THE EFFECT OF HYDROGEN ON THE FRACTURE OF ALLOY X-750

The effect of hydrogen on the fracture of a nickel-base superalloy, al loy X-750, was investigated in the HTH condition. The effect of hydrog en was examined through tensile testing incorporating observations fro m scanning electron microscopy and light microscopy. The ductility at 25 degrees C, as measured by elongation to failure for tensile specime ns, was reduced from 21 pct for noncharged specimens to 7.3 pct for 5. 7 ppm hydrogen and to 3.5 pct for 65 ppm hydrogen. The elongation to f ailure was a function of the strain rate and test temperature. For hyd rogen-charged specimens, the elongation decreased as the strain rate d ecreased at a constant temperature, while for a constant strain rate a nd varying temperature, there was a maximum in embrittlement near 25 d egrees C and no embrittlement at -196 degrees C. For the noncharged sp ecimens, the elongation monotonically increased as temperature increas ed, while there was no noticeable effect of strain rate. Prestraining prior to charging dramatically decreased elongation after hydrogen cha rging. When the strain rate was increased on the prestrained specimens ,more plastic deformation was observed prior to failure. Failure did n ot occur until the flow stress was reached, supporting the proposition that plasticity is required for failure. The intergranular failure me chanism in alloy X-750 was a microvoid initiation process at grain bou ndary carbides followed by void growth and coalescence. The void initi ation strain, as determined from tensile data and from sectioning unfr actured specimens, was observed to be much lower in the hydrogen-charg ed specimens as compared to noncharged specimens. The reduced ductilit y may be explained by either a reduction of the interfacial strength o f the carbide-matrix interface or a local hydrogen pressure at the car bide-matrix interface.