Phase formation and reaction kinetics in M-Sn systems (M = Zr, Hf, Nb, Ta, Mo)

A systematic search for metal-tin systems which show a potentially high remelting temperature in diffusion-soldered bonds was done by investigation of phase formation and reaction kinetics in M-Sn systems (M = Zr, Hf, Nb, Ta, Mo). Phase equilibration experiments were done with powder samples prepared from pure elements, heated in evacuated silica capsules between 300 and 1000degreesC for I h to 200 days, and studied by X-ray diffraction analysis. Diffusion couples M/Sn (M = Zr, Nb, Ta, Mo) were prepared from the foils of pure elements and heated in a bonding furnace or in evacuated silica tubes at 300-700 degreesC for 6-95 h. The couples were cross-sectioned and studied by scanning electron microscopy with energy-dispersive X-ray analysis. The rates of M-Sn reactions can be semiquantitatively ordered in the sequence Zr > Nb > Hf > Ta much greater than Mo (-Sn). The activation energy of the better defined reactions Zr-Sn, Nb-Sn, Hf-Sn are in the range of 67-93 (+/- 15) kJ/mol. A Ta-Sn phase diagram was constructed with similarities to the V-Sn system. Only Zr-Sn is a potential candidate for diffusion soldering of high-temperature stable bonds, Mo is suggested as an effective diffusion barrier against the attack of liquid tin.