No description available.

No description available.

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

M. S.

Acoustic emission activity resulting from plastic deformation of mild steel disks that were clamped and then pressurized from one side with either hydrogen or nitrogen was recorded and analyzed. It was found that during monotonic pressurization of disks in nitrogen gas, more cumulative counts were recorded than for similar disks pressurized in hydrogen gas. Possible signatures of the "births" of cracks were observed during hydrogen pressurization of disks that typically failed by leaking. The records of the nitrogen tests show very high energy and high count events occurring early in the deformation process. These events are believed to be the result of the breaking away of near-surface dislocations that had been pinned by nitrogen. The disks tested in nitrogen typically failed by bursting (ductile failure) while those tested in hydrogen typically failed by leaking ("brittle" failure). / M.S.

Electrolytic charging of hydrogen, at room temperature and in the absence of externally applied stress, induced surface cracking in 316 stainless steel and cobalt based ZIMALOY. Hot Isostatic Pressed (H.I.P.) ZIMALOY showed less susceptibility to surface cracking than 316 stainless steel samples. The susceptibility of 316 stainless steel to surface cracking was determined with samples in the High Energy Rate Forged (HERF), the sensitized, the annealed, and the annealed and sensitized conditions. Investigations showed that surface cracking typically occurred at specific microstructural features. Hence, the relative susceptibilities of twin boundaries, slip bands, grain boundaries, and heavily sensitized regions was established. It was observed that twin boundaries crack most readily in non-sensitized samples, and that both grain boundaries and twin boundaries crack easily in sensitized structures. These observations, coupled with the similarity between hydrogen embrittlement and failure of orthopedic implants, suggest that orthopedic applications should use H.I.P. ZIMALOY in preference to 316 stainless steel whenever possible, and that when the use of 316 stainless steel is unavoidable, HERFed parts should be used. Further investigations are recommended to better assess the hydrogen compatibility of sensitized 316 stainless steel, and to determine the influence of sensitization on the suitability of 316 stainless steel for orthopedic application. / Master of Science

M.S.

No description available.

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / by Subramanian Suresh. / Sc.D.