Shear Ligament Phenomena in Fe_3Al Intermetallics and Micromechanics of Shear Ligament Toughening

摘要

The environment-assisted cracking behavior of a Fe_3Al 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℃, 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, the area fraction V_l, and the work-to-fracture τ_1 γ_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 m~(1/2) at 2.54 x 10~(-5) rnm·s~(-1) to 47 MPa m~(1/2) 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.