The stability of microstructure and the fatigue deformation mechanism of duplex aluminides containing fine precipitates under cyclic loading.

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TiAl alloys are known to be a candidate for high-temperature structure materials because of high strength at high temperatures, low density and superior oxidation resistance. Fatigue propertiy is one of the most important factors for industrial application. In this project, the stability of microstructure and the deformation behavior of TiAl alloys containing fine dispersed precipitates were examined under cyclic loading, and the following summary and conclusions were reached.(1) In TiAl alloys ordinary dislocations perform the to-and-fro motion in γ matrix under cyclic loading and the vein structure composed of highly dense dislocations are formed, resulting in strong cyclic hardening. In the γ matrix, domain boundaries and γ/αィイD22ィエD2 phase boundary act as a strong barrier for the motion of dislocations and the cyclic hardening is accelerated.(2) The persistent slip bands (PSB) are formed depending on the type of γ domains. They act as a nucleation site for a micro-crack and develop … More into a failure of specimens. Formation of the PSB is closely related to twinning. Addition of Nb in TiAl alloys increases the stacking fault energy and suppressed the formation of twninning resulting in improving fatigue life.(3) Refinement of lamellae increased yield stress and improves the ductility of TiAl alloys in monotonic deformation. The refinement of lamellae is effective in suppressing the crack propagation and improvement of fatigue life.(4) In the γ matrix of Al-rich TiAl alloys, fine precipitates of AlィイD25ィエD2TiィイD23ィエD2, r-AlィイD22ィエD2Ti and h-AlィイD22ィエD2Ti with the LlィイD20ィエD2-based superstructures appear. These precipitates are homogenelously distributed and induce improvement of high-temperature strength and creep reststance.(5) When 1/2<110> ordinary and <110> superlattice dislocations enter AlィイD22ィエD2Ti and AlィイD25ィエD2TiィイD23ィエD2 precipitates, an anti-phase boundary (APB) is created behind the dislocation. Since APB energy created by the ordinary dislocation is higher than that by superlattice dislocation, the motion of ordinary dislocation is strongly interrupted by the precipitates and the superlattice dislocation is operative.(6) The h-AlィイD22ィエD2Ti and AlィイD25ィエD2TiィイD23ィエD2 precipitates maintain the good coherency with the LlィイD20ィエD2 matrix.These precipitates exhibit good thermal stability. Less