Transverse Creep of Nickel-Base Superalloy Bicrystals

A liquid metal cooled (LMC) solidification technique was used to produce nickel-base GTD 444 and René N4 superalloy bicrystals with varying degrees of misorientation. Creep experiments with loading normal to the bicrystal boundaries were conducted at 1255 K (982 °C). Despite the similar overall compositions of these two alloys, the GTD 444 alloy with higher levels of carbon and boron displayed significantly higher tolerance to high-angle boundaries. Creep ductilities at rupture of greater than 5 pct were observed in GTD444 for boundaries misoriented by greater than 20 deg. Electron backscatter diffraction analysis showed significant lattice rotation adjacent to the bicrystal boundary in GTD444. In contrast, the René N4 bicrystal accumulated damage along the grain boundary early in creep, failing at less than 2 pct strain.