Publications

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In this project, we performed a preliminary set of sintering experiments to examine nanocrystal-enabled diffusion bonding (NEDB) in Ag-on-Ag and Cu-on-Cu using Ag nanoparticles. The experimental test matrix included the effects of material system, temperature, pressure, and particle size. The nanoparticle compacts were bonded between plates using a customized hot press, tested in shear, and examined post mortem using microscopy techniques. NEDB was found to be a feasible mechanism for low-temperature, low-pressure, solid-state bonding of like materials, creating bonded interfaces that were able to support substantial loads. The maximum supported shear strength varied substantially within sample cohorts due to variation in bonded area; however, systematic variation with fabrication conditions was also observed. Mesoscale sintering simulations were performed in order to understand whether sintering models can aid in understanding the NEDB process. A pressure-assisted sintering model was incorporated into the SPPARKS kinetic Monte Carlo sintering code. Results reproduce most of the qualitative behavior observed in experiments, indicating that simulation can augment experiments during the development of the NEDB process. Because NEDB offers a promising route to low-temperature, low-pressure, solid-state bonding, we recommend further research and development with a goal of devising new NEDB bonding processes to support Sandia's customers.

The effects of heat treatment parameters were examined in complex electrical contact alloys containing Pd-Ag-Cu-Au-Pt. These alloys (Paliney tradename, Deringer-Ney Inc., Bloomfield, CT) are strengthened by precipitation reactions. During processing such as glass-to-metal joining in hermetic connectors, if the cooling rate is too slow, discontinuous precipitation (DP) of lamellar 2nd phases can spoil the strengthening effect. Two different solutionizing temperatures were employed and the effects of cooling rates between 6 C/min and >200 C/min were studied. Novel metallographic techniques were developed to reveal the microstructure of these corrosion resistant alloys and quantitative image analysis (QIA) was used to determine the amount of 2nd phase precipitates. Vickers and Knoop microhardness testing was performed to determine the effects of heat treatment parameters on mechanical properties.

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High speed, high resolution video observations of solidification cracking in longitudinal Varestraint tests of Alloy 718 reveal a number of important features of the test. For large augmented strains, crack initiation occurs at a liquid fraction of approximately 0.2, and crack growth proceeds in both advancing (growth in the torch travel direction) and retreating (growth away from the trailing edge) directions. For the advancing crack, the average growth velocity is identical to the travel speed, indicating that the advancing tip grows at isothermal temperature and consequently, a fixed liquid fraction. Termination of the advancing crack tip occurs as the augmented strain field diminishes. The retreating crack tip grows in an increasing strain field and decreasing temperature, and appears to terminate when the crack tip intersects the solidus. These observations define criteria for the major events governing the maximum crack length at high augmented strains. Coupling these criteria with models of strain development, temperature distribution, and solidification behavior allows for a priori estimation of the maximum crack length. In the present work, the maximum crack length for a variety of Nb-containing Fe and Ni-based superalloys are estimated by using the above models and criteria, and compared with experimental results. Copyright © 2006 ASM International®.

This report describes an inert atmosphere enclosed gas-tungsten arc welding system which has been assembled in support of the MC2730, MC2730A and MC 3500 Radioisotope Thermoelectric Generator (RTG) Enhanced Surveillance Program. One goal of this program is to fabricate welds with microstructures and impurity levels which are similar to production heat source welds previously produced at Los Alamos National Laboratory and the Mound Facility. These welds will subsequently be used for high temperature creep testing as part of the overall component lifetime assessment. In order to maximize the utility of the welding system, means for local control of the arc atmosphere have been incorporated and a wide range of welding environments can easily be evaluated. The gas-tungsten arc welding system used in the assembly is computer controlled, includes two-axis and rotary motion, and can be operated in either continuous or pulsed modes. The system can therefore be used for detailed research studies of welding impurity effects, development of prototype weld schedules, or to mimic a significant range of production-like welding conditions. Fixturing for fabrication of high temperature creep test samples have been designed and constructed, and weld schedules for grip-tab and test welds have been developed. The microstructure of these welds have been evaluated and are consistent with those used during RTG production.