Rapidly solidified surface melts of Ni-B-Si-Cr brazing alloy
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IN view of the interesting physical and chemical properties of amorphous metals, there has been substantial interest during recent years in processing techniques which could produce bulk amorphous metal components or amorphous metal coatings on bulk crystalline components. One potential approach to the formation of amorphous coatings is to deposit a crystalline coating of a glass-forming alloy by some appropriate technique and to use a rapid surface melting technique to transform the coating to an amorphous layer. Breinan, Kear, and Banas have demonstrated that laser glazing can produce amorphous strips on the surface of crystalline Pd-4.2 pct Cu-5.1 pct Si, 1 but there are apparently no reported studies of efforts to produce continuous amorphous surfaces by this approach. The processing method selected in this study for producing the crystalline coating was chosen because it was available to us and because it appeared capable of producing surfaces of good integrity. Because it was deemed desirable to work with nonprecious metals, the investigation employed one of the nickel-based brazing alloys, BNi2. Nickel-based alloys containing Cr, B, Si, or P are used in industry for both brazing and hardfacing applications. 2 They typically exhibit high hardness and relatively low melting points. Their usefulness in furnace brazing applications derives from their low melting points relative to the alloys being joined and the speed with which the melting point depressing metalloids can diffuse out of the joint. Some alloys of these elements, including ones of composition similar to BNi2, can be rapidly solidified into an amorphous solid relatively easily? The combination of the hardening effects at modest processing rates and the ductile behavior of the amorphous phases makes these alloys interesting for the study of microstructures produced at various rates of solidification. The particular alloy used in this study (AWS BNi2, T. R. TUCKER is Physicist,formerly at the Naval Research Laboratory, Washington, DC, and now with Battelle Columbus Laboratories, Columbus, OH 43201, and J. D. AYERS is Metallurgist at the Naval Research Laboratory, Washington, DC 20375. Manuscript submitted June 5, 1980. METALLURGICALTRANSACTIONSA
also designated AMS 4777) has a liquidus of 1313 K and a solidus of 1243 K. The alloy was purchased from a commercial source in the form of an inert gas atomized powder. The composition given by the vendor was Ni-7Cr-2.8B-4.5Si-3Fe. Using coatings of this alloy on a steel substrate, controlled remelt experiments were made using two techniques--an electron-beam source in a "pulsed area" mode, and with a high power CW laser in a scanning mode. The pulsed electron-beam method provides fusion zones with nearly one-dimensional heat flow. By varying the e-beam power and the area scanned, it was possible to heat the samples to the melting point in time ranging from 1 s down to several ms. More rapid melting was achieved with focused CW laser radiation. Together these sources create melt layers with cooling rates sp
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