Near Surface Modification Affected by Hydrogen Interaction: Global Supplemented by Local Approach

The current study is centered on elastic-plastic solid interaction with hydrogen. Here, the environment isfree hydrogen, from either external or internal origins providing as such aggressive effects. In this context, near surface displacement occurred, beside microcracking onset or growth, significant interfacial weakening, as critical forms of mechanical degradation. Metastable austenitic stainless 316L steel was selected, in order to provide a comprehensive study on bulk surfaces. Globalfindings on hydrogen effects were supplemented by nanoscale information. Only for the nanosection, Ti/Cu thinfilms were also included, namely an additional small-volume case. Samples have been charged with hydrogen under lowfugacity conditions and the outcoming effects have been sorted out by mechanical response tracking assisted by contact mechanics methodology. Nanoindentation and continuous scratch tests were utilized supplemented by Scanning Probe Microscopy (SPM) visualization. Local resolution provided remarkable input to the globalfindings, in terms of dislocation nucleation aspects, near surface modification, plastic localization and microfracture onset. In thin layers, the effective work of the adhesion was reduced indicating significant degradation that could be expressed quantitatively. Global/local benefits of the stainless steel system under study made it possible to apply multiscale models describing complex micro­mechanical processes.Рассматривается взаимодействие упруго­-пластического твердого вещества с водоро­дом. Средой служит свободный водород от внешнего или внутреннего источника, что создает агрессивный эффект. В результате происходило приповерхностное смещение, кроме начала образования микротрещин или их роста и значительного межфазного раз­упрочнения, что является основными причи­нами потери механической прочности. Для всестороннего изучения внутренней струк­туры поверхности была выбрана метастабильная аустенитная нержавеющая сталь 316Л. Общие данные о действии водорода были дополнены информацией на наноуровне. Для получения данных на нано­уровне были изучены тонкие пленки Ti/Cu, т.е. проведены испытания на малом объеме материала. Образцы обрабатывали водоро­дом в условиях низкой летучести, а ре­зультаты классифицировали по механи­ческому отклику методом контактной меха­ники. Применяли наноиндентирование и непрерывное царапанье с использованием сканирующей микроскопии. Результаты ло­кальных исследований послужили значи­тельным вкладом в общие выводы, включая зарождение дислокаций, приповерхностную модификацию, начало пластической локали­зации и микроразрушения. Эффективная работа адгезии в тонких слоях уменьшилась, что свидетельствует о существенном сниже­нии механических свойств, выражаемом ко­личественно. Преимущества глобального и локального подходов при изучении нержа­веющей стали позволили использовать многоуровневые модели, описывающие комплексные микромеханические процессы

Hard amorphous hydrogenated carbon films deposited from an expanding thermal plasma

Diamondlike amorphous hydrogenated carbon is deposited from an expanding thermal argon/acetylene plasma. It is observed that the film quality improves with increasing deposition rate. To obtain the best material quality the admixed acetylene flow has to be of comparable magnitude as the argon ion flow from the plasma source (critical loading). A new method to determine the ion density in an argon/acetylene plasma, by probe measurements, is presented. They reveal that the deposition during critical loading is governed by radicals. It is suggested that acetylene is dissociated once and that the C2H radical is formed dominantly

Fracture of hard thin films using nanoindentation and nanoscratch techniques: A materials and mechanics approach

Thin films are used in many applications where special properties are needed to insure performance and reliability. Of particular interest are thin tantalum nitride films. They are used extensively in microelectronic applications because of their long term stability and low thermal coefficient of resistance. They are sputter deposited which produces films with a high structural defect content and high compressive residual stresses both of which can alter the physical and mechanical properties of microelectronic thin films. Although these films are strong heat generators, they exhibit no changes in structure or composition of the interface with aluminum oxide substrates that degrade performance or reliability. However, the use of high power density components is driving a move to replace aluminum oxide with aluminum nitride for greater heat transfer. 6 This replacement substrate creates concern as residual stresses and long-term operation could induce detrimental changes along the thin film interface not observed in aluminum oxide devices. As a result, the authors employed nanoindentation and continuous nanoscratch testing to determine the effects of the intrinsic compressive residual stresses on the properties and fracture resistance of the thin tantalum nitride films. These techniques sample small volumes of material while preserving the production configuration of a free surface. Although nanoscratch tests lack a rigorous derivation of stress distributions and strain energy release rates, good approximations for strain energy release rates can be obtained using mechanics-based models for blister formation where residual stresses dominate interfacial fracture behavior. When combined with scanning and transmission electron microscopy, the results define structure-property relationships and resistance to fracture of these hard films

Effect of deformation schedule on the microstructure and mechanical properties of a thermomechanically processed C-Mn-Si transformation-induced plasticity steel

Thermomechanical processing simulations were performed using a hot-torsion machine, in order to develop a comprehensive understanding of the effect of severe deformation in the recrystallized and nonrecrystallized austenite regions on the microstructural evolution and mechanical properties of the 0.2 wt pct C-1.55 wt pct Mn-1.5 wt pct Si transformation-induced plasticity (TRIP) steel. The deformation schedule affected all constituents (polygonal ferrite, bainite in different morphologies, retained austenite, and martensite) of the multiphased TRIP steel microstructure. The complex relationships between the volume fraction of the retained austenite, the morphology and distribution of all phases present in the microstructure, and the mechanical properties of TRIP steel were revealed. The bainite morphology had a more pronounced effect on the mechanical behavior than the refinement of the microstructure. The improvement of the mechanical properties of TRIP steel was achieved by variation of the volume fraction of the retained austenite rather than the overall refinement of the microstructure. <br /

Effect of microstructure on the stability of retained austenite in transformation-induced-plasticity steels

Two Fe-0.2C-1.55Mn-1.5Si (in wt pet) steels, with and without the addition of 0.039Nb (in wt pet), were studied using laboratory rolling-mill simulations of controlled thermomechanical processing. The microstructures of all samples were characterized by optical metallography, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The microstructural behavior of phases under applied strain was studied using a heat-tinting technique. Despite the similarity in the microstructures of the two steels (equal amounts of polygonal ferrite, carbide-free bainite, and retained austenite), the mechanical properties were different. The mechanical properties of these transformation-induced-plasticity (TRIP) steels depended not only on the individual behavior of all these phases, but also on the interaction between the phases during deformation. The polygonal ferrite and bainite of the C-Mn-Si steel contributed to the elongation more than these phases in the C-Mn-Si-Nb-steel. The stability of retained austenite depends on its location within the microstructure, the morphology of the bainite, and its interaction with other phases during straining. Granular bainite was the bainite morphology that provided the optimum stability of the retained austenite.<br /

Microstructural approach to fatigue crack processes in polycrystalline bcc materials. Annual technical progress report I

Two commercial low-alloy high-strength steels, four Fe-base alloys, and Ti-30 Mo were evaluated. Dislocation dynamics, strain-rate sensitivity and fatigue and fracture characterization were studied. (FS

Micromechanisms of brittle fracture: STM, TEM and electron channeling analysis. Final report

The original thrust of this grant was to apply newly developed techniques in scanning tunneling and transmission electron microscopy to elucidate the mechanism of brittle fracture. This grant spun-off several new directions in that some of the findings on bulk structural materials could be utilized on thin films or intermetallic single crystals. Modeling and material evaluation efforts in this grant are represented in a figure. Out of this grant evolved the field the author has designated as Contact Fracture Mechanics. By appropriate modeling of stress and strain distribution fields around normal indentations or scratch tracks, various measures of thin film fracture or decohesion and brittle fracture of low ductility intermetallics is possible. These measures of fracture resistance in small volumes are still evolving and as such no standard technique or analysis has been uniformly accepted. For brittle ceramics and ceramic films, there are a number of acceptable analyses such as those published by Lawn, Evans and Hutchinson. For more dissipative systems involving metallic or polymeric films and/or substrates, there is still much to be accomplished as can be surmised from some of the findings in the present grant. In Section 2 the author reviews the funding history and accomplishments associated mostly with bulk brittle fracture. This is followed by Section 3 which covers more recent work on using novel techniques to evaluate fracture in low ductility single crystals or thin films using micromechanical probes. Basically Section 3 outlines how the recent work fits in with the goals of defining contact fracture mechanics and gives an overview of how the several examples in Section 4 (the Appendices) fit into this framework