MICROSTRUCTURAL CHANGE IN THE COURSE OF DECOMPOSITION OF AN AMORPHOUS PD82SI18 ALLOY

The medium range order (MRO) and its relation to the structural decomposition on annealing in Pd-Si amorphous alloys have been studied by high resolution electron microscopy, nano-beam electron diffraction and nano-beam elemental analysis. Amorphous Pd82Si18 thin film specimens were prepared by Ar-beam sputtering. The f.c.c. MRO domains were observed in the as-sputtered films. On annealing, observation frequency and the size of the MRO domains increased. The fc.c. MRO domains grew to nano-precipitates of alpha-Pd after annealing at 573 K. At this stage, the second halo diffraction ring tends to split into two in the selected area electron diffraction patterns. The Si atoms are thought to occupy octahedral interstices in the small precipitates and diffuse out to the matrix with increasing the precipitate sizes. The Si composition in the nano-precipitates decreases in the course of growth. It is concluded that the atomic free volume is annihilated by the atomic rearrangement in the formation and development of MRO domains, and that chemical ordering is developed in the surrounding matrix by the local compositional change. The structural relaxation must be corresponding to these local atomic structural changes.