The goal of this LDRD project is to develop a rapid first-order experimental procedure for the testing of advanced cladding materials that may be considered for generation IV nuclear reactors. In order to investigate this, a technique was developed to expose the coupons of potential materials to high displacement damage at elevated temperatures to simulate the neutron environment expected in Generation IV reactors. This was completed through a high temperature high-energy heavy-ion implantation. The mechanical properties of the ion irradiated region were tested by either micropillar compression or nanoindentation to determine the local properties, as a function of the implantation dose and exposure temperature. In order to directly compare the microstructural evolution and property degradation from the accelerated testing and classical neutron testing, 316L, 409, and 420 stainless steels were tested. In addition, two sets of diffusion couples from 316L and HT9 stainless steels with various refractory metals. This study has shown that if the ion irradiation size scale is taken into consideration when developing and analyzing the mechanical property data, significant insight into the structural properties of the potential cladding materials can be gained in about a week.
Recent developments in the ITER experimental fusion reactor require that a 316L stainless steel substructure be bonded to a precipitation strengthened CuCrZr heat sink alloy, C18150. This bond defines the cooling water pressure boundary. Given the importance of this interface, a variety of experiments with fusion welding and solid-state joining techniques have been performed. Analysis of the joints includes mechanical measurements of bond strength and microstructural analysis using optical and electron microscopy techniques. A particular emphasis was placed on the mechanical properties of the CuCrZr, since it undergoes additional thermal processing and cannot be solutionized and aged hardened per standard heat treatments. It was determined that the explosion bonding, of all the techniques examined, maximized the residual mechanical strength of the CuCrZr. The bonding parameters were optimized to minimize the amount of mixing and porosity at the interface. The details of these results and the optimization will be discussed.
Low temperature diffusion bonding of beryllium to CuCrZr was investigated for fusion reactor applications. Hot isostatic pressing was accomplished using various metallic interlayers. Diffusion profiles suggest that titanium is effective at preventing Be-Cu intermetallics. Shear strength measurements suggest that acceptable results were obtained at temperatures as low as 540C.
TufFoam™ is a TDI-free, water-blown, closed-cell, rigid polyurethane foam (PU) initially formulated as an electronics encapsulant to mitigate the effects of harsh mechanical environments. Because it contains no TDI, the handling hazards and chemical sensitization associated with exposure during processing of common, commercial PU foams are obviated. The mechanical properties of TufFoam™ have been found to be comparable or superior to conventional TDI-based foams. Beyond its original intent, it has since found use in a variety of additional applications, including as a structural material and as a thermal and electrical insulating material. TufFoam™ constituents are commercially available in commodity quantities and batch processing schedules have been developed for its preparation at densities ranging from 0.03 to 0.70 g/cc (2 to 40 pcf). TufFoam™ has a uniform, fine cell structure over the entire range of density explored. Its Tg is somewhat dependant on the cure temperature, but is approximately 127°C when cured at 65°C. The coefficient of thermal expansion (CTE) is 7x10 -5 °C -1. TufFoam™ is electrically insulating with a volume resistivity of 3x10 17 ohm-cm at a density of 0.1 g/cc.
Microelectromechanical systems (MEMS) will play an important functional role in future DOE weapon and Homeland Security applications. If these emerging technologies are to be applied successfully, it is imperative that the long-term degradation of the materials of construction be understood. Unlike electrical devices, MEMS devices have a mechanical aspect to their function. Some components (e.g., springs) will be subjected to stresses beyond whatever residual stresses exist from fabrication. These stresses, combined with possible abnormal exposure environments (e.g., humidity, contamination), introduce a vulnerability to environmentally assisted cracking (EAC). EAC is manifested as the nucleation and propagation of a stable crack at mechanical loads/stresses far below what would be expected based solely upon the materials mechanical properties. If not addressed, EAC can lead to sudden, catastrophic failure. Considering the materials of construction and the very small feature size, EAC represents a high-risk environmentally induced degradation mode for MEMS devices. Currently, the lack of applicable characterization techniques is preventing the needed vulnerability assessment. The objective of this work is to address this deficiency by developing techniques to detect and quantify EAC in MEMS materials and structures. Such techniques will allow real-time detection of crack initiation and propagation. The information gained will establish the appropriate combinations of environment (defining packaging requirements), local stress levels, and metallurgical factors (composition, grain size and orientation) that must be achieved to prevent EAC.