Abstract: Based on the results of preloading-unloading and preloading-unloading-reloading compression testing, observations of corresponding cracks and fracture surfaces for TiAl intermetallics based-alloys, compression fracture behaviors are studied in detail. The results show that the compression preloading has no influence on the compression fracture behavior, but has effect on the subsequent tensile fracture behavior, i.e. drastically reducing the tensile fracture stress. For both fully lamellar(FL) and duplex(DP) specimens the shear cracking first starts from the top or /and the bottom surface. In the compression testing, the shear stresses at the top or/and the bottom surfaces are the controlling parameters. The final fracture is induced by the shear cracks produced at the top and bottom surfaces and expanding to the center of specimen and mutually connected. The causes of the longitudinal cracking are different for DP and FL alloys. The much lower values of the b measured in tensile tests are attributed to the difference of the fracture mechanisms between the tensile test and the compression tests of the TiAl alloy. For the tensile test the fracture mechanism is the cleavage fracture caused by the tensile normal stress, while for the compression test it is controlled by the shear cracking.

Key words: Compression tests pecimen, Fracture mechanism, TiAl intermetallics based alloys