Grain Boundary Precipitation in ATI 718Plus® Alloy

ATI 718Plus® Alloy  is a very attractive as a higher-temperature replacement to the most widely-used superalloy IN718 in static and rotating turbine components. The two alloys share characteristic plate-like precipitates at the grain boundaries in addition to small coherent precipitates throughout the fcc matrix.  The precipitation at the grain boundaries is important as it prevents excessive grain growth during processing and also improves resistance to dwell fatigue crack growth along the grain boundaries.

Inconsistencies in the diffraction patterns from these grain boundary precipitates led to the conclusion that the precipitating phase in ATI 718Plus® Alloy was not exclusively the orthorhombic d  phase as found in the alloy IN718, but a doubly ordered hexagonal phase related to the h phase. SEM EDX revealed the chemistry was close to Ni₆AlNb, but to obtain the correct patterns we proposed a second level of ordering of the Nb, Ti and Al atoms within the phase.  Using high-resolution STEM-HAADF on the Titan TEM we were able to confirm both the presence of the h-related phase and also the ordering of the minor element components which form alternate layers in the structure doubling the c lattice parameter of the η-Ni₃Ti prototype. This phase has never been seen in a nickel-based superalloy and this change is likely to have implications for its growth and stability during 718Plus manufacture and use. The figure shows the growth of the phase by discontinuous precipitation (or cellular growth) mechanism which leads to fine closely-spaced precipitates likely to improve the fatigue crack propagation properties in the Alloy.