Deformation and fracture behavior in TiAl alloys under monotonic and cyclic loading conditions

Deformation and fracture behavior of two-phase TiAl alloys was investigated under monotonic as well as cyclic tension loading conditions for duplex and lamellar microstructural forms. The Hall-Perch relationship with an abnormally high constant in fully-lamellar material is explained as a combined function of grain-size and deformation-anisotropy characteristic of the lamellar structure. The effects of microstructure on the behavior are compared and analyzed for deformation temperatures below and above the brittle-ductile transition (BDT). The crack initiation toughness and associated strains near the crack tip are used to explain the inverse relationship between ductility and toughness observed at RT. The effect of temperature on the transition of cleavage-type transgranular fracture to intergranular and/or interlamellar fracture is discussed, along with the boundary weakening which is enhanced with increasing strain rate. The competition between the effects of grain size, lamellar spacing and plastic zone size on tensile and toughness properties is discussed.