Microstructure and behavior of laser-mixed Cr/Ni films on Cu alloys
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J. P. Franey, J. M. Gibson, T.E. Graedel, D. C. Jacobson, G. W. Kammlott, and J. M. Poate A T&TBell Laboratories, Murray Hill, New Jersey 07974 (Received 22 September 1986; accepted 20 November 1986) Deposited thin films of Cr and Ni on Cu substrates have been melted and intermixed with a frequency-doubled 2-switehed Nd:YAG laser. The laser pulses melt the thin films and a shallow portion of the substrate. Resolidification interface volocities are on the order of 1-10 m s" 1 . Rutherford backscattering, Auger spectroscopy, and energy dispersive x-ray mapping have been used to characterize the elemental distribution. Channeling and transmission electron microscopy were used to investigate the microstructure of the surfaces produced. In contrast to the binary Cr-Cu system, where extended solid solutions are produced, the CrNi-Cu system results in a metallic glass surface. We have found that these metallic glass surfaces, which have been dubbed "stainless coppers," exhibit excellent hydrogen sulfide corrosion resistance. Their contact resistance is low and stable over long periods of time and through tens of thousands of electronic dial switching cycles. I. INTRODUCTION There is continued interest in the use of directed energy (laser, electron, and ion beams) to modify the chemical and microstructural nature of metal surfaces. In several instances improved material performance is well documented and commercial applications have resulted. One of the more exciting research areas within the directed energy processing field is laser surface alloying. ' The objective in this case is to melt, intermix, and rapidly resolidify the near surface region of a metallic substrate that is, in general, previously coated with the alloying specie (s) of interest. In concept the method is quite simple and allows great latitude in surface alloying profiles by adjusting film thickness (es) against melt depth. In practice, however, material properties and processing variables such as the thermal diffusivities, normal spectral reflectances, melting points, vapor pressures, pulse duration, incident power density, and the spatial and temporal energy shape of the laser pulse strongly influence the melt-mix-quench process.2 A great deal of the surface alloying work is directed towards producing "equivalent" surface alloys, for example, by the addition of precious metals to base metals3 or the preparation of surface stainless steels.2 These materials, though economically and politically important because of the cost and strategic nature of the alloying specie (s), are technologically less challenging. In most instances there are no thermodynamic constraints inhibiting formation of the surface alloys. Rather, the process results in a surface-engineered version of a bulk alloy that is readily prepared by conventional bulk J. Mater. Res. 2(1), Jan/Feb 1987
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methods. Perhaps the more interesting applications involve taking advantage of the intimate relationship between the molten metal melt and the cold underlying subs
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