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 byJ _l ) was estimated by a micromechanical model. The values of the unknown parameters, which are the average ligament length$\bar l$, the area fractionV _l , and the work-to-fractureτ ₁ γ ₁, were obtained from scanning electron microscopy (SEM) observation. The total fracture toughnessK _c andJ _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$\sqrt m$ 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 onK _Q andK _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.