V.V. Rielli 1*, F. Theska 1, Y. Yao 2, J. P. Best 3, S. Primig 1

1 School of Materials Science & Engineering, UNSW Sydney, NSW 2052, Australia

2 Electron Microscopy Unit (EMU), Mark Wainwright Analytical Centre, UNSW Sydney, NSW 2052, Australia

3 Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straβe 1, 40237 Düsseldorf, Germany

*v.vieirarielli@unsw.edu.au

Ni-based superalloys, such as Alloy 718, are important materials for critical aerospace applications. Elevated strength up to 650°C is enabled by the different phases found in this alloy, in particular the metastable γ′′-precipitates [1]. These precipitates contribute to strength primarily via coherency strengthening [2]. The δ-phase is the stable phase transformation of γ′′-precipitates, and it is commonly found on the grain boundaries. At low volume fractions, the δ-phase can inhibit grain growth, being also beneficial to fatigue properties [3]. The competition for Nb in areas adjacent to the δ-phase is a known phenomenon [4], where the absence of γ′′-precipitates has the potential to deteriorate mechanical strength. Nevertheless, experimental investigations on the width, chemical profile, and mechanical properties in such γ′′-free zones are scarce.

Here, as shown in Figure 1, we study the region adjacent to the δ-phase via correlative highly resolved characterization techniques. The presence of γ′-precipitates and lack of γ′′-precipitates allows the investigation of individual strengthening effects from the nanoscale precipitates in Alloy 718. The nanoindentation response of the material on the γ′′-free zone corroborates the low strengthening contribution from γ′-precipitates in Alloy 718, and the importance of γ′′-precipitates for mechanical strength [5]. Additionally, the study of the nanomechanical properties of the hard δ-phase may guide future modelling on the design of new alloys via microstructural engineering.

References

[1] V.V. Rielli; F. Godor; C. Gruber; A. Stanojevic, B. Oberwinkler; S. Primig. Materials & Design, 2021, 212, 110295.

[2] J.M. Oblak; D.F. Paulonis; D.S. Duvall. Metallurgical and Materials Transactions B, 1974, 5(1), 143–153.

[3] M. Anderson; A.L. Thielin; F. Bridier; P. Bocher; J. Savoie. Materials Science and Engineering A, 2017, 679, 48–55.

[4] V.V. Rielli; F. Theska; S. Primig; Microscopy and Microanalysis, 2021, 1–11.

[5] V.V. Rielli; F. Theska; Y. Yao; J. P. Best; S. Primig. Materials Research Letters, 2022.