HYDROGEN-INDUCED CLEAVAGE FRACTURE OF FE3AL-BASED INTERMETALLICS

Hydrogen-induced fracture of ductile Fe3Al-based intermetallics was st udied through mechanical testing, fracture surface observation, and in situ transmission electron microscopy (TEM) tests of tensile specimen s. Mechanical properties of ordinary ductile X-80 pipeline steel (low- alloy steel) were tested and compared with Fe3Al intermetallics. Elong ations of the Fe3Al alloy decreased from 14 to 10 pct, with increases in the strain rate from 10(-6) to 10(-3)/s. The elongation reduction o f Fe3Al was caused by the hydrogen-induced fracture. There was no elon gation reduction when the testing was done in mineral oil. Non-necking occurred near the fracture section, and the fracture surfaces mainly consist of cleavage and partial intergranular morphologies. Elongation near the fracture surface of the Fe3Al intermetallics was about 14 pc t, which is the same as the total elongation. For the pipeline steel, however, an elongation near the fracture cross section was greater tha n 130 pct, which was much higher than its total elongation of 17 pct. In situ TEM observation on a tensile test sample showed crack propagat ion accompanied by dislocation plasticity. When the Fe3Al was precharg ed cathodically, the crack tip was sharp. Its radius was much less tha n that obtained without hydrogen charging. The crack propagated along the grain boundary for the charged specimens, but penetrated the grain boundary for the specimen without hydrogen charging. Effects of hydro gen on plastic deformation and grain-boundary cracking are discussed i n this article.