Introduction

Quite a number of publications have been devoted to low-cycle fatigue (LCF) of metallic materials including refractory nickel alloys [1,2,3, – 4]. However, the available experimental data on the LCF characteristics of single crystals of refractory nickel alloys under “soft” and “rigid” modes of cyclic loading are insufficient for appropriate computation of the stress-strain state and assessing the life of single crystal blades of gas turbine engines [5].

Rhenium-containing (4 wt.% Re) refractory nickel alloy ZhS32 is used to produce turbine blades with columnar and single crystal structure for advanced gas turbine engines by the method of directed crystallization [6]. According to the international classification, it belongs to the new-generation alloys, has a three-phase structure of γ + γ′ + MC (here γ is an fcc disordered nickel solid solution, γ′ is a phase based on an ordered Ni3Al intermetallic compound, and MC is a tantalum-based carbide phase) and possesses elevated characteristics of long-term strength provided in the first turn by the presence of rhenium [6,7, – 8]. However, castings of single crystal blades from rhenium-containing refractory nickel alloys are characterized by chemical and structural inhomogeneity within the dendrite cells of the single crystal due to the nonequilibrium conditions of the casting by the active processes [9], which causes dendritic segregation of the alloying elements in crystallization. In directed crystallization of nickel alloys, rhenium concentrates in dendrite arms and belongs to actively segregating alloying elements. The segregation of rhenium cannot be removed by long-term homogenizing annealing, which is a cause of formation of binary M6C carbides or topologically close-packed (tcp ) phases during long-term high-temperature impacts on the rhenium-containing refractory nickel alloys [10, 11].

Application of selective laser melting (SLM) to powder compositions of multicomponent alloys for fabrication of parts makes it possible to lower substantially the segregation inhomogeneity of the material [12]. SLM is a widely used kind of additive technologies assumed to be appropriate for making articles for aircraft and other industries [13,14,15, – 16]. A special feature of the SLM process is a very high rate of cooling (on the order of about 106 K/sec) of the melted powder from liquid condition [17].

Most of the studies of high-temperature structural materials obtained by the method of SLM have been devoted to deformable refractory nickel alloys [18,19, – 20]. Special interest is attracted to application of SLM for fabricating materials by 3D-synthesis of powder compositions from castable refractory nickel alloys [21,22,23, – 24]. Creation of a refractory material by melting of a powder composition on a special single crystal substrate with specified crystallographic orientation has high practical importance for repair and production of single crystal blades of gas turbines [25, 26]. It is also interesting to compare the casting properties of refractory nickel alloys obtained by two processes, i.e., single crystal casting and selective laser melting.

The aim of the present work was to assess the LCF characteristics of refractory nickel alloy ZhS32 produced by the methods of single crystal casting and selective laser melting of a powder composition on a single crystal substrate under low-cycle “soft” (the controlled parameter was the stress amplitude ∆σ in a cycle) and “rigid” (the controlled parameter was the strain amplitude ∆ε in a cycle) loading.

Methods of Study

The object of our study was refractory nickel alloy ZhS32 of the following chemical composition (in wt.%): Ni – 6Al – 4.8Cr – 9.5Co – 1.2Mo – 4Ta – 8.4W – 3.9Re – 1.5 Nb – 0.12C – 0.015B. The alloy was melted in a vacuum induction furnace by the method used for melting rhenium-containing refractory nickel alloys [28].

An ingot of alloy ZhS32 was used to prepare a powder composition by gas atomizing (atomizing of the melt by a jet of high-purity argon) in a HERMIGA 10/100VI facility [29] to be used for laser synthesis of samples. The granulometric composition of the ZhS32-VI powder was 10 – 50 μm.

The substrate plates with axial crystallographic directions 〈001〉 and 〈111〉 used for the 3D-synthesis of samples by the method of selective laser melting of the powder composition from alloy ZhS32 were fabricated from single-crystal castings (bars with diameter about 16 mm and length about 170 mm) of the same alloy. The single crystal castings, the longitunal axis of which coincided (within 10 degrees) with one of the crystallographic directions 〈001〉 or 〈111〉, were fabricated the LMC technique (liquid metal cooling) in a commercial UVNK-9A deviceFootnote 1 for directed crystallization [30].

The process of selective laser melting of the powder of ZhS32 was conducted in an EOS M 290 facility in a medium of high-purity argon.Footnote 2 For this purpose, we fabricated a special massive platform and placed the single crystal substrate plates on the latter. The surface of the platform was covered with the powder alloy to provide continuous coating of the single crystal substrate plates with the powder material. The layer-by-layer synthesis of the samples (preforms of SLM-samples with diameter about 15 mm and length about 60 mm) also involved heating of the platform with the single crystal substrate plates to 200°C.

Smooth cylindrical specimens for testing for LCF were fabricated from the obtained single crystal castings with crystallographic orientation (CGO) 〈001〉, 〈111〉 and from preforms of SLM-samples synthesized on the single crystal substrate plates from alloy ZhS32. Prior to the fabrication of the specimens, the single crystal castings of the alloy were subjected to a vacuum homogenizing annealing at a temperature exceeding that of total dissolution of the γ′-phase (1280°C): the preforms of the SLS-samples were subjected to heat and barothermal treatments involving vacuum homogenizing annealing, hot isostatic pressing (HIP), short-term annealing in vacuum with subsequent cooling at a controlled rate from the annealing temperature. The finishing operation consisted in longitudinal mechanical polishing of their functional part.

The microstructure of the single crystals of alloy ZhS32 after the casting and heat treatment and of the preforms of the samples of ZhS32 obtained by SLM followed by heat treatment and HIP has been described in detail in [26, 31,32, – 33].

The tests for LCF were performed in a LFV100 “Walter+Bai” servohydraulic testing machine (an air furnace of type STE-12H) with 102 – 104 cycles in accordance with the requirements of the GOST 25.502 and ASTME-606 Standards. In a “rigid” loading cycle we specified the mean strain and the strain amplitude in a cycle for each sample (controlled the strain during the test and recorded the parameters with the help of an “Epsilon” extensometer (12.5-mm base). The testing mode was as follows: sinusoidal cycle form, asymmetry factor of a cycle Rε = εmin/εmax = 0, frequency f = 0.5 Hz, temperature 850°C. In a “soft” loading cycle we specified the mean stress and the stress amplitude in a cycle for each sample. The testing mode was as follows: sinusoidal cycle form, asymmetry factor of a cycle Rε = εmin/εmax = 0.1, frequency f = 1 Hz, temperature 750°C. Several samples were tested at each level. Under a “rigid” loading cycle the strain of the functional part was controlled until failure of the sample or until formation of a crack in the functional part, which was detected from 50% decrease of the maximum stress in the cycle. Under a “soft” loading cycle the stress was controlled until failure of the sample. The method of testing for LCF has been described in detail in [34].

The results obtained in the tests for LCF were processed statistically using the dependences [34, 35]

$$ N=B\exp \left(-\upbeta \Delta \upvarepsilon \right), $$
(1)
$$ N=C\times {10}^{b\Delta \upsigma}, $$
(2)

where N is the number of cycles before failure, ∆ε is the strain amplitude, ∆σ is the stress amplitude; B, C, β and b are constant factors determined from the results of the tests.

The microstructure of the samples of the alloy tested for LCF was studied using a Zeiss EVO MA 10 scanning electron microscope.Footnote 3

Results and Discussion

Single crystals of alloy ZhS32. The results of the LCF tests of single crystals of alloy ZhS32 with two orientations at a “soft” loading cycle at 750°C and at a “rigid” loading cycle at 850°C are presented in Fig. 1. We can observe considerable anisotropy of the LCF of the single crystals of ZhS32. The computed estimates of the mean values of the LCF parameters for a base N = 1 × 104 cycles for single crystals of alloy ZhS32 at 750 and 850°C for every statistical sample are presented in the Table. These data show that the preferred crystallographic orientation for single crystals of ZhS32 with respect to the resistance to low-cycle fatigue at a base of 1 × 104 cycles for “soft” loading at 750°C is 〈111〉; for “rigid” loading at 850°C the preferred crystallographic orientation is 〈001〉. A qualitatively similar effect of the CGO on the LCF characteristics at 850°C has been obtained for single crystals of refractory rhenium-containing nickel alloys ZhS32U [4], VZhM7 [36] and DD6 at 760°C, for rhenium-ruthenium-containing alloys VZhM4 [37] and VZhM8 [36], and for an intermetallic nickel-based alloy of type VKNA [38].

Fig. 1.
figure 1

Curves of low-cycle fatigue of single crystals of alloy ZhS32 with orientations 〈001〉 (1 ) and 〈111〉 (2 ): a) “soft” loading cycle at 750°C; b ) “rigid” loading cycle at 850°C.

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Analysis of the microstructure of the fractured samples of alloy ZhS32 has shown that the main sources of microcrack nucleation are located in the bulk close to precipitates of a eutectic γ′-phase independently of the crystallographic orientation of the single crystals and are commonly connected with presence of casting micropores.

SLM-samples of alloy ZhS32. The results of the tests of SLM-samples of alloy ZhS32 for LCF at 750°C with a “soft” loading cycle and at 850°C with a “rigid” loading cycle are presented in Fig. 2. It can be seen that the experimental data are arranged quite compactly in a common interval of endurances with narrow enough scattering. A wider scattering is observed for the tests of SLM-samples under “soft” loading. It follows from Fig. 2 that behavior of the curve of the low-cycle fatigue of the SLM-alloy is the same as in the single crystals of ZhS32 (see Fig. 1). The computed estimates of the LCF parameters for a base of N = 1 × 104 cycles for the SLM-alloy ZhS32 at 750 and 850°C are presented in Table 1. For comparison, we also present there the data on the LCF of some domestic castable refractory nickel alloys with a polycrystalline structure and intermetallic-carbide (γ′, MeC) reinforcement and with a single crystal structure and intermetallic (γ′) reinforcement. The former type is represented by carbon alloy VZhL21 and the latter type is represented by alloys VZhM7, VZhM4 and VZhM8. VZhM7 is alloyed with 2.6 wt.% rhenium, and VZhM4 and VZhM8 are alloyed with 6 and 6.3 wt.% rhenium and 4 and 6% ruthenium, respectively. It follows from the Table 1 that the LCF parameters of alloy ZhS32 obtained by SLM on a single crystal substrate with CGO 〈001〉 is inferior to the alloy with same composition having a single crystal structure with orientation 〈001〉 and to the other alloys with similar structures at all the temperatures studied. It should be noted that for the studied SLM-alloy tested by the “rigid” loading scheme the LCF parameter is close to that obtained for the single crystals of ZhS32 and for the other refractory nickel alloys with orientation 〈111〉.

Fig. 2.
figure 2

Curves of low-cycle fatigue of samples of alloy ZhS32 obtained by SLM on a single crystal substrate with CGO 〈001〉: a) “soft” loading cycle at 750°C; b ) “rigid” loading cycle at 850°C.

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Table 1. Low-Cycle Fatigue of Refractory Nickel Alloys Obtained by Directed (Equiaxed) Crystallization and Selective Laser Melting on a Single Crystal Substrate
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Analysis of the fracture surfaces and of the microstructure of the SLM-samples fractured in the LCF tests has shown that independently of the loading cycle (“soft” or “rigid”) the fracture mechanism was brittle (Fig. 3a ). The main sources of crack nucleation were located in the bulk close to the pores extended over the longitudinal boundaries of subgrains and oriented in the loading direction (Fig. 3b and c). The cracks propagated chiefly over the γ-phase (Fig. 3d ), which is typical for high-temperature cyclic tests of single crystals of refractory nickel alloys [4].

Fig. 3.
figure 3

Fracture appearance (a ) and longitudinal microstructure (b – d ) of SLM-samples of alloy ZhS32 after fatigue fracture by the “rigid” loading cycle at 850°C (∆ε = 0.4%, N = 32,418 cycles): b, c ) cracks propagation at the boundary of columnar subgraines; c, d ) white particles at the alloy structure—carbide tantalum-base phase.

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Conclusions

1. We have studied the low-cycle fatigue (LCF) of refractory nickel alloy ZhS32 obtained by the method of directed crystallization in the form of single crystals with crystallographic orientations 〈001〉 and 〈111〉 and by the method of selective laser meting (SLM) of a powder composition of alloy ZhS32 on single crystal substrates from the same alloy with orientations 〈001〉 and 〈111〉. The tests for the LCF were conducted in the modes of “soft” (at 750°C) and “rigid” (at 850°C) loading cycles.

2. The preferred crystallographic orientation for single crystals of ZhS32 with respect to the resistance to low-cycle fatigue at a base of 1 × 104 cycles under the “soft” loading at 750°C is 〈111〉; under the conditions of “rigid” loading at 850°C the preferred crystallographic orientation is 〈001〉.

3. The computed mean values of the LCF parameters for N = 1 × 104 loading cycles are ∆σ = 880 MPa and ∆ε = 0.86% for the single crystals of ZhS32 with CGO 〈001〉 and ∆σ = 960 MPa and ∆ε = 0.36% for the single crystals of ZhS32 with CGO 〈111〉.

4. The behavior of the curve of the LCF of the SLM-alloy ZhS32 after vacuum homogenizing annealing and hot isostatic pressing is the same as that of the heat treated single crystals of the same alloy.

5. The computed mean values of the LCF parameters after N = 1 × 104 loading cycles for the SLM-samples of alloy ZhS32 obtained on a single crystal substrate with CGO 〈001〉 are ∆σ = 820 MPa and ∆ε = 0.44%.

6. The main sources of crack nucleation in the LCF tests of the SLM-samples of ZhS32 are located in the bulk near the pores extended over the longitudinal boundaries of subgrains and oriented over the loading direction.