SHEAR LIGAMENT PHENOMENA IN FE3AL INTERMETALLICS AND MICROMECHANICS OF SHEAR LIGAMENT TOUGHENING

The environment-assisted cracking behavior of a Fe3Al intermetallic in an air moisture environment was studied. At room temperature, tensile ductility was found to be increased with strain rate, from 10.1 pct a t 1 X 10(-6) s(-1) to 14.3 pct at 2 X 10(-3) s(-1). When tensile tests were done in heat-treated mineral oil on specimens that have been hea ted in the oil for 4 hours at 200 degrees C, ductility was found to be recovered. These results suggest the existence of hydrogen embrittlem ent. Shear ligaments, which are ligament-like structures connected bet ween microcracks, were observed on the tensile specimens. They undergo ductile fracture by shearing and enhance fracture toughness. This tou ghness enhancement (represented by J(l)) was estimated by a micromecha nical model. The values of the unknown parameters, which are the avera ge ligament length (l) over bar, the area fraction V-l, and the work-t o-fracture tau(1) gamma(1), were obtained from scanning electron micro scopy (SEM) observation. The total fracture toughness K-c and J(l) wer e reduced toward a slower strain rate. The experimental fracture tough ness, K-Q, was found to be increased with strain rate, from 35 MPa roo t m at 2.54 X 10(-5) mm . s(-1) to 47 MPa root m at 2.54 X 10(-2) mm . s(-1). The fact that strain rate has a similar effect on K-Q and K-c verifies the importance of shear ligament in determining fracture toug hness of the alloy. With the presence of hydrogen, length and work-to- fracture of the shear ligament were reduced. The toughening effect cau sed by shear ligament was reduced, and the alloy would behave in a bri ttle manner.