John J. Stephens

Intermetallic compound layer development during the solid state thermal aging of 63Sn-37Pb solder/Au-Pt-Pd thick film couples

A study was performed which examined the solid state, intermetallic compound layer growth kinetics between 63Sn-37Pb solder and a 76Au-21Pt-3Pd (wt.%) thick film conductor on 96% alumina substrates. A linear, multivariable regression analysis was used to assess the experimental data according to the following empirical relationship: x-x0=At/sup n/ exp(-/spl Delta/H/RT). A time exponent of n=0.78/spl plusmn/0.08 was observed, suggesting that a combination of bulk diffusion and interface reaction mechanisms were responsible for layer growth. The apparent activation energy, /spl Delta/H, was 106/spl ges/8 kJ/mol. Parallel aging experiments were performed on diffusion couples fabricated between 63Sn-37Pb solder and bulk alloy stock having the same Au-Pt-Pd composition as the thick film. Similar growth kinetic parameters were computed. Intermetallic compound layer growth was accelerated under thermal cycling and thermal shock conditions due to residual stresses generated by the thermal expansion mismatch between the solder and the ceramic substrate.

The effect of reinforcement stability on composition redistribution in cast aluminum metal matrix composites

Abstract A detailed microstructural characterization was performed on cast Al-7Si particulate-reinforced metal matrix composites (where the composition is in approximate weight per cent) which included extensive examination of particulate-matrix interfacial precipitation reactions. Essentially no reaction products were observed at the interface of SiC particulates. In sharp contrast, however, an extensive reaction product zone was observed at the interface and at near-interface regions of B4C reinforcement. The type of reinforcement was shown to have a profound effect on the interfacial precipitation, the elemental composition distribution and the as-cast and solution-treated-and-aged microstructures of the matrix alloy. The stability of SiC particulates in the A356 matrix alloy (Al-7Si-0.35Mg-0.2Ti) was substantially greater in comparison with highly reactive B4C particulates. Electron microprobe, scanning electron microscopy and scanning transmission electron microscopy were used to identify and analyze phases associated with the bulk matrix microstructure and the particulate-matrix interfacial reaction zones. The extensive matrix-reinforcement reactions caused selective partitioning of matrix alloying elements, thereby modifying the matrix microstructure in the composites by compositional redistribution in the matrix. B4C significantly modified the matrix microstructure of the composite, whereas the matrix containing SiC particulates remained essentially unchanged.

The active metal brazing of TZM-Mo and SiN ceramics

The active metal brazing of ceramics holds the opportunity to design metal-ceramic brazing processes without conventional metallization and nickel-plating steps. Because of their intermediate thermal coefficient of expansion (TCE) properties, molybdenum and the TZM-Mo alloy (Mo-0.5Ti-0.08Zr) are attractive candidates for joining to silicon- nitride ceramics. The recently developed Fe-Ni-Co-based Thermo-Span™ alloy, with reduced TCE from room temperature up to ≈400°C, is also an attractive candidate material for 700°C joint service applications. This article discusses the wetting, solid-state aging, and mechanical behavior of unalloyed molybdenum, TZM-Moalloy, Thermo-Span alloy, and silicon-nitride ceramic brazes made with three different active-metal braze alloys.

Short term creep rupture predictions for tantalum alloy T‐111

A knowledge of the short term creep rupture behavior of Tantalum alloy T‐111 is necessary to predict device integrity in the heat source section of Radioisotope Thermoelectric Generators (RTGs) at the end of service life, in the event of a fuel fire. High pressures exist in RTGs near the end of service life, these are caused by gas generation resulting from radioactive decay of the nuclear fuel. The internal pressure exerts a significant hoop stress on the T‐111 alloy structural containment member. This paper analyses the short term creep behavior (rupture times up to ∼2×103 hrs.) of cold worked (CW) T‐111 alloy, using the existing data of Stephenson (1967). Corellations for the time to rupture, time to 1% strain and minimum creep rate have been obtained from this data using multivariable linear regression analysis. These results are compared to other short term rupture data for T‐111 alloy. Finally, at the stress/temperature levels relevant to the RTG fuel fire scenario near the end of service life, the ...

Joining SI3N4 for Advanced Turbomachinery Applications

The main objective of this project was to develop reliable, low-cost techniques for joining silicon nitride (Si{sub 3}N{sub 4}) to itself and to metals. For Si{sub 3}N{sub 4} to be widely used in advanced turbomachinery applications, joining techniques must be developed that are reliable, cost-effective, and manufacturable. This project addressed those needs by developing and testing two Si{sub 3}N{sub 4} joining systems; oxynitride glass joining materials and high temperature braze alloys. Extensive measurements were also made of the mechanical properties and oxidation resistance of the braze materials. Finite element models were used to predict the magnitudes and positions of the stresses in the ceramic regions of ceramic-to-metal joints sleeve and butt joints, similar to the geometries used for stator assemblies.

Ceramic-Metal Brazing, From Fundamentals to Applications: A Review of Sandia National Laboratories Brazing Capabilities, Needs and Opportunities

The purpose of the report is to summarize discussions from a Ceramic/Metal Brazing: From Fundamentals to Applications Workshop that was held at Sandia National Laboratories in Albuquerque, NM on April 4, 2001. Brazing experts and users who bridge common areas of research, design, and manufacturing participated in the exercise. External perspectives on the general state of the science and technology for ceramics and metal brazing were given. Other discussions highlighted and critiqued Sandias brazing research and engineering programs, including the latest advances in braze modeling and materials characterization. The workshop concluded with a facilitated dialogue that identified critical brazing research needs and opportunities.

Interfaces in High-Strength, High-Temperature Ceramic Joints

Development of stronger, more refractory joints in non-oxide ceramics has been accompanied by a better understanding of diffusion and reactivity at ceramic interfaces. Control of the chemistry and interfacial reactivity of refractory oxide joining compositions has produced silicon nitride joints with four-point bend strengths of 550 MPa at 1000°C. The oxide compositions were chosen to be compatible with the silicon nitride grain boundary phase, which enhances bond strength. Au-Pd braze alloys with small additions of V have been used to join silicon nitride between 1200 and 1300°C, giving strengths of 520 MPa at room temperature. Microprobe and SEM analysis suggests the interfacial reaction is Si3N4 + 6Pd + 4V = 4VN + 3Pd2Si. Initial tests of the suitability of both the refractory oxide and Au-Pd-V compositions for joining SiC have been conducted.