Preparation and phase transformations of melt-spun Al-Ge-Si brazing foils

A series of Al-Ge-Si alloys was melt spun into ribbons of about 40 mu m thickness. The alloy compositions were selected so as to be suitable as filler metals with brazing temperatures < 500 degrees C. In the as-quenched state the foils were relatively brittle due to the occurrence of metastable phases. After appropriate annealing treatments between 300-400 degrees C the metastable phases were transformed into a fine-grained microstructure of beta-Ge(Si) particles within the alpha-Al matrix. This led to considerably improved mechanical properties, which are manifested in decreased microhardness levels near 100 HV0.02 and bend radii < 1 mm. The transformation process of foils on annealing was investigated by differential scanning calorimetry, transmission electron microscopy and X-ray diffraction methods. The intensity of X-ray reflections of the metastable and equilibrium phases as well as the lattice parameters of alpha-Al and beta-Ge(Si) were evaluated as functions of the annealing temperature. Differences in the transformation behaviour of binary Al-Ge and ternary Al-Ge-Si alloys, in particular, the decreased transformation temperature for the decay of metastable phases in ternary alloys, were revealed.