Single Crystal Nickel Base Research Articles

Behaviour in oxidation at 1200°C of two nickel-based alloys reinforced by (Ta, Hf)C carbides

ABSTRACT Unlike gamma/gamma prime nickel-based single crystals, some carbide-strengthened polycrystalline superalloys can be efficiently helped by their reinforcing particles to resist creep deformation at temperatures above 1100°C. Various MC carbides in high quantity demonstrated recently such potential interest. Among them, HfC monocarbides are remarkable for their morphological resistance to heat, thus by their strengthening effect in the long term. Unfortunately, more wt-% of the expensive Hf elements may cause problems. In this work, two limited replacements of Hf, by Ta in a Ni–Cr alloy, are envisaged for a base of 6 wt-% M (M = Hf + Ta). Their resistance to microstructure changes and their oxidation properties is investigated at 1200°C. The results show that, after 50 h at 1200°C, microstructures have been evolved a little more than an HfC-reinforced Ni-based superalloys studied earlier. In contrast, the oxidation behaviours stayed good, except for the resistance against oxide spallation at cooling if the Ta content becomes higher (between 2 and 4 wt-%) than the Hf content. The optimal value of Ta will be found in future work.

Nickel-based superalloy single crystals fabricated via electron beam melting

Additive manufacturing technologies have emerged as potentially disruptive processes whose possible impacts range across supply chain logistics, prototyping, and novel materials synthesis. Numerous works illustrate the ability to control microstructure in fusion based processes and a few recent authors have even produced single crystals. However, a number of questions remain open regarding the process window which enables printing of single crystals. Furthermore, it has been observed that these additively manufactured single crystals exhibit a preferred 〈011〉 secondary orientation parallel to the scanning direction. In this work we investigate the fabrication conditions that enable printing of single crystals via electron beam melting. A space filling design of experiments is utilized to efficiently explore the fabrication space. Single crystals are successfully obtained using both commercially available powders and custom melt alloys. Microstructures obtained via these exploratory experiments exhibited a continuum of columnar structures ranging from weakly textured polycrystals, near single crystal, and fully single crystalline material. Complex geometry experiments are performed to study the grain selection mechanism. We find that the grain selection mechanism is independent of the bulk scale geometry and must therefore be driven by local heat transfer and solidification dynamics. Furthermore, grain selection is shown to be driven by competing driving forces; one which prefers epitaxial growth and another which is driven by the imposed processing conditions.

The effect of porosity size on the high cycle fatigue life of nickel-based single crystal superalloy at 980 °C

The high-cycle fatigue (HCF) lives of nickel-based single crystal (N-SC) superalloys with different porosity sizes were investigated at 980°C. The test results show that the failure cracks are prone to initiate from large pores, and the oxide layer can also act as initiation site for secondary cracks when under low stress level. Despite the observation of oxide layer, the pore defect size remains the dominant parameter controlling fatigue life. Therefore, a prediction model based on the critical plane was used to quantitatively evaluate the effect of pure porosity size on the HCF property of N-SC superalloys. The prediction results show that the proposed model is effective and applicable in engineering.

Thermophysical properties of nickel-based single crystal of René N5

Nickel-based superalloys have been widely used for various high-temperature and high-pressure applications such as gas-turbines, power plants, and boiler housings. In this study, we report new experimental results for thermophysical properties of René N5 alloy in a temperature range of room temperature to 1000 °C. Especially, specimens of René N5 alloy were studied in directions [001] and [111] from the same batch of commercial alloy bar. Thermal diffusivity, specific heat capacity, thermal conductivity, and coefficient of thermal expansion (CTE) with correction data of density were evaluated for each measurement method, and detailed data are provided in tables. Thermal conductivity of the [001] alloy had a higher trend than that of the [111] alloy, with relative deviation of 0.7% to 4.8%. Coefficient of thermal expansion values showed good agreement (within 5.3%) and had a curve similar to that of the specific heat capacity. All thermophysical property results were described and compared with those of single crystal alloy of CMSX-4, René N5 and conventional cast René 80 of reference data. The microstructures of alloys [001] and [111] of René N5 were observed by SEM and found to have phase of γ/γ′, which affects the thermophysical properties.

A fatigue life prediction method distinguishing fracture modes for Ni-based single crystal superalloys considering porosity defect

Microporosity is one of the most common defects in nickel base single crystal (N-SC) superalloys, which is a great threat to fatigue performance. In this paper, nine porosity-related fatigue damage parameters for N-SC superalloys were selected by referring to those parameters originally proposed for casting and 3D printing metal materials as well as the critical plane damage parameters commonly used in N-SC superalloys. According to the plastic constitutive model of single crystals, a crystal plastic finite element (CPFE) model was established to evaluate these different fatigue damage parameters, and the parameters of CPFE were calibrated by the low cycle fatigue (LCF) hysteresis loop obtained from the fatigue test. The evaluation results showed that the geometric mean of stress–strain concentration factor (GCF) kg was a satisfactory parameter to quantify the effect of porosity defects on fatigue. Inspired by the discovery and considering the slip systems characteristics of N-SC superalloys, the geometric mean of resolved stress–strain concentration factor (GRCF) krg on slip systems was proposed, and it was found that the max(krg,kg) could be used to predict the fracture modes of N-SC superalloys. Based on the max(krg,kg), a porosity-related fatigue life prediction method which can distinguish the fracture modes was put forward. The prediction results showed that the fracture mode prediction results were consistent with the test results, and the life prediction results were also improved compared with the GCF parameter.

Effects of process parameters on microstructure and cracking susceptibility of a single crystal superalloy fabricated by directed energy deposition

As an advanced material forming method, additive manufacturing (AM) has been rapidly developed in many industry fields. Due to the non-weldability of nickel-based single crystal (SX) superalloys and complex solidification and stress conditions of AM process, hot cracks can easily form in depositions. Previous research focused on the additive manufacturing of single-track thin-walled SX samples, while effective solutions to fabricate crack-free multi-track SX block samples are still limited. Directed energy deposition (DED) technique was used to fabricate multi-track block samples in this work. The effects of the overlapping ratio, scanning velocity, and laser power on the microstructure and cracking susceptibility were analyzed. The experimental results demonstrated that improper process parameters play significant roles in the formation of high-angle grain boundaries and large internal stress, both of which will increase the cracking susceptibility. Crack-free SX multi-track block was successfully fabricated after adjusting the process parameters. These findings provide a guide to reduce the cracking susceptibility by controlling the process parameters, which aids in fabrication of large-scale crack-free SX blocks through DED technique.

Orientation-dependent low cycle fatigue performance of nickel-base single crystal superalloy at intermediate temperature range

Slip deformation is the predominant fatigue damage form of nickel-based single crystals at intermediate temperature range (near 760℃). Under different crystal orientations, the dominant slip system may be an octahedral or cubic slip system. This paper explores the dominant slip systems under different crystal orientations through crystal plastic finite element analysis, and determines the fatigue damage parameter that can characterize both the octahedral deformation and the cubic deformation. The results show that within a certain orientation range close to the <111> orientation, the dominant slip system of the nickel-based single crystal is the cubic slip system. For an orientation far deviating from the <111> orientation, the dominant slip system is the octahedral slip system. Through finite element calculation, this paper gives the distribution diagram of the dominant slip system in the standard pole figure. The fatigue data of nickel-based single crystals with different orientations in the existing literature are used for verification, and the results show that the dominant slip system can be effectively determined by using the distribution diagram, and the proposed fatigue damage parameter can effectively characterize the fatigue damage of different slip systems. Compared with the condition where the distribution of the dominant slip system is not considered, the method proposed in this paper has effectively improved the fatigue life prediction ability.

Fatigue Fracture Mechanism of a Nickel-Based Single Crystal Superalloy with Partially Recrystallized Grains at 550 °C by In Situ SEM Studies

The fatigue fracture mechanism of a nickel-based single crystal (NBSC) superalloy with recrystallized grains was studied at 550 °C by in situ observation with a scanning electron microscope (SEM) for the first time. Multiple crack initiations associated with recrystallized grain boundaries and carbides were observed. By analysis of the slip traces and crack propagation planes, the operated slip systems were identified to be octahedral for both single crystal substrate and recrystallized grains. Distinct crystallographic fractures dominated, accompanied by recrystallized grain boundary associated crack initiations. This is different from the widely reported solely intergranular cracking at high temperature. Fatigue crack growth rate curves showed evident fluctuation, due to the interaction of fatigue cracks with local microstructures and the crack coalescence mechanism. Both the recrystallized grains and the competition between different slip systems were responsible for the deceleration and acceleration of fatigue microstructurally small crack behavior.

Nondestructive effect of the cusp magnetic field on the dendritic microstructure during the directional solidification of Nickel-based single crystal superalloy

The mechanical-property improvement of directionally-solidified Nickel-based single crystal (SC) superalloy with the single-direction magnetic fields is limited by their destructiveness on the dendritic microstructure. Here, the work present breaks through the bottleneck. It shows that the application of the cusp magnetic field (CMF) ensures that the dendrites are not destroyed. This feature embodies that the primary dendrite trunks arrange regularly and orderly, as well the secondary dendrite arms grow symmetrically. By contrast, both the unidirectional transverse and longitudinal magnetic field destroy the dendrite morphology, and there are a number of stray grains near the totally-remelted interface. The nondestructive effect is achieved mainly by the combined action of the thermoelectromagnetic force on the dendrites and thermoelectromagnetic convection in the melt during directional solidification. The investigation should contribute a new route for dramatically and effectively improving the crystal quality and mechanical properties of the directionally-solidified alloys.

Fretting fatigue damage mechanism of Nickel-based single crystal superalloys at high temperature

A novel high temperature fretting fatigue test apparatus was developed to investigate the fretting fatigue mechanism of Nickel-based single crystal (NBSX) superalloys at elevated temperature. The fretting fatigue tests were carried out under 5 loading conditions at 600°C. Results showed that the fretting fatigue life decreased with the increase of axial load and normal load. Metallographic observations revealed that surface peeling, delamination occur at the contact area. Energy dispersive spectrum (EDS) analysis showed that the surface material was oxidized, and the oxides will be compacted onto the contact surface and form a glazed layer. Besides, many micro cracks are observed at the contact surface that are nearly perpendicular to the fretting direction. These micro cracks would either be eliminated due to the large relative displacement or grow into the main crack, which are supposed to be the dominant source of fretting fatigue failure. Multiple fretting fatigue cracks initiate at the contact leading edge area and grow along the (100) plane that is nearly perpendicular to the fretting direction. Then the crack propagation direction will deflect from (100) plane to a series of {111} planes and the specimen eventually failed. The combined effect of fretting wear and crystallographic slip is the cause of fretting fatigue failure.

Anisotropic creep fracture mechanism and microstructural evolution in nickel-based single crystal specimen with a center film hole

The influence of orientation on the creep rupture properties of nickel-based single crystal superalloy with a center film hole was carried out at 980 °C and 300 MPa. The mechanisms of creep rupture and microstructural evolution of the specimens were studied through macro- and micro-morphological observations and finite element analysis. The central film hole exhibited the creep-strengthening effect on the nickel-based single crystal plate specimens. The degree of strengthening was related to crystal orientations, and the most prominent effect was observed in the [011] orientation. The steady-state creep rate of the specimens with central film holes decreased in the order of [001] > [011] > [111], which was completely opposite of the specimens without central holes. The creep rupture mechanism occurred due to the growth and aggregation of micro-cracks and voids. The microstructural evolution in the strengthening phase γ’ of the specimens with central film holes was closely related to the local stress state. In the high-stress zone where fractures originated, the rafting directions of the [011] and [111] orientations formed a 50° angle with the loading axis, and that of the [001] orientation was perpendicular to the loading axis. The modified crystal plastic-damage model was employed to perform the analysis of stress and damage around the center hole. The simulation results concorded well with the experimentally observed fracture behaviors.

Effects of Hot Isostatic Pressure on Microdefects and Stress Rupture Life of Second-Generation Nickel-Based Single Crystal Superalloy in As-Cast and As-Solid-Solution States

Effects of Hot Isostatic Pressure on Microdefects and Stress Rupture Life of Second-Generation Nickel-Based Single Crystal Superalloy in As-Cast and As-Solid-Solution States

High-Temperature Friction and Wear Features of Nickel-Based Single Crystal Superalloy

The nickel-based single crystal (NBSC) superalloy has been widely used in aero engines, gas turbines, and other power plants due to its excellent high-temperature performance. During the working process, wear often occurs on the contact surface of the NBSC superalloy, which will promote crack nucleation and reduce its working life. In this work, the dry-sliding friction and wear behaviors of NBSC superalloy with crystallographic orientation at different temperature and loading conditions are investigated by using a ball-on-disc tribometer. Results show that both the friction coefficient and wear rate of the NBSC superalloy are high at room temperature, and both them are significantly reduced as the temperature rises to 600 ℃ and 700 ℃. Analysis of the worn morphologies and cross-section SEM micrographs shows that a compacted glaze-layer is formed at high temperatures like 600 ℃ and 700 ℃ during the early stage of wear process, acting like a lubricant between the superalloy and rubbing ball. Hence, the change in wear mechanism caused by temperature leads to significant change of wear behaviors. In contrast to temperature, the magnitude of load has less effect on the wear mechanism.

Application research progress of hot isostatic pressing technology in nickel-based single crystal superalloy

Since the introduction of hot isostatic pressing (HIP) technology, it has been widely used in powder metallurgy forming and improving the densification of castings. With the increasing demand for the performance of nickel-based single crystal superalloy, more and more scholars started research on the application of hot isostatic pressing in nickel-based single crystal superalloy. This article summarizes the current research progress in the application of hot isostatic pressing to the high temperature of nickel-based single crystals, explains the elimination effect of hot isostatic pressing on porosity and other pore defects in nickel-based single crystal superalloy, analyzes the pore healing mechanism, and the changes in mechanical properties such as tensile, endurance, and fatigue of the alloy after applying hot isostatic pressing are shown. Finally, the future development of China’s hot isostatic pressing technology is prospected.

Observation of a[100] Dislocations in the γ′ Phase in a [011]-Oriented Nickel-Base Single Crystal Superalloy During High Temperature Creep

Superdislocation configurations in a [011]-oriented single crystal superalloy after creep at 1100 °C/150 MPa have been investigated. Besides a〈110〉 superdislocations, a[100] superdislocations are observed. TEM analysis shows that these a[100] superdislocations are pure edge dislocations and mainly lie on (100) plane with 〈011〉 directions. They form by the reactions of two matrix dislocations with different Burgers vectors, and they may move in a similar way to a〈100〉 dislocations in [001] specimens owing to the zero Schmid factors.

Improvement of grain boundary tolerance by minor additions of Hf and B in a second generation single crystal superalloy

Nowadays, it is challenging to completely eliminate low angle or high angle grain boundaries (LAGBs or HAGBs) from Nickel-based single crystal (SX) superalloys manufactured using the conventional directional solidification technique. The additions of C, B and Hf have been found to be an effective measure in improving the damage resistance of grain boundary (GB) defects, and thus increasing the creep resistance. However, the strengthening mechanism through their additions is still unclear. In this study, a double-seed solidification technique with two misorientation levels, i.e., 5° and 20°, was used to produce a series of bicrystal superalloys with different contents of Hf and B. It is the first report of an alloy with joint Hf and B addition that demonstrates tolerance to GBs with a misorientation as high as ∼20° under all of the creep conditions: 1100 °C/130 MPa, 980 °C/250 MPa and 760 °C/785 MPa. Interestingly, the effect of individual additions of Hf or B was not as pronounced as that of the joint Hf and B addition. To understand the influence of these additions on the creep mechanism in nickel-based superalloys with GB defects, a detailed characterization of the microstructures in the vicinity of the LAGBs or HAGBs was carried out, and the elemental distribution at the HAGBs was analyzed with various techniques. This study will be beneficial for understanding the role of Hf and B additions on improving the GB tolerance, and optimizing the Hf and B additions in nickel-based single crystal superalloys.

Typical characteristics for creep fracture cleavage plane of nickel-based single crystal

Typical characteristics for creep fracture cleavage plane of nickel-based single crystal were investigated through the experimental observations. The results suggest that the topological shape of cleavage planes is related to the crystal graphical orientation. The cleavage surface exhibit four-fold symmetric in [001] orientation, two-fold symmetric in [011] orientation, and triple-symmetric in [111] orientation. It is proved that the topological morphology is associated with the stress field near void through crystal plasticity finite element method. Void size dependence and external stress magnitude dependence of cleavage plane morphology was studied as well. The void size and external stress magnitude have a significant effect on the magnitude of the axial strain, but do not change visually with respect to the distribution of axial strain. The formation of typical characteristics of cleavage plane was discussed by the slip systems activation. The characteristics of dislocation motion around the void were described by molecular dynamics simulation.

Atomic simulation of void location effect on the void growth in nickel-based single crystal

Void growth in metallic materials are strongly affected by local microstructure. The location of defect produces a great influence on the mechanical properties of material, especially if defect occurs at the interface. Considering the complex microstructure of nickel-based single crystal, the Ni, Ni3Al and Ni/Ni3Al interface model were established to analyze the expansion dynamics of void using molecular dynamics (MD) method. The expansion behavior of void shows that a/6〈1 1 2〉 Shockley partial dislocations initially nucleates on the free surface of void during the stretching of Ni and Ni3Al model. Whereas, dislocations nucleate from the interface of Ni/Ni3Al interface model. Evolution of void suggested that the plasticity deformation is dominated by movable Shockley partial dislocations and immovable Stair-rod dislocations. Void size effect analysis revealed that the larger void radius is, the smaller yield stress and Young’s modulus are. The mechanical properties of Ni/Ni3Al interface model were controlled by the interaction between void and interface.

Improvement of Grain Boundary Tolerance by Minor Additions of Hf and B in a Second Generation Single Crystal Superalloy

Nowadays, the low angle or high angle grain boundary (LAB or HAB) are difficult to be fully removed from Nickel-based single crystal (SX) superalloys by using the conventional directional solidification technique. The additions of C, B and Hf have been found to be an effective measure in improving the damage resistance of grain boundary (GB), and thus increasing the creep resistance. However, the strengthening mechanism by their additions is still unclear. In this study, a double-seed solidification technique with two levels of misorientation, i.e. 5° and 20°,was used to produce a series of bicrystal superalloys with different content of Hf and B . It is the first report of an alloy with the joint addition of Hf and B, that demonstrates tolerance to GBs with a misorientation as high as 20°under all the creep conditions: 1100 °C/130 MPa, 980 °C/250 MPa and 760 °C/785 MPa. However, the effect of the individual addition of Hf or B was not as pronounced as or even worse than that of the Hf and B joint addition. To understand the influence of Hf and B additions on the creep mechanism in the nickel-based superalloys with GB defects, a detailed characterization of the misorientation-related microstructures at the GBs was carried out and the elemental distribution at the GBs was analyzed. This study will be beneficial for understanding the role of Hf and B additions on improving the GB tolerance, and optimizing the Hf and B additions in nickel-based single crystal superalloys.

In-situ SEM study of slip-controlled short-crack growth in single-crystal nickel superalloy

Initiation and growth of short cracks in a nickel-based single crystal were studied by carrying out in-situ fatigue experiments within a scanning electron microscope (SEM). Specimens with two different crystallographic orientations, i.e., [001] and [111], were tested under load-controlled tension fatigue in vacuum. Slip-caused crack initiation was identified at room temperature while initiation of a mode-I crack was observed at 650 °C. Slip traces continuously developed ahead of the crack tip once initiated and acted as nuclei for early-stage crack growth at both room and high temperature (650 °C). These slip traces were caused by accumulated shear deformation of activated octahedral slip systems, which were specifically identified by analysing the surface slip traces and crack-propagation planes. The crack-growth rates were evaluated against stress intensity factor range, revealing the anomaly of slip-controlled short-crack growth. The effects of crystallographic orientations and temperature on fatigue crack growth were subsequently analysed and discussed, including the influence of microstructural features such as carbides and pores.