TENSILE PROPERTIES AND FRACTURE-TOUGHNESS OF A TI-45AL-1 1.6MN ALLOY AT LOADING VELOCITIES OF UP TO 12 M S/

A gamma-base TiAl alloy with duplex microstructure of lamellar colonie s and equiaxed gamma grains was prepared with a reactive sintering met hod. Tensile tests and fracture toughness tests at loading velocities up to 12 m/s (strain rate for tensile tests up to 3.2 x 10(2)/s) were carried out. The microstructure of the alloy before and after tensile deformation was carefully examined with a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The fractography of the tensile specimens and fracture toughness specimens was studied. The experimental results demonstrated that the ultimate tensile stren gth (UTS) and yield strength (YS) increase with increasing strain rate up to 10/s and subsequently level off. The UTS and YS exhibited simil ar strain rate sensitivity. The strain rate sensitivity exponent at st rain rates lower than 10/s is about 1.5 x 10(-2) and at higher strain rates is almost zero. In this study, fracture toughness was found to b e less sensitive to the loading velocity, having values of around 25 M Pa root m, which is believed to be attributed to the high strain rate experienced at the crack tip. The predominant deformation mechanism fo r the strain rates used in this study was found to be twinning. Howeve r, in the low strain rate range, the dislocation motion mechanism was operative at the initial deformation stage and twinning dominated the later stage of the deformation process. In the high strain rate range, the entire deformation process was dominated by twinning. The interac tion between deformation twinning and grain boundaries resulted in int ergranular fracture in the gamma grains and delamination of alpha(2)/g amma interfaces in the lamellar colonies.