In situ micromechanical testing in environmental scanning electron microscope: A new insight into hydrogen-assisted cracking

Abstract

The susceptibility of Fe–26Al–0.5Cr (at.%) intermetallic alloy to hydrogen assisted cracking was evaluated by micromechanical fracture mechanics specimens. The notched micro-beams were loaded in situ in an environmental scanning electron microscope under two conditions: one with low pressure to avoid any hydrogen effect and the other with water vapour to promote hydrogen uptake and hydrogen assisted cracking. Fractographic and electron backscatter diffraction analysis carried out on the fracture surfaces of the ruptured beams revealed the mechanisms of the crack propagation. Under both conditions, the failure was cleavage-like, accommodated with plastic deformation. The results show the influence of hydrogen-induced embrittlement on the initiation of the fracture and the plasticity of the crack tip during the propagation of the crack, where the latter becomes localised and uniform over the fracture surface. From the observations, a three stage crack propagation process is proposed. Also, the fracture toughness of the samples was evaluated by linear elastic fracture mechanics and the validity of the results are discussed.