Laser alloying of Cu and Cr
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N. M. van der Pers and Th. H. de Keijser Laboratory of Metallurgy, Delft University of Technology, Rotterdamseweg 137, 2628 AL, Delft, The Netherlands (Received 30 May 1986; accepted 8 September 1986) CuCr multilayers, 0.5-1 [xm total thickness, on Cu substrates have been laser irradiated. Threshold energy densities for complete alloying with different laser wavelengths and different multilayer structures were determined using Rutherford backscattering. Results are discussed in terms of absorbance of Cu and Cr as a function of laser wavelength, overall chemical composition, and thicknesses of the individual Cu and Cr layers. Also, x-ray diffraction was used to study the microstructure of the CuCr before and after laser irradiation. A method is outlined for unraveling the contributions to peak shift of stacking faults, stresses, and change in chemical composition. The CuCr alloy produced by the laser irradiation consisted of small, very defective Cu-rich and Cr-rich crystallites. The CuCr layer was subjected to a high tensile stress. The distinct change in preferred orientation of crystallites on laser irradiation indicated a complete melting of the CuCr multilayer. A high tensile strength ( > 935 MPa) of the CuCr before and after laser alloying is suggested by the microstructure as observed by x-ray diffraction and sustained by hardness measurements. In the Cu-rich crystals 4.0 at. % Cr was in solid solution, i.e., five times the maximum equilibrium solid solubility. I. INTRODUCTION The Cu-Cr system exhibits a irascibility gap in the liquid phase for large Cr contents and a (small) solid solubility of about 0.8 at. % Cr in Cu at higher temperatures, decreasing to 0.02 at. % Cr at room temperature.' Therefore conventional metallurgical methods cannot be used to manufacture Cu-Cr solid solutions containing more than 1 at. % Cr. There is, however, a strong interest in Cu-Cr solid solutions with relatively high Cr contents. The addition of Cr is not only expected to increase the mechanical strength and the corrosion resistance of Cu but also to improve the performance of Cu in electrical high-power switches.2 Accepted methods for obtaining supersaturated solid solutions are liquid quenching (splat cooling, melt spinning, etc.), coevaporation, and ion-, electron-, and laser-beam surface irradiation. Laser surface irradiation seems to be very promising because of (i) its high quenching rate (10 9 -10 10 K/s) as compared to splat cooling (10 5 -10 7 K/s), (ii) its ability to affect layer thicknesses ( s 1 /im) far in excess of those affected by ion beams (zzO. 1 /nm), (iii) the better adhesion of the alloyed layers to the substrate as compared with coevaporated layers, (iv) the fact that samples can be processed in open air: experimental procedures are relatively easy as compared to electron- and ion-beam irradiation, and, finally, (v) the possibility of producing (metastable) surface layers on bulk substrates not restricted in shape, while splat cooling yields samples re652
J. Mater. Res. 1 (5), Sep/Oct 1986
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