The objective of this study was to characterize hydrogen-assisted crack propagation in gastungsten arc (GTA) welds of the nitrogen-strengthened, austenitic stainless steel 21Cr-6Ni-9Mn (21-6-9), using fracture mechanics methods. The fracture initiation toughness and crack growth resistance curves were measured using fracture mechanics specimens that were thermally precharged with 230 wppm (1.3 at. pct) hydrogen. The fracture initiation toughness and slope of the crack growth resistance curve for the hydrogen-precharged weld were reduced by as much as 60 and 90 pct, respectively, relative to the noncharged weld. A physical model for hydrogenassisted crack propagation in the welds was formulated from microscopy evidence and finiteelement modeling. Hydrogen-assisted crack propagation proceeded by a sequence of microcrack formation at the weld ferrite, intense shear deformation in the ligaments separating microcracks, and then fracture of the ligaments. One salient role of hydrogen in the crack propagation process was promoting microcrack formation at austenite/ferrite interfaces and within the ferrite. In addition, hydrogen may have facilitated intense shear deformation in the ligaments separating microcracks. The intense shear deformation could be related to the development of a nonuniform distribution of hydrogen trapped at dislocations between microcracks, which in turn created a gradient in the local flow stress.
This paper is a review of the current joining technologies for plasma facing components in the US for the International Thermonuclear Experimental Reactor (ITER) project. Many facilities are involved in this project. All of those facilities are not represented in the authors list but all contributions will be noted throughout the report and in the acknowledgements. Many unique and innovative joining techniques are being considered in the quest to join two candidate armor plate materials (beryllium and tungsten) to a copper base alloy heat sink (CuNiBe, OD copper, CuCrZr). These techniques include brazing and diffusion bonding, compliant layers at the bond interface, and the use of diffusion barrier coatings and diffusion enhancing coatings at the bond interfaces. The development and status of these joining techniques will be detailed in this report. Ultimately, these assemblies will be considered for possible use in selected regions of a full-scale, tokamakdesigned fusion reactor.Because beryllium reacts with all but a few elements to form intermetallics and is a strong oxide former, this study considered several different surface coatings as a means of both inhibiting these reactions and promoting a good diffusion bond between the two substrates. All bonded assemblies used aluminum and/or an aluminum-beryllium composite (AlBeMet 150) as the interfacial material in contact with beryllium. In all cases, a hot iso-static pressing (HIP) furnace was used to reduce oxidation and apply sufficient pressure to the bond area to produce metal-metal contact and subsequent bonding. Several different processing schedules were evaluated during the course of this study. Tensile testing followed by postmortem examination of the fractures and cross-sections showed that several of the trial assemblies produced excellent strength (100-200 MPa) and ductility. Two of these assemblies including one brazed assembly and one diffusion bonded assembly were tested in the electron beam test system (EBTS facility) at SNL-NM. These actively-cooled mock-ups were subjected to heat loads of up to 2000 cycles at 5 MW/m2 and 1000 cycles at 10 MW/m2 without bond failure or damage to any tiles. Several tiles were subjected to short term heat loads as high as 250 MJ/m2 (0.5s) to simulate vertical disruption events which caused surface melting of the beryllium tiles.Several different techniques and geometries are being evaluated in joining a tungsten armor rod bundle (tungsten brush) to a CuCrZr heat sink. Methods being used to coat the tungsten rod bundles with copper include plasma spraying, electroplating, and ion sputtering. Also, in collaboration with the Efremov Institute in Saint Petersburg, Russia, research is underway to coat tungsten rod bundles using a copper casting technique. Following the coating process, these bundles are bonded to the copper substrate using a low temperature diffusion bonding technique which employs a HIP process at 400-500°C. Thin coatings (100-200 nm) of nickel are used at the bond line to enhance diffusion ...
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