Strengthening of Nano-lamellar Materials by Controlling of Nano-structure of Lamellar Interface

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TiAl alloys are the most promising light materials for high temperature structural applications. The alloys take a lamellar microstructure consisting of α_2Ti_3Al and γTiAl phases, and their strength is affected significantly by the microstructure of lamellar interfaces. It was examined in this study how to control the microstructure of the interfaces.1. Misfit dislocation is absent on α_2/7 lamellar interfaces when a y lamella is thin, and the lamellar alloys are in a coherent state (with high internal stress state). Misfit dislocations are introduced onto lamellar interfaces above a critical γ thickness of γ_c and the alloys change to a semi-coherent state. The misfit dislocation network consists of 1/2<110] ordinary dislocations and 1/6<112] super partial dislocations.2. 1/2<110] dislocations are introduced ahead of the others, because of the larger lattice misfit in <110] directions and the higher mobility of 1/2<110] dislocations. The density of misfit dislocations at a given lamellar thickness increases with increasing amount of lattice misfit.3. Spacing of misfit dislocations decreases with increasing misfit strain, and is finer in Zr added TiAl alloys than in Nb added alloys. The critical γ thickness γ_c for the introduction of misfit dislocations decreases in the Zr added alloys. Therefore, one can control the introduction of misfit dislocations by changing lattice misfit with alloy addition.4. Thermal stability of lamellar interfaces is crucial to high creep strength, and α_2/γ interfaces have the highest stability among several types of interfaces in TiAl lamellar alloys. High heating rate during hear treatment results in a high density of α_2/γ interfaces.