抄録
The effects of total strain range and test temperature on the fatigue life of a cast Intermetallic compound, Ti-34wt.%Al, which possesses a lamellar structure, γ+α2, were investigated. The behaviors of initiation and propagation of small cracks were observed in the smooth bar specimens subjected to push-pull cyclic loading. The results observed are summarized as follows.
(1) A small amount of inelastic strain is observed even in fatigue under high strain range up to 1%. The fatigue life is sensitive to the total strain range.
(2) The fatigue life shows a positive temperature dependency.
(3) Small cracks observed on/near the surface of specimens are classified into three types, Type A cracks which initiate from defects introduced during casting process and form a main crack in fatigue below 1073K. Type B cracks which initiate along an interface of lamellae being parallel to the stress axis and are blocked by grain boundaries never to grow as a long crack. Type C cracks which initiate from the cracking of the surface oxide-layer and bring numerous fracture sites in the matrix over 1173K.
(4) Although the initiation and early propagation of Type A cracks show an eminent fluctuation caused by the distribution of casting defects and the orientation of lamellar structure, they follow by a continuous propagation with an increase in crack length. Therefore, Type A cracks are typical of this material where by the fatigue fracture is brought about at the early stage of life.
