Effects of Oxidation on the Cracking Behavior of Additive-Manufactured Cobalt-Based Alloys Under Thermal Fatigue Conditions

Abstract

Stellite alloys are widely used in the aerospace field owing to their excellent high-temperature strength and thermal fatigue resistance. However, with the rapid development of the aerospace industry, there is an urgent demand to further enhance the mechanical properties and thermal fatigue resistance of Stellite alloys. In the present study, we prepared a conventional CoCrW alloy (classified as a Stellite alloy) and a novel CoCrWAlNi alloy, which was formulated by introducing aluminum and nickel into the CoCrW matrix, using the direct laser deposition technique. Their microstructural characteristics, mechanical properties, and thermal fatigue performance were systematically investigated. The results indicated that the additions of aluminum and nickel contribute to stabilizing the γ-Co phase. Compared with the CoCrW alloy, the CoCrWAlNi alloy exhibited higher elongation at fracture. In situ observation was employed to study the initiation and propagation of thermal fatigue cracks. Meanwhile, the effects of oxidation on thermal fatigue resistance were analyzed through experimental tests and theoretical calculations based on the Huntz model. Finally, an optimized thermal fatigue mechanism tailored for cobalt-based alloys was established, which yields deeper insights into the failure mechanisms of these alloys under complex thermal-cycling fatigue conditions.

1. Introduction

As a critical component of rocket engines, the regenerative cooling nozzle participates in key processes including the generation of high-temperature combustion gas and its expansion, acceleration, and ejection [1]. By flowing through the internal cooling channels of the nozzle before entering the injector, the fuel or oxidizer effectively reduces the inner wall temperature, improves energy utilization efficiency, and prolongs the service life of the nozzle. However, such severe thermal cycling tends to induce thermal fatigue (TF) failure, as the temperature rapidly rises to several hundred degrees Celsius within seconds and then drops just as quickly [2]. TF is one of the primary factors responsible for performance degradation and service life reduction in rocket engines for aerospace applications [3]. Furthermore, as space endeavors (e.g., deep space exploration) continue to evolve, more stringent performance criteria have been placed on rocket engines, such as enhanced specific impulse and increased thrust output; these demands, in turn, entail a far more extreme operating environment. Cobalt-based alloys are extensively employed as rocket engine nozzles, owing to their exceptional mechanical strength and toughness [4,5,6]. Extensive research has been conducted on the microstructural characteristics, phase transformation behaviors, and mechanical properties of CoCrW alloys, while recent studies have focused on strategies to enhance their TF resistance [4]. Specifically, the TF resistance of cobalt-based alloys can be enhanced via alloying approaches. For instance, the addition of manganese stabilizes the ε-Co phase during the TF process, thereby reducing crack propagation rates [7]. In addition, Wu et al. [8] demonstrated that the additions of titanium and nickel (Ni) to CoCrW alloys effectively suppress TF crack propagation by reducing carbide precipitation and improving material toughness. Furthermore, the CoNiTi alloy has been developed, which presents excellent TF resistance compared with CoCrMoW and Stellite 6 alloys [9].

A defining feature of TF is the rapid cycling of temperatures, which generates substantial thermal stress throughout the process [10]. When alloys undergo TF process, volume variation, thermal expansion coefficient mismatch, and temperature gradients are the core drivers of damage initiation and evolution [11]. Wen et al. [12] investigated the microstructural evolution of cobalt-based alloys during thermal fatigue and found that new precipitates appeared around the carbides under thermal fatigue cycling conditions, thereby causing stress concentration. Oxidation also plays a critical role in TF, contributing synergistically to the processes of crack nucleation and propagation [13,14,15]. The formation and spalling of oxide films exert a significant impact on TF resistance, whereas the growth rate and interfacial adhesion of the oxide layer determine its protective efficacy [16]. Cracks form on the sample surface during thermal shock, and their propagation is driven by multiple stresses, including those induced by volume variation, mismatch in coefficients of thermal expansion (CTEs), and temperature gradients between the matrix and internal oxides [17].

Therefore, creep deformation, cyclic fatigue damage, oxidation reactions, and oxidation-induced stress must all be considered as synergistic factors during TF analysis [18,19]. Wagner was the first to investigate transient oxidation, and he deduced the formation conditions of the most thermodynamically stable oxide in binary alloys where both constituent elements are oxidizable [20]. Subsequently, Gesmundo et al. [21,22,23,24] explored the complex oxidation behaviors of binary two-phase alloys and proposed a semiquantitative criterion for the formation of protective oxide layers. However, the oxidation process is governed by multiple factors, including the composition, spatial distribution, volume fraction, and microstructural characteristics of the two phases [25]. Considering these influences, the potential modes of high-temperature oxidation under low oxidant partial pressures have been examined, revealing that oxygen diffusion kinetics and the interdiffusion coefficients of the two alloying elements exert a dominant effect [26]. Notably, internal oxidation—an important oxidation process in binary two-phase alloys—is far more complex than that in single-phase alloys, as it involves the selective conversion of specific alloy components into oxides [27]. It has been reported that a critical chromium (Cr) concentration of 15–25 wt.% is required to form a continuous and stable chromium oxide layer during the oxidation of Co-Cr alloys [28,29]. Nevertheless, the presence of other alloying elements and the precipitation status of secondary phases can alter this critical Cr concentration and modify the morphology of the formed oxides [30]. Furthermore, oxidation during TF is synergistically affected by thermal stress and creep deformation, making it far more complex than oxidation under static, isothermal conditions; to date, no precise mechanism has been established to describe this process [31].

In the present study, CoCrWAlNi and CoCrW alloys were fabricated via direct laser deposition (DLD)—a representative additive manufacturing technology. The primary objective was to add aluminum (Al) and Ni in cobalt-based alloys to inhibit the rapid oxidation of carbides and thereby improve TF resistance. The TF process was systematically characterized, and microstructural changes before and after TF testing were analyzed in detail. In addition, the coupled creep-diffusion-oxidation stress field was quantitatively calculated, and a corresponding TF model was established. This work elucidates the mechanism by which Al and Ni additions modulate TF resistance, providing a theoretical basis for the compositional optimization of cobalt-based alloys.

2. Materials and Methods

2.1. Materials and DLD Process

The powders were prepared by adding 2 wt.% Al and 25 wt.% Ni into CoCrW commercial powders and mixing in a planetary ball mill (QM-2SP12, NanDa Instrument, Nanjing, China) for 5 h at 100 rpm.

Table 1. Chemical composites of CoCrW and CoCrWAlNi powders (wt.%).

Alloy	Cr	W	C	Si	Fe	Al	Ni	Co
CoCrW	31.74	4.48	0.88	1.18	1.41	/	/	Bal.
CoCrWAlNi	23.17	3.27	0.64	0.86	1.03	2	25	Bal.

lists the chemical compositions of the powders used to fabricate the alloys by DLD. Figure 1a shows that the DLD equipment comprises a 1070 nm-wavelength ytterbium fiber laser system (YLS-2000, IPG, Marlborough, MA, USA) and a robot (Rx160, Staubli, Pfäffikon, Switzerland) to mechanically operate the movement. The substrate was AISI H13 steel and a two-layer deposition was manufactured by DLD under argon for protection. A zigzag scanning strategy was adopted, where the deposition speed of the first (bottom) layer was 6 mm/s and that for the second (top) layer was 4 mm/s. The laser power, powder flow rate, and gas flow were set as 800 W, 8 g/min, and 12 L/min, respectively.

2.2. Microtensile Testing and Thermal Fatigue Testing

Microtensile tests were conducted at room temperature, and the samples were machined to the geometry shown in Figure 1b, which was designed according to International Standard ISO-6892-1 [32]. The specimen thickness was set to 0.5 mm, since TF cracks predominantly initiate from the surface. Thus, surface-layer metals were carefully extracted to enable a reliable comparison of mechanical properties. All the samples were ground and polished to ensure the same surface roughness. Microtensile tests were conducted on a 5 kN screw machine (MTEST5000W Tensile Stage, GATAN, Abingdon, UK) at a displacement rate of 0.1 mm/min under an in situ scanning electron microscope (SEM, LYRA3GMU, TESCAN, Brno, Czech Republic). The deformation was measured by the movement of the speckle pattern on the gauge section, which was deposited before the test. Three specimens were prepared for each alloy and subjected to microtensile testing, and the tensile data presented are the average values obtained from three independent measurements.

The TF test parameters were set with reference to the operating conditions of a certain type of engine: the temperature was raised to 600 °C within 5.5 s and then cooled down to room temperature within 10 s. The TF test was performed using the testing apparatus, and the dimensions are shown in Figure 1d. The test involved cyclic heating and water cooling, and a schematic diagram is shown in Figure 1c; the TF cycle is illustrated in Figure 1e. The temperature was measured using a K-type thermocouple. Three replicate specimens were also prepared for each material for thermal fatigue testing.

2.3. Microstructure Characterization

The microstructural morphologies of the samples before and after the tensile and TF tests were observed by scanning electron microscope (SEM) equipped with an energy dispersive spectroscope (EDS; Aztec X-MaxN, Oxford, UK) and electron backscatter diffraction (EBSD; HKL Nordlys Max3, Oxford, UK). The phase constituents and surface oxides of the alloys were characterized by X-ray diffraction (XRD; Ultima IV, Rigaku, Tokyo, Japan) and X-ray photoelectron spectroscopy (XPS; AXIS Ultra DLD, Tokyo, Japan), respectively. The TF process was observed using SEM and a laser scanning microscope (LSM, 900, Zeiss, Oberkochen, Germany).

3. Results

3.1. Microstructures and Tensile Properties of DLD Manufactured Alloys

As shown in Figure 2a,b, both alloys consist of fine dendrites and network-shaped eutectic structures. Under backscattered electron (BSE) imaging, the contrast in image intensity corresponds to chemical composition differences between the matrix and precipitated phases. The area fraction of precipitates in the CoCrW alloy is 20%, which is higher than the 14% measured in the CoCrWAlNi alloy. The overall cross-sectional morphologies of the two alloys are shown in Figure 2c,d, with a thickness of approximately 1 mm. The tensile specimens were machined from the surface to a depth of 0.5 mm in the middle region. As listed in

Table 2. Chemical composites of the points in Figure 2c,d (wt.%).

Location	Cr	W	C	Si	Fe	Al	Ni	Co
Sp1	30.59	4.05	1.02	2.14	1.05	/	/	61.15
Sp2	25.18	4.01	0.96	1.21	0.95	1.82	25.81	40.06

, the compositions of the cladded alloys are close to that of the original powder according to the EDS analysis.

XRD analysis (Figure 2e) reveals that both alloys contain γ-Co, M₂₃C₆, and M₇C₃ phases, whereas ε-Co is only detected in the CoCrW alloy. The γ-Co matrix is formed during solidification, and eutectic carbides precipitate within the interdendritic regions, mainly consisting of M₂₃C₆ and M₇C₃. Owing to the low stacking fault energy (SFE) of the CoCrW alloy [33], ε-Co is generated from γ-Co via strain-induced martensitic transformation during cooling. The additions of Al and Ni elements increase the stacking fault energy of the alloy, which favors the nucleation and propagation of twins and dislocations rather than martensitic phase transformation [8].

Figure 2f displays the engineering stress–strain curves of the CoCrW and CoCrWAlNi alloys. Although the CoCrW alloy exhibited higher ultimate tensile strength and yield strength than the CoCrWAlNi alloy, its plasticity was considerably poor. The additions of Al and Ni elements improved the ductility effectively, and the CoCrWAlNi alloy achieved an elongation of 6%.

Figure 3a,b present the fracture morphologies of the two alloys, which were analyzed to characterize their deformation and fracture behaviors. Although the two metals exhibit similar microscopic fracture characteristics with dimples observed in both, it can be seen from Figure 3c,d that the fracture surface of the CoCrW alloy is relatively flat with low elongation. In contrast, the CoCrWAlNi alloy shows a more tortuous fracture surface, indicating improved ductility.

3.2. Thermal Fatigue Performance of DLD Manufactured Alloys

Figure 4 presents the initiation and propagation of cracks in the CoCrW and CoCrWAlNi alloys. The surface morphologies were observed by SEM and LSM at intervals of every 100 TF cycles. The critical TF cycles were defined as the number of cycles required for the crack length to exceed 50 μm at the central region of the samples. The average crack growth rate was determined as the ratio of the change in surface crack length to the corresponding number of cycles. Compared with the CoCrW alloy, the CoCrWAlNi alloy exhibited an increase in critical TF cycles from 1700 to 2200. Meanwhile, the average crack growth rate decreased from 0.023 μm/cycle to 0.015 μm/cycle along the length direction and from 0.04 μm/cycle to 0.035 μm/cycle along the depth direction. In order to reveal the TF mechanism and the differences in TF resistance between the two alloys, in situ TF tests were carried out.

Figure 5 and Figure 6 respectively illustrate the crack evolution of the CoCrW and CoCrWAlNi alloys from their critical TF cycles up to 3000 cycles. All samples were subjected to 3000 TF cycles to monitor the initiation and propagation of cracks. It can be observed that TF cracks propagate rapidly once initiated. The TF cracks initiated earlier and grew faster in the CoCrW alloy than in the CoCrWAlNi alloy. Moreover, surface oxidation was found to develop preferentially along the cracks, indicating that TF behavior is associated with oxidation, in addition to the abrupt temperature variation.

3.3. Oxidation and Cracking During Thermal Fatigue Process

Figure 7 presents high-resolution XPS spectra obtained from the TF surfaces of the CoCrW and CoCrWAlNi alloys. High-resolution scans were acquired for the Cr 2p3/2 and Co 2p3/2 regions. The spectra can be deconvoluted into metallic and oxidized states: Cr (573.1 eV), Cr₂O₃ (577 eV), Co (777.8 eV), and Co₃O₄ (780.3 eV), as shown in Figure 7(a₁,a₂,b₁,b₂). For the CoCrWAlNi alloy, the Al 2p3/2 spectrum was also fitted into two components, corresponding to metallic Al (73 eV) and Al₂O₃ (77.3 eV), while the Ni 3p signal (66.3 eV) was detected as well (Figure 7(b₃)). The high-resolution Ni 2p3/2 spectrum was further deconvoluted into metallic Ni (852.1 eV) and NiO (854.6 eV) (Figure 7(b₄)). Based on the cation fraction analysis of Co, Cr (Figure 8), and their corresponding oxides, the additions of Al and Ni elements promote the formation of a continuous and compact Cr-rich oxide film. This effect is attributed to the enhanced self-diffusivity of Cr [34].

Figure 9 and Figure 10 illustrate the cross-sectional morphologies of the TF damaged surfaces, which reflect the differences in oxidation behavior between the two alloys. The blue line in Figure 9 denotes the scanning path of the line scan. At 1500 TF cycles, a thin and continuous oxide layer was formed on the CoCrWAlNi alloy. In contrast, a significant oxygen signal was detected in the network-shaped eutectic structures of the CoCrW alloy, indicating more severe internal oxidation compared to the CoCrWAlNi alloy. After 3000 TF cycles, cracks had propagated into the interior of both alloys. Notably, such internal oxidation is not typical of single-phase alloys but is prevalent in multi-phase alloys. This internal oxidation induces substantial internal stress, which adversely impairs the TF resistance of the alloys. Furthermore, oxidation progresses with crack propagation and, in turn, accelerates the crack growth process.

As shown in Figure 11, EBSD and EDS analyses were performed on the specimens after 3000 TF cycles. The CoCrWAlNi alloy exhibited superior TF crack propagation resistance compared to the CoCrW alloy. Inverse pole figure (IPF) maps (Figure 11a,e) represent crystallographic orientations via distinct colors, indicating that both cobalt-based alloys consist of equiaxed dendrites without significant texture. Kernel average misorientation (KAM) maps and geometrically necessary dislocation (GND) calculations reveal the destructive effects of TF cracks on the alloy substrates. Specifically, regions adjacent to cracks and grain boundaries displayed considerably higher KAM values than the matrix, accompanied by enhanced stress concentration (Figure 11b,c,f,g). GND densities were derived from the average KAM values, which are strongly correlated with material deformation. Notably, the CoCrW alloy exhibited higher KAM and GND values than the CoCrWAlNi alloy, suggesting that more severe TF cracking induces greater deformation and internal strain. As TF cracks propagate rapidly after initiation, oxidation is further accelerated. EDS mappings (Figure 11d,h) confirm the enrichment of oxygen within the TF cracks.

4. Discussion

The TF process involves cyclic thermal shock induced by heating-cooling cycles, which gives rise to thermal stress, plastic strain, oxidation, and elemental diffusion. As detailed in the previous section, the initiation and propagation behaviors of TF cracks have been systematically characterized. In this section, the factors influencing TF resistance will be discussed through stress analysis, with a particular focus on the effects of internal oxidation.

Both alloys consist of a γ-Co matrix and carbide precipitates, where the network-shaped carbides are more susceptible to oxidation than the matrix. This preferential oxidation of carbides leads to internal oxidation and oxygen diffusion along the carbide networks. As illustrated in Figure 8, a thin and continuous external oxide layer forms on the surface of the CoCrWAlNi alloy, while internal oxidation occurs in both alloys—with the CoCrW alloy exhibiting more severe internal oxidation. This phenomenon can be attributed to the additions of Al and Ni elements, which promote Cr diffusion in the matrix. Such enhanced Cr diffusion facilitates the formation of a continuous, thin Cr-rich oxide layer on the surface, rather than the exclusive oxidation of carbides observed in the CoCrW alloy. Nevertheless, the Cr content in the matrix of the CoCrWAlNi alloy is insufficient to form a relatively thick protective oxide film.

Only internal oxidation was considered in the oxidation models of the two alloys. According to Huntz [35], the stress induced by internal oxidation mainly arises from the volume expansion of the oxides (${\sigma }_v$).

$${\sigma }_v={\displaystyle \frac{E_m(1-\sqrt[3]{{\lambda }_{PB}})}{\left(1-{\nu }_m\right)d_m/d_o}},$$

(1)

where the subscripts $m$ and $o$ denote the matrix and oxide, respectively. $E$ is the elasticity modulus, $\nu$ is Poisson’s ratio, d is the grain size, ${\lambda }_{PB}$ is the Pilling Bedworth Ratio (Cr₂O₃/Cr 2.07). The related data is listed in

Table 3. Physical parameters for internal oxidation.

Alloy	E/(GPa)	ν	${\mathit{d}}_{\mathit{m}}$/μm	${\mathit{d}}_{\mathit{o}}$/μm
CoCrW	208	0.31	5	0.35
CoCrWAlNi	194	0.38	6	0.25
Co	305 [36]	0.303 [36]	/	/

.

The oxide volume expansion-induced stresses in the CoCrW and CoCrWAlNi alloys were determined to be 8.41 GPa and 5.01 GPa, respectively. The volume expansion of oxides gave rise to compressive stress within the alloy matrix, which further generated high tensile stress at the Cr₂O₃/Cr/matrix interface. Notably, the calculated stress values exceeded the yield strength of the alloys, which was inconsistent with the corresponding experimental observations. In fact, the stress generated by internal oxidation was partially relaxed via creep deformation, and the deformation rate of polycrystalline materials can be described by the model proposed by Karch [37]. The internal oxidation-induced stress in the CoCrW alloy was significantly higher than that in the CoCrWAlNi alloy, which constituted the dominant factor responsible for the TF failure of the material.

As shown in Figure 10, the strain and stress concentration at the crack tip facilitated oxygen diffusion and oxide formation. Meanwhile, the formed oxides further elevated the local stress, thereby accelerating crack propagation. Cracks tended to nucleate at phase boundaries, while their subsequent growth exhibited a mixed intergranular and transgranular mode, with propagation directions perpendicular to the sample surface. This behavior suggests that the applied stress was sufficiently high to drive cracks to propagate directly through the matrix rather than along preferential pathways. The resistance of the matrix to cracking dominates the overall crack growth behavior. The additions of Al and Ni elements enhanced the stability of the γ-Co matrix, endowing it with superior deformability. Consequently, the matrix was capable of accommodating a higher density of dislocations and sustaining greater crack-tip stress. Therefore, the crack propagation rate of the CoCrWAlNi alloy was lower than that of the CoCrW alloy.

The TF process of the CoCrWAlNi alloy can be divided into four stages. (1) A thin oxide layer forms on the alloy surface. (2) Carbide precipitates are oxidized as oxygen diffuses through the thin oxide layer, leading to the generation of high internal stress within the alloy. (3) Cracks initiate at the oxide–substrate interface under cyclic thermal stress. (4) Oxidation proceeds along the crack paths, which further grow and aggravate stress concentration at the crack tip, thereby promoting crack propagation. By contrast, the TF process of the CoCrW alloy skips the first stage, resulting in the rapid oxidation of carbides.

In summary, the additions of Al and Ni elements to the CoCrW alloy improve ductility and stabilize the γ-Co matrix. The TF resistance is closely associated with the oxidation behavior of the alloy. The CoCrWAlNi alloy possesses a lower carbide fraction than the CoCrW alloy, and a continuous thin oxide layer forms on its surface. These factors are beneficial in suppressing internal oxidation and encouraging crack propagation along phase boundaries.

5. Conclusions

In this study, Al and Ni elements were incorporated into the CoCrW alloy fabricated by DLD to tailor its microstructure and performance. The microstructure, mechanical properties, and TF resistance of the two alloys were systematically characterized, and the evolution behavior and underlying mechanism of the TF process were analyzed in detail. The main conclusions are summarized as follows.

(1)

The CoCrWAlNi alloy was composed of γ-Co, M₂₃C₆, and M₇C₃ phases, while the ε-Co phase was only present in the CoCrW alloy. The additions of Al and Ni elements reduced the carbide fraction and increased the SFEs, thereby stabilizing the γ-Co matrix. Consequently, the CoCrWAlNi alloy achieved a higher elongation (6%) than the CoCrW alloy (1.75%).

(2)

The CoCrWAlNi alloy exhibited superior TF resistance. The alloying elements Al and Ni facilitated the formation of a protective external oxide layer and suppressed internal oxidation, which effectively retarded the initiation of TF cracks.

(3)

Oxidation during the TF process was detrimental, and internal oxidation was significantly more harmful than external oxidation with respect to crack initiation. Reducing the carbide content decreased the rapid diffusion path of oxygen and inhibited crack propagation.