On the Effect of Nb on the Microstructure and Properties of Next Generation Polycrystalline Powder Metallurgy Ni-Based Superalloys

Katerina A Christofidou,Olivier M D M Messé,Nicholas G Jones,Alison S Wilson,Paul M Mignanelli,Paul Bowen,Ed J Pickering,Mark C Hardy,Howard J Stone,Suyang Yu,Alex Evans,Hang-Yue Li,Daniel Child,Robert J Gilbert,Cathie M F Rae,Christos Argyrakis,Hiroto Kitaguchi

doi:10.1007/s11661-018-4682-4

Abstract

The effect of Nb on the properties and microstructure of two novel powder metallurgy (P/M) Ni-based superalloys was evaluated, and the results critically compared with the Rolls-Royce alloy RR1000. The Nb-containing alloy was found to exhibit improved tensile and creep properties as well as superior oxidation resistance compared with both RR1000 and the Nb-free variant tested. The beneficial effect of Nb on the tensile and creep properties was due to the microstructures obtained following the post-solution heat treatments, which led to a higher γ′ volume fraction and a finer tertiary γ′ distribution. In addition, an increase in the anti-phase-boundary energy of the γ′ phase is also expected with the addition of Nb, further contributing to the strength of the material. However, these modifications in the γ′ distribution detrimentally affect the dwell fatigue crack-growth behavior of the material, although this behavior can be improved through modified heat treatments. The oxidation resistance of the Nb-containing alloy was also enhanced as Nb is believed to accelerate the formation of a defect-free Cr2O3 scale. Overall, both developmental alloys, with and without the addition of Nb, were found to exhibit superior properties than RR1000.

Highlights

THE development of Ni-based superalloys has been inextricably linked to the needs of the aerospace industry and its regulators
Alloy A was found to exhibit markedly different behavior compared with Alloy B following P-SHT1, as Alloy A showed a bimodal tertiary c¢ distribution with coarser and finer particles forming in the microstructure, in contrast to the monomodal, fine distribution obtained in Alloy B
The results indicated that both developmental alloys were more resistant to creep deformation than RR1000, with Alloy B displaying a superior resistance than Alloy A, under lower-stress/higher-temperature conditions, their behaviors began to converge

Summary

Introduction

THE development of Ni-based superalloys has been inextricably linked to the needs of the aerospace industry and its regulators. 3896—VOLUME 49A, SEPTEMBER 2018 reduced environmental and acoustic emissions.[1,2] while aerodynamic design optimizations of aircraft engines are possible, the service conditions that can be tolerated by the superalloys used in the critical turbine parts limit the efficiency improvements that can be achieved. The desire to reduce the size and weight of components requires materials with higher strength levels, which can be achieved through a fine grain size and increases in the volume fraction of the strengthening c¢ phase. Such microstructures cause deterioration in the time dependent crack growth performance of the material that may limit the design life of the component. Alloy design and processing is further complicated by likely changes in microstructure and phase stability as a result of long duration exposures at temperatures above 700 °C

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