The grain boundary pinning effect of the Mue phase in an advanced polycrystalline Gamma/Gamma′ Co-base superalloy

Abstract

A grain boundary pinning phase was introduced into a γ/γ′ Co-base superalloy by the addition of Mo. The intermetallic phase was identified as μ phase (Co,Ni,Cr)7(W,Mo)6 by high-energy X-ray diffraction and energy-dispersive X-ray spectroscopy in the transmission electron microscope. Synchrotron radiation in-situ heating experiments confirmed the stability of the grain structure up to 1050 °C and therefore the grain boundary pinning effect of this phase. Recrystallization heat treatments at different temperatures resulted in varying amounts of μ phase. The grain size analysis revealed that at least 1% of the phase is necessary to maintain a pinning effect during recrystallization. Atom probe tomography confirmed that Mo partitioned predominantly to the γ phase. Since the μ phase and the γʹ phase are competing for W, a lower γ′ volume fraction was formed in comparison to a Mo-free polycrystalline γ/γʹ Co-base superalloy variant. However, the homogeneous distribution of small grains resulted in a higher strength compared to the Mo-free alloy without grain boundary pinning phases. The creep resistance at 750 °C was similar to the Mo-free alloy as well as the polycrystalline Ni-base superalloy U720Li with a creep rate minimum of 1.5 × 10−7 s−1 at 620 MPa.