Investigation of the \u03b3\u2032 Precipitates Dissolution in a Ni-Based Superalloy During Stress-Free Short-Term Annealing at High Homologous Temperatures

Łukasz Rakoczy,Anna Zielińska-Lipiec,Fabian Hanning,Joel Andersson,Rafał Cygan,Grzegorz Cempura,Małgorzata Grudzień-Rakoczy

doi:10.1007/s11661-021-06420-4

Abstract

The equiaxed Ni-based superalloy René 108 was subjected to short-term annealing at five temperatures between 900 °C and 1100 °C. The phase composition, phase lattice parameters, microstructure, stereological parameters, and chemical composition of γ′ precipitates were investigated by thermodynamic simulations, X-ray diffraction, scanning and transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Analysis of the γ and γ′ lattice parameters using the Nelson-Riley extrapolation function showed that the misfit parameter for temperatures 900 °C to 1050 °C is positive (decreasing from 0.32 to 0.11 pct). At 1100 °C, the parameter becomes negative, δ = − 0.18 pct. During the short-term annealing, γ′ precipitates dissolution occurred progressing more rapidly with increasing temperatures. The surface fraction of γ′ precipitates decreased with increasing temperature from 0.52 to 0.34. The dissolution of γ′ precipitates did not only proceed through uninterrupted thinning of each individual precipitate, but also included more complex mechanisms, including splitting. Based on transmission electron microscopy, it was shown that after γ′ precipitates dissolution, the matrix close to the γ/γ′ interface is strongly enriched in Co and Cr and depleted in Al.

Highlights

NICKEL-BASED superalloys are characterized by very high mechanical properties, as well as resistance to corrosion and oxidation at high homologous temperatures, making them an important group of materials in the energy, nuclear, and aviation industries.[1]
Cormier et al.[10] indicated that since the material can be subjected to short-time exposure at very high temperatures during in-service conditions, dissolution kinetics may be an essential feature, i.e., long times at low temperatures may be less harmful than shorter times at very high temperatures
During heating and holding at elevated temperature, it is expected that the strengthening precipitates will start to dissolve in the matrix, enriching it with alloying elements

Summary

Introduction

NICKEL-BASED superalloys are characterized by very high mechanical properties, as well as resistance to corrosion and oxidation at high homologous temperatures, making them an important group of materials in the energy, nuclear, and aviation industries.[1] The Ni-based superalloys are usually strengthened by the intermetallic c¢ phase, which has an L12-type ordered structure. As stated, based on the MC2 superalloy investigations, a slight increase in temperature significantly affects the stability of the c¢ precipitates, likely to cause a drastic decrease in mechanical properties due to the drop in structural hardening. Giraud et al.[11] based on the study of the CMSX-4 superalloy stated that the dissolution kinetics depends on the initial c¢ precipitates’ morphology, coherency stress relaxation, as well as accumulated plastic strain. An increase in the accumulated plastic strain leads to an increase in the dissolution kinetics relative to the variant without the applied stress

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