Transgranular cleavage fracture of Fe3Al intermetallics induced by moisture and aqueous environments

Moisture and aqueous environment-induced transgranular cleavage fracture of ductile Fe3Al intermetallics were studied through mechanical testing, frac ture surface observation and in situ TEM tensile observation. Elongations o f the Fe3Al alloy decreased from 14 to 10% with a decrease of strain rate f rom 10(-3) to 10(-6)/s in air environment. The reduction in elongation of F e3Al was caused by the hydrogen generated on the specimen surface. It could be recovered when the testing was done in mineral oil. Necking was not fou nd in the tensile specimen close to the fracture section and the fracture s urfaces mainly consist of cleavage and partial intergranular morphologies. In situ TEM observation on a tensile test showed crack propagation accompan ied by a certain plastic deformation. When the Fe3Al was precharged cathodi cally, the crack tip was sharp. Its radius was much less than that without hydrogen. The environment-assisted cracking behavior of an Fe3Al intermetal lics in a 3.5% NaCl solution was studied by slow strain rate under potentia l control ranging from - 1000 to 0 mV vs. SCE. When tested at anodic potent ials, from - 500 to 0 mV vs. SCE, ductility reduced from 8.7 to 3.9%. When tested in the cathodic region, from -500 to -1000 mV, ductility was between 7.3 and 9.1%. Results of tests done on preimmersed specimens and notched t ensile specimens confirmed this material degradation to be caused by stress corrosion cracking (SCC). To identify the mechanism, an electrochemical pe rmeation technique was employed. By measuring the diffusible hydrogen conce ntration, sensitivity to hydrogen embrittlement can be assessed at differen t potentials. Anodic dissolution is believed to be the controlling mechanis m of the SCC as the alloy is less sensitive to hydrogen embrittlement at an odic potentials. (C) 1998 Elsevier Science S.A. All rights reserved.