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 at 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 heated in the oil for 4 hours at 200 degrees C, ductility was found to be recovered. These results suggest the existence of hydrogen embrittlement. Shear ligaments, which are ligament-like structures connected between microcracks, were observed on the tensile specimens. They undergo ductile fracture by shearing and enhance fracture toughness. This toughness enhancement (represented by J(l)) was estimated by a micromechanical model. The values of the unknown parameters, which are the average ligament length (l) over bar, the area fraction V-l, and the work-to-fracture tau(1) gamma(1), were obtained from scanning electron microscopy (SEM) observation. The total fracture toughness K-c and J(l) were reduced toward a slower strain rate. The experimental fracture toughness, K-Q, was found to be increased with strain rate, from 35 MPa root 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 toughness of the alloy. With the presence of hydrogen, length and work-to-fracture of the shear ligament were reduced. The toughening effect caused by shear ligament was reduced, and the alloy would behave in a brittle manner.