Elsevier

Acta Materialia

Volume 60, Issues 6–7, April 2012, Pages 2729-2738
Acta Materialia

Lattice strain evolution during creep in single-crystal superalloys

https://doi.org/10.1016/j.actamat.2012.01.039Get rights and content

Abstract

In situ neutron diffraction studies are carried out to characterize the micromechanical deformation occurring during tensile creep of a typical single-crystal nickel-based superalloy, CMSX-4. The loading responses of the matrix γ phase and the precipitate γ′ are distinct. Moreover, the behaviour in the tertiary creep regime (in which the γ′ phase remains intact) is qualitatively different from that in the primary creep regime (when γ′ is sheared). In tertiary creep, initial deformation of the matrix leads to a release of misfit between the phases in the (1 0 0), resulting in elastic compression of the γ in the loading direction. The load state then remains fairly constant during creep. During the initial stages of primary creep, elastic compression of the γ phase is observed until at around 2–4% creep strain this compression stabilizes as the (1 0 0) misfit is released. This is the point at which γ′ shearing is thought to begin. Subsequently, the load in the γ increases by around 200 MPa until a maximum is reached at around 8% creep strain. This load is then suddenly released, which may be due to the release of back-stress.

Introduction

Certain questions remain unanswered about the micromechanics of deformation of single-crystal superalloys. For example, to what extent is deformation on the microscale restricted to the face-centred cubic γ matrix which resides between the strengthening, ordered γ′ precipitates? Or alternatively, under what conditions are the γ′ particles sheared by dislocation activity? Moreover, how do the different modes of deformation on the microscale depend upon the temperature T and the applied stress level σ? And from an engineering perspective, how does the underlying micromechanical mode of deformation influence the macroscopic response? Answers to these questions would allow a greater understanding of the deformation characteristics of these materials, and would allow more physically faithful constitutive models to be constructed [1], [2]. In turn, such understanding would enable more accurate estimates to be made of the deformation induced under engine operating conditions.

To answer these questions unequivocally, novel and high-resolution characterization techniques are needed. Unfortunately, the micromechanics of deformation are most usually studied by post-mortem characterization of the microstructure and dislocation activity, often under conditions very different from those at which deformation was induced in the first place. For example, in the case of creep in nickel-based superalloys, the regimes of temperature and stress relevant to practical applications are 700–1100 °C and 100–850 MPa, as experienced in gas turbine applications. Yet is probably true to state that transmission electron microscopy (TEM) has been, until now, the most viable and widely used tool for studying the modes of dislocation activity [3], [4]. However, a weakness of this approach is that one is unable to probe the kinematics of deformation. Moreover, it is necessary to make sometimes rather subjective interpretations based upon the dislocation configurations, which are often relaxed because of the necessity to examine thin foils at room temperature. Finally, it is necessary to be sure that one samples a volume of material whose deformation characteristics are representative of the bulk behaviour, which might not always be the case.

In superalloys, conventionally creep is studied in three regimes [2], [3], [4]. At low temperatures and high stresses, dislocation shear of the γ′ precipitates is observed and the creep rate initially decreases in a conventional manner. This is followed by a so-called steady-state regime and finally, acceleration of the creep rate associated with void formation. Low-temperature and high-stress creep is termed the primary creep regime due to the large primary creep strains observed. At intermediate temperatures and stresses, shear of the precipitates does not occur and little initial primary creep is observed. Instead, the creep rate increases continuously with time; this is termed the tertiary regime since only the final accelerating creep rate is observed. Morphological changes of the γ′ occur at the highest temperatures. This change is termed rafting and therefore this regime is called the rafting creep regime.

In this paper, neutron diffractometry is used to directly study the micromechanisms occurring during the creep deformation of a typical single-crystal superalloy. Whilst neutron diffractometry is now a widely used method for characterizing engineering alloys [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], it is still rare for the measurements to be made in situ at temperatures and loading conditions beyond simple tensile or compression testing, as is attempted here. Moreover, the nature of specimens which are monocrystalline in form means that the experimental set-up is more complicated than that used conventionally for polycrystalline materials [7], [9], [10], [11], [19], [20]. Here, it is demonstrated that neutron diffractometry can be used to elucidate the micromechanisms of creep deformation in these materials; moreover some of the first in situ measurements made on this class of material are presented. This is done by making phase-specific determinations of the load–strain response during creep deformation.

Section snippets

Experimental details

Tensile-creep bars of the single-crystal nickel superalloy CMSX-4 were machined from homogenized, heat-treated and aged castings provided by Rolls-Royce plc, Derby, UK. The orientation of the three bars were with the loading axis within θ < 5.8° of the [1 0 0] direction. The mosaic spread of the crystals was ∼1° in the casting [1 0 0] direction and around ∼7° in the secondary [0 1 0] and [0 0 1] orientations, as is typical for industrial Bridgman castings.

In time-of-flight diffraction, the measurement is

Results

The macroscopic creep behaviour observed during creep in the primary regime, at 650 °C and 825 MPa, is quite different to that observed during tertiary creep at 900 °C and 460 MPa (Fig. 3). The macroscopic strain is defined by the standard formula ε = (l  l0)/l0, where l is the instantaneous length of the sample and l0 is the original sample length at 0 MPa. The macroscopic creep strain is defined as εcreep = (l  lx)/lx, where lx is the sample length following initial elastic loading when the applied

Summary and conclusions

The elastic lattice strain evolution during the tertiary and primary creep of single crystals of the nickel-base superalloy CMSX-4 has been examined using in situ neutron diffraction and analyzed in light of our current understanding of the dislocation mechanisms operative during creep from TEM studies. The following conclusions can be drawn from this work.

  • 1.

    In situ neutron diffractometry can be used to examine the evolution of loading state in the two phases in a single-crystal superalloy during

Acknowledgements

J.C. would like to acknowledge funding from EPSRC and QinetiQ plc of a CASE studentship. D.D. and R.C.R. gratefully acknowledge funding from EPSRC and Rolls-Royce plc for this work, and for the provision of the test bars and useful discussions by Drs. D.W. MacLachlan and N. Jones at Rolls-Royce plc. Beamtime at ISIS was generously provided by STFC, assistance in performing the experiments by Dr. E.C. Oliver and with profile fitting by Dr. H.J. Stone.

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