Strength degradation mechanisms in NiAl alloy coated sapphire fibers

The efficiency of a gas turbine can be increased by increasing the combustion zone temperature. For this new materials are needed. NiAl strengthened with single crystal alpha-Al2O3 (sapphire) fibers is considered as a load bearing component in the combustion zone turbine blades. However, due to strength degradation of the fibers during composite fabrication, sufficient strengthening of NiAl can not be achieved. The goal of this thesis is to identify strength degradation relevant mechanisms in order to minimize the strength degradation during the production of Intermetallc Matrix Composites (IMC’s) in the future. NiAl and IP75 (Ni45Al45Cr7,5Ta2,5) were considered as matrix materials. The presence of an interlayer on the fiber strength was studied; hexagonal BN (h-BN) for a weak interface, Y and Hf for enhanced interface strength by compound formation. The strength of coated fibers was evaluated by tensile testing and compared to the strength of uncoated fibers. The effect of temperature during diffusion bonding was studied. None of the investigated coupling concepts yields the improvement in tensile strength of NiAl or IP75 necessary for high temperature structural applications due to process related fiber strength degradation. By systematic examination of fracture and fiber surfaces as well as chemical analysis, the following strength degradation mechanisms have been identified: Twinning of the rhombohedral plane (r-plane) of the sapphire crystal as a consequence of the evolving thermal stress during cooling. Chemical reactions between fiber and matrix material and/or impurities and surface diffusion of Al2O3 into irregularities in the adjacent matrix material like cracks in the h BN interlayer both lead to altering of the fiber surface morphology. In combination with the evolving thermal stress during cooling, fracture mirror formation at the surface flaws takes place, degrading the fiber strength. In this work the fiber strength degradation mechanisms active during individual process steps are identified. The significance of thermal stress induced fiber damage is emphasized and it is concluded that the elimination or a significant reduction thereof is the largest challenge which has to be addressed to explore the strengthening potential of sapphire fibers for IMC´s in the future. Based on the results presented here, a strategy for obtaining high strength IMC´s in the future is compiled. It is suggested that the strength degradation may be avoided or minimized by: lowering the thermal stress by increasing the fiber volume fraction and/or by using a ductile interlayer.

Grantor dc:publisher

Shaker

Year dc:date

2008

Author dc:creator

• Hajas, David E.

Contributors dc:contributor

• Schneider, Jochen M.

• info:eu-repo/classification/ddc/550
• Faserfestigkeit
• Faser
• Saphir
• Geowissenschaften
• NiAl
• Komposite
• Saphire fiber
• Tensile strength
• Coating
• Intermetallic Matrix Composite

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-112809%22

OAI record oai:identifier

https://publications.rwth-aachen.de/oai2d?verb=GetRecord&metadataPrefix=oai_dc&identifier=oai%3Apublications.rwth-aachen.de%3A50258