Transformation of borides in directionally solidified nickel base superalloy and its mechanical response

Abstract

Directionally solidified (DS) GTD444 is a boron-modified nickel base superalloy which is used in later stage tubine blades. Boron is added as a grain boundary strengthener and it modifies the grain boundary microchemistry. During service, the blades undergo thermal exposure that could lead to further changes in the grain boundary chemistry. This is known to influence the grain boundary strength and affects the overall mechanical response of the superalloys. In the present study high resolution characterization has been carried out to investigate the evolution of grain boundary precipitates in DS GTD444 alloy under thermal aging at 900 ºC. In addition, in situ picoindentation and high-temperature tensile tests were performed to determine the effect of changes in microstructure on the mechanical properties. Atom probe tomography, in conjunction with electron microscopy, shows that nano-sized borides present along grain boundaries in the as-received DS blades completely transform into carbides after aging. These carbides are of two types: M23C6, and M6C. These carbides are present discretely along the grain boundary. The evolved microstructure after 320 h of aging shows higher transverse ductility as compared to the as-received DS blades, and the failure in all tested samples was transgranular. This suggests that the presence of discrete precipitates (borides or carbides) along the DS boundaries strengthens the grain boundaries.