Ultra-high temperature deformation in a single crystal superalloy: Mesoscale process simulation and micromechanisms
Graphical abstract
Introduction
Single crystal nickel-base superalloy turbine blades in modern aero-engines operate under harsh conditions, experiencing temperatures close to their melting point, high stresses that induce plastic deformation and surface degradation via oxidation & corrosion mechanisms [1]. Survival of these components has led to design strategies that provide active cooling; this includes thermal barrier coatings applied to the aerofoil surface, surface film cooling holes or intricate internal cooling passages. The latter presents the most difficult processing challenge, since it results in very complex internal cooling geometries with discontinuities in the component cross-section [2]. During directional solidification, stress concentration features accumulate significant plastic strain, known to prevail in the temperature range below the incipient melting point, but well above the solvus temperature of the strengthening precipitate phase, [3]. The resultant plastic strain presents a significant problem during subsequent homogenisation heat treatments of the as-cast material as recrystallisation can occur [4]. In single crystal alloys, the formation of recrystallised grains is unacceptable as they are highly detrimental to both creep resistance [5] and thermal-mechanical fatigue [6]. The economic impact of the resultant scrap rate motivates a mechanism informed understanding of recrystallisation that will guide developments in casting technology.
To uncover why recrystallisation occurs, a broad range of studies have been conducted to identify the influencing factors. The onset of recrystallisation in particular has received a fair amount of attention, where it is agreed that there is a plastic strain threshold and a minimum critical temperature when this plastic strain is induced [7], [8]. It is further noted that the process will prevail as a result of a migrating recrystallised grain boundary, where the presence of precipitates will slow this process [9] and will dissolve at the migrating recrystallisation boundary front [10]. Similarly, a eutectic is considered to act as pinning sites for advancing grain boundaries, limiting growth of recrystallised grains [11]. Hence, the microstructure must play a key role in determining the susceptibility of an alloy to recrystallise. This has a direct relationship to the bulk alloy composition, which has been demonstrated to have some dependency on recrystallisation [12]. Finally, the dissolution of phases during recrystallisation is enhanced by the presence of residual stresses [13].
Both the in-service and processing behaviour of single crystal nickel-base superalloys have been traditionally derived from isothermal stress-strain measurements, often performed under constant stress or constant strain rate [14], [15], [16], [17]. The transient temperature regimes subjected to nickel-base superalloys during casting may be better represented by non-isothermal creep deformation experiments, where there is extensive data in the high temperature (up to 1200 C), low stress condition in several alloy systems including CMSX4 [18], [19], MC2 [20], [21], [22] & MC-NG [23], [24]. In creep behaviour, capturing the thermo-mechanical history dependence of deformation and microstructure is known to be critical in predicting subsequent plasticity [25], [26], which must also play a governing role in the behaviour of transient micromechanical and microstructural phenomena during an investment casting process. This is further supported by measurements obtained during cooling experiments from casting relevant temperatures where stress relaxation, stress partitioning and work hardening including dislocation structures are shown to be highly dependent on the microstructural history during cooling & creep experiments on CMSX-4 [27], [28]. Furthermore, the degree to which these properties factors vary and subsequently control microscopy behaviour is location dependent within the cast structure, due to micro-segregation remnant from the solidification [29].
In spite of extensive work already performed to date, understanding the mechanisms that lead to recrystallisation in single crystal nickel-base superalloys has not yet reached a consensus. Reasons behind may be threefold. Firstly, there exists a lack of thermo-mechanical simulation data with suitable fidelity to conditions experienced by the alloy during casting, in particular to replicate the extreme temperatures (1300 °C) and time scales of the process. In addition, the influence of the deformation history, which may vary with location, on subsequent deformation is seldom reported – conventional testing designs typically comprise a large sample volume, which averages out the contribution of individual dendrites. Finally, the micromechanisms that are operative under these conditions remain unclear due to an inadequacy of direct evidence. Therefore, it has proven challenging to predict, control and prevent the formation of secondary grains in susceptible casting geometries.
This study utilises high resolution electron backscatter diffraction (HR-EBSD) datasets to newly understand the development of the stress states and dislocation structures relevant to ultra-high temperature deformation, at length scales finer than the microstructure. This was performed on a single crystal nickel-base superalloy subjected to the macroscopic stress states considered most critical for mechanisms relevant to recrystallisation. This has been possible via the design of an industrially-relevant process simulation using an electro-thermal mechanical testing (ETMT) system with miniaturised samples. The simulation replicates the thermo-mechanical environment subjected to alloys in the solid state, that proceeds soon after solidification in the investment casting process, as the alloy cools from the eutectic temperature to just below the solvus temperature. The resultant structure was preserved by cooling rapidly under zero load, followed by post-mortem characterisation, enabling the dynamic interplay between the governing micromechanics and microstructure to be revealed.
Section snippets
Electro-thermal mechanical testing (ETMT)
This study focuses on the thermo-mechanical behaviour of the single crystal Ni-base superalloy CMSX-4, as the temperatures falls in the solid-state from the eutectic temperature and through the solvus, with deformation simultaneously applied. This procedure represents an ultra-high temperature region of an industrial casting process; gaining knowledge here is deemed technologically critical for optimised control of microstructure whilst avoiding casting defects. Experiments were performed on
Macroscopic behaviour
CMSX-4 specimens were subjected to 3 thermal-loading cycles, denoted hereon as Samples 1, 2 & 3, as described in Table 2. The corresponding stress-strain response during the three casting simulations are shown in Fig. 2a, and the accumulation of macroscopic strain as the sample cools is shown in Fig. 2b. It can be seen that as Sample 3 was held at °C at 88 MPa for 20 mins, it accumulated an additional 3 plastic strain. During this creep hold, the creep strain rate is shown in Fig. 2c. It is
Macroscopic Behaviour
A variation in mechanical response including strain hardening rate and resultant macroscopic stress state, combined with a sharp strain hardening rate increase at the different formation temperatures indicate a significant influence of differing compositions between the samples. The origin of the dramatic sample-to-sample variation of such mm-scale specimens has been previously seen to result from significant micro-segregation in the cast bar from which these samples were machined [29]. This
Summary and conclusions
A mesoscale thermal-mechanical process simulation on the single crystal nickel-base superalloy CMSX-4 was conducted to understand the microstructure and deformation patterning that develops during an industrial casting process. As-cast material was cooled from a near solidus temperature whilst subjecting samples to controlled tensile loads, and understood using a combination of macroscopic measurements and mortem electron microscopy including HR-EBSD characterisation. The following specific
Research data
HR-EBSD data for Fig. 7, Fig. 9 and 10 can be downloaded from http://doi.org/10.5281/zenodo.4115991.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors would like to acknowledge the provision of material from Rolls-Royce. D.C. acknowledges financial support from his Birmingham Fellowship and C.P. & Y.T. would like to acknowledge the funding from the Engineering and Physical Science Research Council (EPSRC) under UKRI Innovation Fellowship EP/S000828/1.
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