Mechanisms of Microstructure Evolution in an Austenitic Stainless Steel Bond Generated Using a Quaternary Braze Alloy
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has been used extensively to create attachments between metallic structures. It has been especially successful for bonding such materials as Ni alloys,[1] steels,[2] and structural ceramics.[3] The method has re-emerged as a research topic largely because of its role in the fabrication of large-scale, lattice-based, multifunctional structures of the type depicted on Figure 1. These structures are leading candidates for applications that require ultralight weight,[4] blast resistance,[5] active cooling,[6] and shape morphing.[7] The fabrication involves the bending of thin sheets to create the core members, followed by either brazing or welding the faces to the interlayers.[8] Welding is often not feasible for topologically complex systems (Figure 1), leaving brazing as the preferred option. The concern is that, because of the multiplicity of bonds, large variability might ensue, diminishing the overall damage tolerance of the structure. This issue has motivated the present assessment. Most lattice systems have been fabricated from stainless steels or Al alloys for naval and land vehicles, respectively, with recent interest in Ni alloy lattices for aero-turbines. This investigation has a focus on austenitic stainless steels (such as 304). Several commercial brazes
exist for these materials. The most commonly used include the silver-based brazes and the nickel-based alloys.[2] In a typical brazing operation, after melting, the system is immediately cooled to ambient, and intermetallic phases form. Moreover, the intermetallics are spatially continuous (Figure 2), causing the joint to be brittle. To minimize this limitation and eliminate the intermetallics, the system may be maintained at high temperature for a time sufficient to diffuse the melting point depressants into the steel host (Figure 2 inset). This has a dramatic effect on the ductility (Figure 3). Note, however, that the ductile joint (Figure 2 inset) is narrow (of order 50 lm). When the joints are much thicker, an inordinate time is needed to eliminate the intermetallic phases.[9] When fabricating complex structures (Figure 1), it is not possible to maintain tolerances that ensure most joints are within a dimensional range consistent with elimination of the intermetallics in a practical time.[10] An important research goal for structures of the type depicted in Figure 1 is to devise brazing cycles and heat treatments that provide robust joints even when the original spacing between the steel members is several hundred micrometers. Accordingly, the objective of the present study is to sufficiently understand the mechanisms governing the microstructure that approaches for modifying the fabrication to eliminate brittleness can be identified.*
N.R. PHILIPS, Doctoral Candidate, Department of Materials, and C.G. LEVI and A.G. EVANS, Professors, Departments of Materials and Mechanical Engineering, are with the University of California, Santa Barbara, CA 93106-5050 USA. Contact e-mail: nphilips@ engr.ucsb.edu Manuscript submitted July 13, 2007. Article published online D
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