The room temperature burst pressure of 316L stainless steel burst discs exhibited increases of about 10% over 90 days. This increase may be associated with a strain-aging phenomenon requiring the presence of carbon since tensile property instability in worked austenitic stainless steels has been reported.[1–5] The cold worked material directly beneath the score root on the burst disc could undergo the strain aging process, thus causing the observed increase in burst strength. Characterization and analysis were therefore undertaken to identify the controlling phenomena in the small heterogeneous volume that controls rupture of the burst disc. Optical metallography and magnetic measurements confirmed the presence of martensite. Nanoindentation hardness measurements were correlated with finite element simulation of the as-formed mechanical properties. A representative portion of the microstructure was then recreated through cold rolling, and subjected to real-time and accelerated thermal aging treatments and mechanical activation analysis. Saturation of strengthening was observed, and a low temperature martensite reversion anneal was found to prevent or reverse the aging process. The results are consistent with previous observations of strain aging, although in this instance the effects are observed over a 10,000-fold greater aging time. Aging mechanisms are discussed, incorporating the phenomenologies of activation enthalpy and aging kinetics. A model explaining the sensitivity of aging rate to extreme cold work-induced dislocation densities and cold work-induced vacancy content is proposed.

The following document describes the processing, testing and post-test analysis of two Be-Cu assemblies that have successfully met the heat load requirements for the first wall and dome sections for the International Thermonuclear Experimental Reactor (ITER) fusion reactor. Several different joint assemblies were evaluated in support of a manufacturing technology investigation aimed at diffusion bonding or brazing a beryllium armor tile to a copper alloy heat sink for fusion reactor applications. Judicious selection of materials and coatings for these assemblies was essential to eliminate or minimize interactions with the highly reactive beryllium armor material. A thin titanium layer was used as a diffusion barrier to isolate the copper heat sink from the beryllium armor. To reduce residual stresses produced by differences in the expansion coefficients between the beryllium and copper, a compliant layer of aluminum or aluminum-beryllium (AlBeMet-150) was used. Aluminum was chosen because it does not chemically react with, and exhibits limited solubility in, beryllium. Two bonding processes were used to produce the assemblies. The primary process was a diffusion bonding technique. In this case, undesirable metallurgical reactions were minimized by keeping the materials in a solid state throughout the fabrication cycle. The other process employed an aluminum-silicon layer as a brazing filler material. In both cases, a hot isostatic press (HIP) furnace was used in conjunction with vacuum-canned assemblies in order to minimize oxidation and provide sufficient pressure on the assemblies for full metal-to-metal contact and subsequent bonding. The two final assemblies were subjected to a suite of tests including: tensile tests and electron and optical metallography. Finally, high heat flux testing was conducted at the electron beam testing system (EBTS) at Sandia National Laboratories, NM. Here, test mockups were fabricated and subjected to normal heat loads to 10 MW/m(2) (3 Hz) and abnormal heat loads to 250 MJ/m(2) (0.5 s) to determine their performance under simulated fusion reactor conditions for first wall components. Both assemblies survived the normal heat loads with no visual damage. Optical and electron microscopy were used to evaluate the extent of the damage at the interfaces following the VDE simulations. (C) 2000 Elsevier Science B.V. All rights reserved.

The potential use of high temperature coolant (e.g. 900 degreesC He) in first wall structures would preclude the applicability of copper alloy heat sink materials and refractory metals would be potential replacements. Brazing trials were conducted in order to examine techniques to join tungsten armor to high tungsten (90-95 wt%) or molybdenum TZM heat sink materials. Palladium-, nickel- and zirconium-based filler metals were investigated using brazing temperatures ranging from 1000 degreesC to 1275 degreesC. Palladium-nickel and palladium-cobalt braze alloys were successful in producing generally sound metallurgical joints in tungsten alloy/tungsten couples, although there was an observed tendency for the pure tungsten armor material to exhibit grain boundary cracking after bonding. The zirconium- and nickel-based filler metals produced defect-containing joints, specifically cracking and porosity, respectively. The palladium-nickel braze alloy produced sound joints in the Mo TZM/tungsten couple. :Substitution of a lanthanum oxide-containing, fine-grained tungsten material (for the pure tungsten) eliminated the observed tungsten grain boundary cracking. (C) 2000 Elsevier Science B.V. All rights reserved.

Several different joint assemblies were evaluated in support of a manufacturing technology for diffusion bonding a beryllium armor tile to a copper alloy heat sink for fusion reactor applications. Because beryllium reacts with all but a few elements to form intermetallic compounds, this study considered several different surface treatments as a means of both inhibiting these reactions and promoting a good diffusion bond between the two substrates. All diffusion bonded assemblies used aluminum or an aluminum-beryllium composite (AlBeMet-150) as the interfacial material in contact with beryllium. In most cases, explosive bonding was utilized as a technique for joining the copper alloy heat sink to an aluminum or AlBeMet-150 substrate, which was subsequently diffusion bonded to an aluminum coated beryllium tile. In this approach, a 250 /spl mu/m thick titanium foil is used as a diffusion barrier between the copper and aluminum to prevent the formation of Cu-Al intermetallic phases. In all cases, a hot isostatic pressing (HIP) furnace was used in conjunction with canned assemblies in order to minimize oxidation and apply sufficient pressure on the assembly for excellent metal-to-metal contact and subsequent bonding. Several different processing schedules were evaluated during the course of this study; bonded assemblies were produced with up to 100% joint efficiency. At this writing, a beryllium-copper divertor mock-up has survived 1000 thermal cycles at 10 MW/m/sup 2/ without damage during testing at the electron beam test system (EBTS) facility at Sandia National Laboratory in New Mexico.

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 (Glidcop, Elbrador). 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.

Thermo-mechanical modeling has indicated the desirability of using 'brush' structures (clusters of small filaments or rods) for armor in plasma facing components as a means of reducing stresses at the joint with the heat sink. Fabrication challenges have prevented use of this armor configuration to date. Development work in the US has resulted in methods for fabricating tungsten (W) brush structures suitable for scale-up on the ITER targets and dome. Methods developed use 1.6 mm and 3.2 mm diameter W welding electrode as stock for the armor, and welded metallic honeycomb for fixturing. Three approaches to fabricating said structures are: (1) Cu or Cu/W functionally gradient materials (FGMs) are plasma sprayed to the W brushes followed by diffusion bonding to the Cu alloy heat sink, (2) Cu is cast to the tips of the W brushes followed by diffusion bonding to the Cu alloy heat sink, and (3) W rods are coated with a bonding aid and are directly pressed into and diffusion bonded to the Cu alloy heat sink. All three methods utilize low temperature Cu/Cu diffusion bonding techniques also developed in the US under ITER. The honeycomb core may be left in place to provide indications of armor wear.

Beryllium–copper reactivity was studied using test parameters being considered for use in the ITER reactor. In this application, beryllium–copper tiles are produced using a low-temperature copper–copper diffusion bonding technique. Beryllium is joined to copper by first plating the beryllium with copper followed by diffusion bonding the electrodeposited (ED) copper to a wrought copper alloy (CuNiBe) at 450°C, 1–3 h using a hot isostatic press (HIP). In this bonded assembly, beryllium is the armor material and the CuNiBe alloy is the heat sink material. Interface temperatures in service are not expected to exceed 350°C. For this study, an ED copper–beryllium interface was subjected to diffusion bonding temperatures and times to study the reaction products. Beryllium–copper assemblies were subjected to 350, 450 and 550°C for times up to 200 h. Both BeCu and Be2Cu intermetallic phases were detected using scanning electron microscopy and quantitative microprobe analysis. Growth rates were determined experimentally for each phase and activation energies for formation were calculated. The activation energies were 66 mol and 62 kJ mol−1 for the BeCu and Be2Cu, respectively. Tensile bars were produced from assemblies consisting of coated beryllium (both sides) sandwiched between two blocks of Hycon-3. Tensile tests were conducted to evaluate the influence of these intermetallics on the bond strength. Failure occurred at the beryllium–copper interface at fracture strengths greater than 300 MPa for the room-temperature tests. At 300°C, the fracture strength was decreased significantly and, in contrast to the room-temperature tests, the fracture initiated in the copper–copper bond. The change in fracture initiation is attributed to a decrease in the residual stresses at the beryllium–copper interface at the higher temperatures and a decrease in the intrinsic fracture strength of the ED copper.

During the ITER EDA (Engineering Design Activity), the US Home Team developed improved methods for fabricating tungsten armored plasma facing components. Thermo-mechanical modeling indicated the desirability of using "brush-like" structures (clusters of small filaments or rods) as a means of reducing thermal stresses. The commercial availability of tungsten welding electrode rods (1.6 mm and 3.2 mm diameter) significantly reduced the raw material costs. Three approaches were developed: (1) Cu is plasma sprayed to the W brushes followed by diffusion bonding or e-beam welding the copper backing to the CuCrZr heat sink, (2) Molten Cu is cast directly on the tips of the W brushes followed by the diffusion bonding step, and(3) W rods are sputter-coated with a bonding aid and are directly bonded to the CuCrZr heat sink using HIP or vacuum hot pressing. High heat flux testing was performed up to 18 MW/m(2) without damage to two small-scale divertor mockups.

Five different brazing techniques were evaluated in the process of joining beryllium to copper. Aluminum-based filler metals were used in conjunction with aluminum coatings on both beryllium and copper substrates. This innovative approach was born out of the necessity to inhibit the formation of oxides and intermetallics on the aluminum and beryllium surfaces both before and during the joining process. Several bonding techniques, diffusion barriers, and oxide inhibitors were employed to reduce the bonding problem to that of joining aluminum to aluminum. The volume of aluminum in the joint was found to be an important factor in reducing the segregation of secondary alloying elements at the beryllium interface. Plasma sprayed aluminum coatings were too porous to use in the as-sprayed condition and were further processed using a hot isostatic press (HIP) to accomplish full density. The use of plasma sprayed aluminum coatings, Al-12%Si filler metal (Alloy 718), and the HIP process produced excellent bonds between the aluminum coated beryllium and 1100-Al alloy plate which was explosively bonded to a copper alloy. Bond strengths were measured at 100% of the strength of the 1100-Al plate strength (90 MPa). The ductility of the aluminum bond was sufficient to produce extensive necking prior to fracture.

An attempt was made to review the current joining techniques being considered for the ITER plasma facing components (PFC). This review describes the general characteristics of two of the joining techniques (brazing and diffusion bonding) being used to joint a beryllium armor to a copper alloy heat sink and describes the issues associated with these joining processes. Much of the information is relevant to the other ITER material candidates. Most important is a list of the relevant activities either completed or in progress addressing the beryllium—copper joining issues.

High energy rate forming (HERF) was used to study the thermo-mechanical behavior of 22-13-5 stainless steel. Tensile yield strengths in excess of 150 k.s.i. (1033 MPa) were achieved with 15% elongations. Tests comparing 21-6-9 stainless with 22-13-5 showed the strengthening response of the latter to be better. Tests to evaluate the hydrogen compatibility of 22-13-5 in the annealed and HERFed condition showed a lack of sensitivity to both external and internal hydrogen environments.

A surface martensite was observed in a 304L stainless steel. The morphological details as influenced by polishing techniques and thermal quenching are discussed. The quantity of martensite was found to be greatly amplified by the strain energy produced by mechanical polishing.