keywords: Hydrogen embrittlement; Aerospace materials; Ariane 5 combustion chamber; Pulse plated nickel; Nickel based super alloy 718; SSRT test; Weldability test; TDA; SEM/EBSD; Hydrogen induced hot and cold cracking; Multiscale modelling and experimental methods.

Hydrogen transport during a test and hydrogen segregation to twins, second phase particles and precipitation products prior to testing are shown to adversely effect the mechanical properties of metals. Hydrogen embrittlement processes in austenitic stainless steel, mild steel and aluminum occurred primarily by hydrogen induced weakening of the interfaces associated with specific metallographic features. In impact and slow bend tests of 21-6-9 and 304L stainless steels, the effect of hydrogen manifests itself in hydrogen induced faceted fracture along interfaces in the metal lattice. The extent of this weakening increases as the hydrogen content in the test sample is increased and during slow strain rate studies which promote hydrogen redistribution during the test. Disk rupture studies with 1015 and 1018 steels show that hydrogen segregation to the inclusion-matrix interface weakens the interface to such a degree that rapid fracture occurs. Studies with aluminum also indicate that hydrogen segregation to an interface degrades the mechanical properties. In age hardening experiments, hydrogen segregation caused an increase in the overaging kinetics in 2024 Al. This caused local softening of the aluminum and was probably due to the effect of hydrogen in promoting a loss of coherency at precipitate-matrix interfaces. The combined results of these tests support a decohesion type embrittlement mechanism, with the decohesion occurring at the interfaces. The results also suggest that any decohesion type mechanism must take into account the importance of hydrogen segregation and dislocation transport of hydrogen in the embrittlement process. / Master of Science

In many manufacturing and service industries, a need exists for a nondestructive test to determine the presence of hydrogen in a material system. The feasibility of such a system is examined here. Acoustic emission activity resulting from a microhardness indentation is employed to detect hydrogen in A106 and 4340 steel bars following cathodic, gaseous, and chemical charging. These tests show a large increase in emission energy after charging followed by a drop to precharge levels with time. These activity levels are used to calculate hydrogen diffusivity and binding energy of hydrogen to traps in the steel. A mechanism of acoustic emission generation is proposed involving the breakaway of dislocations from Cottrell-like hydrogen atmospheres. The effects of surface roughness and microstructure are also evaluated. Testing of various surfaces indicates that limited surface preparation is necessary prior to implementing the test procedure. Low activity levels before and after charging in 4340, and in martensitic and bainitic A106 indicate possible difficulties in applying the test to harder, more dispersed structures. Despite this limitation and a large amount of scatter in the acquired data, the results indicate that acoustic emission monitoring of microhardness indentations may be of value in detecting the presence of hydrogen in metals and as a research tool in the study of hydrogen transport and embrittlement mechanisms. / M.S.