Resistance against Creep-Fatigue Realized by Anisotropic and Composite Structure

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In order to examine a method for attaining higher resistance of heat-resisting alloys against fatigue and creep-fatigue at high temperatures, fatigue and creep-fatigue tests were carried out for an intermetallic compound TiAl and a unidirectianally solidified Ni-based superalloy Mar-M247LC.Both alloys are composed of anisotropic structure, such as lamellate structure of gamma and alpha2 phases, and columnar(or elongated)grains. The followings are the results obtainde for TiAl. (1)In the early stage of fatigue at high temperature, microcracks were initiated on the surface of the specimen both at casting defects and at lamellar interfaces in the grain. (2)As most of the microcracks initiated at lamellar interfaces were arrested, the main crack was formed by one of the cracks initiated at relatively large casting defects. (3)While the growth rate of the microcracs was scattered widely, the aberage rate agreed with the fracture mechanics law which was obtained by propagation tests of macro … More rack using notched specimens. It is therefore possible to estimate the fatigue life of TiAl by fracture mechanics.The followings were clarified by the creep-fatigue tests for unidirectionally solidified Mar-M247LC.(1)In creep-fatigue with uniaxial loading parallel to the solidification direction(i.e., O degree-specimen), microcracks were initiated at small segments of wavy grain boundary where the grain boundary was locally perpendicular to the stress axis direction. (2)Most of the cracks were arrested when the crack tips reached the end of the perpendicular segment. The crack, which grew into the grain out of the grain boundary, became one of the largest cracks and brought about the final fracture of the specimen. (3)Creep-fatigue tests were also carried out changing the loading direction against solidification direction(i.e., using 45 degree-and 90 degree-specimens). Independent of the loading direction, microcracks were always initiated at the perpendicular segments. As the proportion of the perpendicular segments becomes the largest, the number of cracks was maximum in 90 degree-specimen, and this specimen showed the minimum life among three specimens under the same creep-fatigue condition. Less