Gavens, A. J. (JHU) Subramanian, P. R. (GE Corporate Research and) van Heerden, D. (Johns Hopkins U) Weihs, T. P. (Johns Hopkins U) Foecke, T. J. (METALLURGY DIVISION - 855)
The Stability of Nb/Nb^d5^Si^d3^ Microlaminates at High Temperatures
Metallurgical and Materials Transactions. A: Physical Metallurgy and Materials Science - September 01, 2001

The chemical, phase and microstructural stability of Nb/Nb^d5^Si^d3^ microlaminates was investigated at temperatures ranging from 1200°C to 1600°C. The microlamintes were vapor deposited at room temperature and subsequently annealed at 1200°C to crystallize the as-deposited amorphous Nb-Si layers. After annealing the Nb layers have large (1-2µm), equiaxed grains while Nb^d5^Si^d3^ layers have small (200nm), equiaxed grains and oxygen stabilized intergranular amorphous phase. The predominant breakdown mechanism of the microlaminates at elevated temperatures was a chemical instability: the loss of Si via sublimation to the environment. Si loss was partially suppressed by annealing in a Si-rich atmosphere and by annealing in Ar. The microlaminates exhibited considerable phase stability up to 1400°C with no discernable dissolution of Si from Nb^d5^Si^d3^ layers and no silicide precipitates in the Nb layers on cooling. Annealing at higher temperatures, though, resulted in the formation of Nb^d3^Si on the Nb/Nb^d5^Si^d3^ interfaces. This phase is thought to precipitate from the supersaturated Nb-Si solid solution on cooling, and is stabilized by the development of tensile stresses in the Nd layer. The layering within the microlanimates exhibited considerable microstructural stability with no evidence of pinch-off, even after extended annealing at 1600°C. Nevertheless improved processing routes for the manufacture of microlaminates will be required to increase the grain size of the silicide phase as well as to suppress the formation of porosity during processing.
Keywords: intermetallic composites , creep , deformation , NB , toughness , silicides , fracture , strength , alloys, behavior
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