Elsevier

Scripta Materialia

Volume 52, Issue 6, March 2005, Pages 519-524
Scripta Materialia

Effects of Ru and Re additions on the high temperature flow stresses of Ni-base single crystal superalloys

https://doi.org/10.1016/j.scriptamat.2004.10.039Get rights and content

Abstract

The strengthening effects of Re and Ru additions in high refractory content single crystal Ni-base superalloys were investigated. An electro-thermal mechanical testing (ETMT) unit was used to measure the high temperature flow stresses of five experimental Ni-based superalloys containing varying levels of Re and Ru. Flow stresses of single phase γ and γ′ alloys with compositions representative of the constituent phases found in superalloy single crystals were also measured. Significant increases in flow stress were observed with the addition of Ru or Re. For similar alloying levels in wt%, additions of Re were more effective in strengthening the single crystal alloy when compared to addition of Ru.

Introduction

Advanced Ni-base single crystal superalloys utilize extremely high levels of refractory alloying additions to enhance structural properties at elevated temperatures. In these complex multi-component engineering alloys, additions of Mo, W and Re are commonly incorporated to provide a high degree of solid solution strengthening [1], [2], [3]. This allows remarkable improvements in high temperature creep resistance. However, alloys containing high levels of these refractory alloying additions are also susceptible to the formation of topological close packed (TCP) phases during service at high temperatures [3], [4], [5]. Microstructural and chemical changes associated with the precipitation of these refractory rich intermetallic precipitates degrade the structural integrity of the component and results in a substantial reduction of high temperature creep properties. Recently, addition of Ru in Ni-base alloys has been used to improve the microstructural stability and suppress the formation of these TCP phases, resulting in much-improved high temperature creep properties [6], [7].

At lower temperatures, when creep performance is not dictated by microstructural instabilities, the intrinsic properties of the γ and γ′ phases govern the deformation characteristics. The strength of the alloy then becomes dependent upon a number of factors including, stacking fault energies (SFE), anti-phase boundary energies (APB), lattice misfit and volume fraction of γ′. Due to the potential improvements in microstructural stability associated with Ru additions, assessing the contribution of this particular platinum group metal addition to the overall strength of the alloy is also interesting. In the present investigation, a miniaturized electro-thermal mechanical testing system [8], [9]—ETMT was used to evaluate the high temperature flow stresses of five experimental Ni-based single crystal superalloys with varying amounts of Re and Ru content. Furthermore, the flow stresses of single phase γ and γ′ alloys with compositions representative of Ni-base superalloys were also measured. Changes in flow stress associated with additions of Ru and Re are compared and discussed.

Section snippets

Experimental materials and procedures

High temperature flow stresses were investigated using five experimental single crystal Ni-base superalloys containing varying levels of Ru or Re. To minimize the effect of minor elemental additions (B, C, N, S and O) and compositional variations due to processing, chemistries of the experimental alloys (Table 1), were obtained by doping a vacuum induction melted (VIM) solute-lean base alloy with varying levels of Re or Ru. Other than the absence of Re from AC2000, levels of refractory alloying

Results and analysis

Prior to mechanical testing, the influence of the systematic additions of Re and Ru on the microstructures of the experimental single crystal Ni-base superalloys was assessed. No changes in the morphology of γ′ precipitates were observed with the additions of Re or Ru. Micrographs depicting the structures of AC2002 and AC2004 are shown in Fig. 2. However, changes in the volume fraction of γ′ precipitates, did occur as a result of the Re and Ru additions. Both elements reduced the overall volume

Discussion

The high temperature mechanical properties of Ni-base single crystal superalloys are primarily reliant on precipitation and solid solution strengthening. With γ′ precipitate volume fractions in advanced single crystal superalloys approaching ∼70%, little opportunity exists for further optimization of the microstructures. Hence, recent initiatives to increase strength and extend the temperature capability of single crystal superalloys have relied on solid solution strengthening of both the γ and

Conclusions

An ETMT unit was used to characterize the flow stresses of a number of experimental single crystal superalloys containing additions of Ru or Re. Flow stresses of single phase γ and γ′ alloys representative of the constituent phases in actual superalloys were also measured. The following conclusions could be drawn from this work:

  • 1.

    Tensile tests conducted at 900 °C and 1100 °C reveal that both Re and Ru additions provide a high degree of solid solution strengthening. Despite a lower overall content

References (14)

  • J.X. Zhang et al.

    Acta. Mater.

    (2003)
  • R.C. Reed et al.

    Scripta Mater.

    (2004)
  • W.S. Walston et al.

    Superalloys

    (1996)
  • A.F. Giamei et al.

    Metall. Trans.

    (1985)
  • G.L. Erickson

    Superalloys

    (1996)
  • R. Darolia et al.

    Superalloys

    (1988)
  • C.M.F. Rae et al.

    Superalloys

    (2000)
There are more references available in the full text version of this article.

Cited by (188)

View all citing articles on Scopus
View full text