Residual stresses and interface strengths in MoSi 2 /stainless steel 316L joints produced by metallic-glass brazing
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I. INTRODUCTION
MOLYBDENUM disilicide (MoSi2) is a potential hightemperature structural material owing to its excellent oxidation resistance, high melting temperature (2030 ⬚C), relative low density (6.24 g/cm3), high thermal conductivity (52 W/ m K), brittle-to-ductile transition near 1000 ⬚C, and stability in a variety of corrosive and oxidative environments.[1–5] Some potential uses for MoSi2 include furnace components, gas burners and ignitors, gas injection tubes, high-temperature nozzles, temperature sensor sheaths, and periscope sight tubes.[1,2,6] In order for MoSi2 to be used in many of the aforementioned applications, it must first be joined to other materials, in particular, to ferrous alloys such as stainless steels. However, direct bonding of MoSi2 to stainless steels is difficult due to the large differences in the coefficients of thermal expansion (CTE) between MoSi2 and stainless steel. The large thermal expansion mismatch coupled with the necessity of using high joining temperatures (in the case of refractory brazes) results in large residual stresses and leads to joint failure upon cooling. Low-temperature brazing techniques and the use of ductile interlayers of intermediate CTE can alleviate the problem of large thermal stresses developed upon cooling from the bonding temperatures.[7,8,9] However, the addition of the interlayers adds to the cost and complexity of the joining process. From a joining standpoint, use of metallic glasses as brazing foils provides a number of practical advantages. The use of metallic glasses reduces the size of the brazement gaps as those used with brazing pastes and powders, to achieve RAJENDRA U. VAIDYA, PARTHA RANGASWAMY, and AMIT MISRA, Technical Staff Members, are with the Materials Science and Technology Department, Los Alamos National Laboratory, Los Alamos, NM 87545. MARC VERDIER, Technical Staff Member, is with LTPCMENSEEG, Domaine Universitaire, St. Martin D’Heres, Cedex, France. Manuscript submitted August 8, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
complete filling of the braze cross section. The high flexibility and ductility of these amorphous foils allows them to be used as a preplaced preform. These metallic glasses also melt over a narrow temperature range (during transient heating). The result is less erosion of the base materials being joined, lower sensitization of the base materials due to the shorter brazing times, absence of organic solvents (as with brazing pastes), and a more uniformly brazed joint. Furthermore, these foils have a significantly smaller amount of surface oxide film, unlike the gas-atomized powders used in filler brazes. These surface oxides prevent fusion of individual particles and may result in nonuniform melting. Although metallic glasses have been used as brazes in various metal-metal systems, there have been no other studies in the literature demonstrating their use in ceramic-metal joining. We have been the first to demonstrate the successful use of a cobalt-based metallic glass (METGLAS* 2714A) *METGLAS is a tr
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