FATIGUE PRECRACKING AND FRACTURE-TOUGHNESS TESTING OF TIAL INTERMETALLICS

The fatigue crack initiation and growth in as-cast, two-phase near gam ma (gamma) TiAl allays with and without 2 at%Cr were studied. Prefatig ued single edge notch bend (SENB) and compact tension (CT) specimens w ere tested at room temperature and 700 degrees C to determine the frac ture toughness of the alloys. The effect of microstructure on the defo rmation, crack initiation and propagation were studied on the side and fracture surfaces of the specimens tested. The fatigue precracks gene rated by cyclic compression in SENB specimens were initiated at a grai n boundary, grew up to 50 mu m in size before they became arrested. Ex tensive damage and microcracks of 20-30 mu m length were observed at t he notch tip. However, both SENB and CT specimens were successfully fa tigue precracked in cyclic tension. The cracks were initiated at stres s intensity factor (K) levels of 9 MPa root m and 20 MPa root m in SEN B and CT specimens, respectively. The fatigue cracks were then arreste d after 0.8-1.0 mm growth. Crack growth occurred with increased load, with associated increased deformation particularly in the gamma phase, as well as crack branching and crack deviation in lamellae. SEM (Scan ning Electron Microscope) examination of fracture surfaces showed that the fatigue fracture surface was not distinguishable from the rest of the fracture surface, where the fracture mode in both regions was bri ttle. Thus, crack initiation and growth are strongly affected by the m icrostructural component and the orientation of the lamellae at the no tch tip. The composite-like fracture behaviour of the material with la rge lamellar grains leads to higher initiation toughness due to increa sed fracture surface roughness and crack deviation in lamellae.