Evaluation of vapor deposited Nb/Nb5Si3 microlaminates

Alternating layers of Nb and amorphous Nb-Si were sputter deposited at room temperature and then annealed at elevated temperatures to produce microlaminates with flat, discrete layering and an equiaxed grain structure. The amorphous Nb-Si layers crystallize at temperatures between 740 and 800 degrees C depending upon whether the silicide was co-deposited from elemental targets, or directly deposited from a single composite target. The first phase to form on crystallization is Nb5Si3, but upon heating to 1000 degrees C for 3 h a second phase, metastable Nb3Si, forms. Annealing at 1200 degrees C for 3 h though, eliminates the Nb3Si phase. The microstructural stability of the microlaminates was examined by annealing samples at temperatures up to 1400 degrees C. In all cases very limited grooving was observed and there was no pinch-off of the layering. There was also no evidence of silicide precipitates in the Nb grains after high temperature anneals. The mechanical properties of the microlaminates were examined using room temperature tensile tests. The ultimate tensile strength of the microlaminates is 590-640 MPa. Examination of fracture surfaces from the samples reveals that the Nb layers blunt cracks in the silicide layers and contribute significantly to the observed room temperature strength of these microlaminates. (C) 1999 Elsevier Science S.A. All rights reserved.