The effect of long-term thermal exposures on the microstructure and properties of CMSX-10 single crystal Ni-base superalloys

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Abstract

The long-term thermal stability of CMSX-10 single crystal superalloys has been studied as a function of heat treatment and thermal exposure. The CMSX-10 alloys were subjected to long-term unstressed heat treatments at temperatures of 950 and 1050 °C for periods of 1000 and 10,000 h. The microstructures and creep properties of the samples before and after long-term thermal exposures were characterized. The γ′ coarsening was observed after all exposures and the formation of topologically close packed (TCP) phases was observed in both samples exposed at 1050 °C and the 10 kh exposure at 950 °C. High temperature creep tests were carried on these samples at 950 °C at an engineering stress of 310 MPa. The creep lives were found to deteriorate with the increase in exposure time and temperature as a result of the microstructural degradation. The coarsening of the γ′ precipitates was observed to be the main factor in the degradation of the properties.

Introduction

CMSX-10 alloy is a third generation single crystal Ni-base superalloy with a composition characterized by 6 wt.% rhenium content, high refractory element levels, and low chromium contents [1], [2], [3]. The alloy provides a creep strength improvement of about 30 °C relative to the second generation single crystal Ni-base superalloys, such as CMSX-4 and PWA 1484. The alloy also shows an attractive blend of fatigue strength, tensile and impact strengths, foundry performance, heat treatability, and environmental resistance. Microstructual instability at elevated temperatures, causing γ′ coarsening and the formation of topologically close packed (TCP) phases, can be a concern for second and third generation alloys, since the stress-rupture properties can be adversely affected. Frequently, graphical representations of the creep properties exhibit breaks in the logarithmic plots of stress versus creep life, which generally coincide with the formation of TCP phases or γ′ coarsening [4], [5], [6], [7], [8], [9]. The Larsen–Miller curves also shift to the left with increased levels of TCP phase formation, indicating a reduction in rupture properties.

During γ′ coarsening in Ni-base superalloys, the particle size of γ′ increases with increasing service temperature and exposure time. The growth of γ′ is generally found to follow cubic kinetics: a=[k(tt0)]1/3, where a is half the mean length of the γ′ cubic particle edge, k is a constant, t is the time, and t0 a time zero adjustment [5]. The coarsening occurs above about 0.6 TM, where TM is the absolute melting temperature, facilitating dislocation bypassing and thus lowering the long-time creep strength. The driving forces for coarsening are the reduction in surface area at the γ–γ′ interface, a reduction in the γ−γ′ lattice mismatch strain, and a reduction in the modulus misfit [10]. The increase in the size of the γ′ will result in a loss of the precipitate coherency and a significant decrease in strength. The coarsening of γ′ has been shown to be retarded by reducing the γ/γ′ misfit strain and modulus misfit by increasing the aluminum/titanium ratio and by adding slow diffusing elements that partition to γ′, such as niobium and tantalum [5]. Furthermore, the addition of Re has been shown to slow the γ′ coarsening rate [11].

In alloys whose composition includes significant levels of refractory metals, such as Re, Cr, Mo, and W, undesirable hard, brittle, platelike intermetallic phases can form either during heat treatment or service. These hard phases have been identified as the topologically close packed phases, such as σ, μ, P, R, or Laves phases, which are often reported to result in deterioration of stress-rupture properties. The stress-rupture life of the polycrystalline nickel base superalloy, U-700, was reduced by about 50% of its expected life due to σ-phase formation [4]. Reduction in the creep rupture lives of single crystal samples have also been attributed to the presence of TCP phases [1], [2], [3], [8]. The TCP phase formation is observed to be promoted by strain in some alloys and delayed by heat treatment or processing that improves homogeneity [5]. However, no processing approach will eliminate the formation of TCP phases in an alloy chemistry that is prone to the precipitation of these phases. The physical hardness of TCP phases and their platelike morphology can cause premature cracking leading to reduced levels of ductility and brittle failure. The formation of the TCP phases also depletes refractory metals in the γ matrix causing loss of strength in the matrix [8], [12]. Also, high temperature fracture can occur along the interface between the TCP plates and the γ/γ′ matrix, resulting in a severe loss in rupture life [4], [6], [7]. The second and third generation SX superalloys show more propensity to form TCP phases than the first generation due to presence of rhenium [8], [12], which strongly influences the kinetics of TCP formation and the temperatures at which precipitation will most rapidly occur. Generally, the TCP phases form more rapidly and at higher temperatures in alloys with increased Re content [8], [12].

This study focuses on the effect of long-term thermal exposure on the microstructure and the creep properties of CMSX-10. The degradation of creep properties with the exposure temperature and time has been examined and correlated with the microstructural changes, such as the increase in γ′ precipitate size and TCP precipitation, during the long-term thermal exposure.

Section snippets

Materials and experimental procedures

The CMSX-10 superalloy samples used in this study were from a single master heat (Table 1), procured from Cannon–Muskegon Corporation (Muskegon, MI). The directionally solidified single crystal bars (1.27 cm in diameter and 20 cm long) were produced in two (2) investment casting cluster molds at PCC Airfoils, Minerva, OH. Each mold contained 19 bars and was processed using standard high gradient industry practices in a Bridgeman-type withdrawal furnace. The samples were cast in the <001>

Microstructures

The baseline samples, without any long-term exposure, exhibited a uniform distribution of cuboidal γ′ precipitates with an edge length of 0.56 μm (Fig. 1). No effect of solution heat treatment was observed, since as-heat treated microstructures for both solution heat treatments were very similar in appearance and in the γ′ size. The microstructures of the samples subjected to long-term heat treatments are shown in Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, which indicates a growth in the γ′

Discussion

The rupture life of the unexposed standard heat treated CMSX-10 alloy at 310 MPa and 950 °C was found to be about 660 h on average; whereas the samples given the alternate solution heat treatment exhibit rupture lives of about 820 h (Table 6). The properties observed in the as-heat treated samples were similar to reported rupture strengths of CMSX-10 [2], [13]; therefore, the properties of the unexposed samples will be used as the baseline properties for all subsequent comparisons.Once samples were

Conclusions

The CMSX-10 samples used in this study were found to be susceptible to σ-phase formation during long-term thermal exposure at 950 and 1050 °C. The extent of the σ-phases as well as the extent of γ′ coarsening increased when the exposure temperature was increased from 950 to 1050 °C and/or the time increased from 1 to 10 kh. The creep strength of the samples was also found to deteriorate with the increase in exposure temperature and/or time. The drop in creep strength is found to be explained

Acknowledgements

This material is based upon work supported by the National Science Foundation under Grant No. 0072671.

References (15)

  • G.E. Fuchs

    Mater. Sci. Eng. A

    (2001)
  • G.L. Erickson, in: R.D. Kissinger, D.J. Deye, D.L. Anton, A.D. Cetel, M.V. Nathal, T.M. Pollock, D.A. Woodford (Eds.),...
  • G.L. Erickson, Presented at the Second Pacific Rim International Conference on Advanced Materials and Processing...
  • G.L. Erickson

    JOM

    (1995)
  • Heat resistant materials, in: J.R. Davis and Associates (Eds.), ASM Specialty Handbook, ASM International, Materials...
  • S.T. Wlodek, in: G.E. Fuchs, K.A. Dannemann, T.A. Deragon (Eds.), Long-Term Stability of High Temperature Materials,...
  • G. Chen, C. Yao, Z. Zhong, in: J.K. Tien, S.T. Wlodek, H. Morrow, M. Gell, G.E. Maurer (Eds.), Superalloys 1980, ASM,...
There are more references available in the full text version of this article.

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