Shear ligament phenomena in Fe3Al intermetallics and micromechanics of shear ligament toughening

Abstract

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 1×10−6 s−1 to 14.3 pct at 2 × 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°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 byJl) was estimated by a micromechanical model. The values of the unknown parameters, which are the average ligament length\(\bar l\), the area fractionVl, and the work-to-fractureτ1γ1, were obtained from scanning electron microscopy (SEM) observation. The total fracture toughnessKc andJl were reduced toward a slower strain rate. The experimental fracture toughness,KQ, was found to be increased with strain rate, from 35 MPa\(\sqrt m \) at 2.54×10−5 mm·s−1 to 47 MPa\(\sqrt m \) at 2.54×10−2 mm·s−1. The fact that strain rate has a similar effect onKQ andKc 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.