Improve the ductility of a nickel based superalloy by optimizing secondary phase distribution through grain boundary engineering

Abstract

The microstructure and high temperature tensile property of grain boundary engineering (GBE) treated GH984 alloy was studied by electron backscattering diffraction, scanning electron microscopy and transmission electron microscopy. After GBE treatment, the fraction of low Σ coincidence site lattice (CSL) grain boundary increases from 43.1% to 76.9%. The grain boundary network and grain boundary precipitation distribution were optimized by GBE treatment. During aging at 800 °C for 160, 320 and 640 h, the needle-like η phase and M23C6 carbide precipitated at grain boundaries, both have coherent orientation relationship with matrix. The morphology and distribution of precipitates at grain boundaries with various characters are different. The η phase is parallel distributed along coherent Σ3 grain boundary, perpendicularly precipitated at both side of incoherent Σ3 grain boundary, while only precipitated at one side of Σ9, Σ27 and random grain boundaries. Few fine carbide particles precipitated at Σ3 grain boundary, the larger carbide precipitated at Σ9, Σ27 and random grain boundaries. The high temperature ductility of GBE sample is improved compared with that of solution annealed sample under the same aging treatment. According to the experimental results, the effects of grain boundary characters on the precipitation behaviors of η phase and carbide, and their influences on the mechanical properties were discussed.