The stability of Nb/Nb5Si3 microlaminates at high temperatures

The microstructural, phase, and chemical stability of Nb/Nb5Si3 microlamina tes was investigated at temperatures ranging from 1200 degreesC to 1600 deg reesC. Freestanding Nb/Nb5Si3 microlaminates were prepared by sputter depos ition and their stability was investigated by annealing either in vacuum or in an Ar atmosphere. The microlaminates were generally structurally stable , with no evidence of layer pinchoff, even after annealing at 1600 degreesC . However, a small volume fraction (<2 pct) of voids formed in the silicide layers at 1500 degreesC and 1600 degreesC, which are attributed either to the Kirkendall diffusion of Si or to the growth of silicide grains. In term s of phase stability, there was no discernible dissolution of the Nb5Si3 la yers and no silicide precipitates in the Nb layers following anneals at 140 0 degreesC. Annealing at higher temperatures, though, resulted in the forma tion of non-equilibrium Nb3Si on the Nb/Nb5Si3 interfaces. This phase is th ought to precipitate from the supersaturated Nb-Si solid solution on coolin g, and is stabilized by the development of tensile stresses in the Nb layer s. The most pervasive observed high-temperature breakdown mechanism was che mical in nature, namely, the loss of Si via sublimation to the environment. The Si loss was partially suppressed either by annealing in a Si-rich atmo sphere or by annealing in Ar.