Interfacial Dislocation Networks Research Articles

Degradation effects of the rafts and dislocation network on creep property of single crystal superalloy at medium temperature

Generally, raft structure and interfacial dislocation networks enhance the creep resistance at elevated temperatures above 1000 °C and low stresses in Ni-based single crystal superalloys. However, the formation of raft structure and dislocation networks by raising the Mo content increased the creep rate at 900 °C and 392 MPa, accelerating the final failure. The formation of rafts was closely related to the creep rate acceleration, promoting dislocations piling up at the γ′/γ interface and the shearing events in the γ′ phase. Moreover, the dislocation networks formed at 900 °C and 392 MPa did not differentiate significantly with different Mo content, which could not enhance the creep resistance as expected. This work revealed the microstructural evolution at medium temperature and provided a new perspective for understanding the creep mechanisms and alloy design.

Effect of carbon content on the microstructure and stress rupture properties of a 4th-generation nickel-based single crystal superalloy

The balance between the stress rupture properties and castability of a 4th-generation Ni-based single crystal superalloy can be achieved by addition of appropriate carbon content. In this work, the effects of carbon content on the microstructure and stress rupture properties of a 4th-generation Ni-based single crystal superalloy were investigated in detail. The results showed that minor carbon addition could aggravate the dendrite segregation, while the addition of carbon had no obvious effect on the rafting and distortion of γ′ phase during stress rupture deformation. At 760 °C/810 MPa and 1100 °C/210 MPa, the stress rupture lives of the three experimental alloys initially increased and subsequently decreased with the increasing carbon content. However, under condition of 1140 °C/137 MPa, the increased carbon content resulted in the reduction of the stress rupture lives of alloy. It was discovered that minor carbon elements addition (0.045 wt%) can promote formation of interfacial dislocation networks and improve creep resistance at high temperatures. At intermediate temperature, the introduction of carbon could reduce the stacking fault (SF) energy of γ matrix, thus facilitating the formation of SFs in γ matrix. The M6Cgranular carbides precipitated at high temperature, which was considered to be beneficial to stress rupture properties.

Stress sensitivity and its influence on high-temperature creep behavior of a novel 2nd-generation low-cost nickel based single crystal superalloy

To meet the application requirements under complex service conditions for aero engine blades, the influence of stress-state on creep behavior of a self-designed Re-low Ni-based single crystal superalloy at 1038 °C over a wide stress range of 137–207 MPa was investigated. With applied stress decreasing from 172 to 137 MPa, the creep life rapidly extends by a factor of 2.41, showing strong stress dependence. Further, the creep mechanisms under different stresses are discussed based on microstructural characterization and threshold stress calculation results. With the increasing applied stress, interfacial dislocation networks exhibit sparse and irregular shapes. Particularly, in necking zone of the fractured specimen at 1038 °C/137 MPa, due to the long-duration accumulation of deformation storage energy, dislocation arrays within γ′ rafts gradually evolve into dislocation walls through recombination and annihilation of dislocations, then develop into subgrain boundaries. Finally, some recrystallization grains with large misorientations (>10°) form by subgrain-rotation. Moreover, based on the calculated threshold stress, dislocations climbing can play a major role in creep deformation throughout the steady-state stage under a stress range of 137–207 MPa. Nevertheless, with the arrival of tertiary stage, the increasing actual stress resulted from necking deformation can also promote dislocations to cut or bypass γ′ rafts under lower stresses (137 and 172 MPa), which is consistent with the final evolution of dislocation substructures. Overall, the findings from this work are helpful to understand the high-temperature creep mechanism of low-cost single crystal superalloys, so as to improve the creep resistance of materials.

Spatial arrangements and types of dislocations in interfacial networks obtained by Si(001) wafer bonding at low twist angle: A TEM characterization

Two interfacial dislocation networks were obtained in a Si–Si material system by wafer bonding process for low twist angles around 0.2°. For one sample, one of the two initial wafers was rotated with an additional angle of 90° before the bonding. TEM analyses of samples prepared for planar and transverse views show different characteristics in terms of spatial arrangement and dislocation types. Long climbing segments slightly deviated from the [100] axis and short segments a/2<101> are arranged in the dislocation network obtained with an additional rotation and at the lowest twist angle (Ψ = 0.18°) from a <110> axis. Long screw segments a/2<110> and short partial Shockley segments are arranged in the dislocation network obtained without any additional rotation at the highest twist angle (Ψ = 0.27°) from a <110> axis. In both cases, the long and short segments form elongated hexagonal patterns, which are accurately ordered in one set paving the <100> directions or randomly ordered in two perpendicular sets paving the <110> directions, respectively. The results are discussed taking into account the arangement of the surface miscut steps in the initial wafers before the bonding.

The unique influence of 1000 °C high-speed rotation simulated turbine blade working conditions on microstructural degradation of single-crystal superalloy NiAlReRu

Turbine blades are mainly subjected to thermal stress and gradient centrifugal stress in operational conditions. The effects of centrifugal stress on the microstructural degradation of single crystal superalloys under a multiaxial stress state remain unclear. Here, the NiAlReRu component was exposed to high-speed rotation at 20,000 rpm and a high temperature of 1000 °C for 100 h. The analysis indicates that the centrifugal stress causes the distortion of dendrite stems and the aggregation of micropores. The centrifugal stress promotes the formation of interfacial dislocation networks, accompanied by more severe rafting of γ′ phases. These findings provide a comprehensive experimental understanding of the microstructural evolution under simulated turbine blade working conditions, which provides a reference for the subsequent tests under near-service conditions.

Deformation Mechanisms Dominated by Decomposition of an Interfacial Misfit Dislocation Network in Ni/Ni3Al Multilayer Structures.

Ni/Ni3Al heterogeneous multilayer structures are widely used in aerospace manufacturing because of their unique coherent interfaces and excellent mechanical properties. Revealing the deformation mechanisms of interfacial structures is of great significance for microstructural design and their engineering applications. Thus, this work aims to establish the connection between the evolution of an interfacial misfit dislocation (IMD) network and tensile deformation mechanisms of Ni/Ni3Al multilayer structures. It is shown that the decomposition of IMD networks dominates the deformation of Ni/Ni3Al multilayer structures, which exhibits distinct effects on crystallographic orientation and layer thickness. Specifically, the Ni/Ni3Al (100) multilayer structure achieves its maximum yield strength of 5.28 GPa at the layer thickness of 3.19 nm. As a comparison, the (110) case has a maximum yield strength of 4.35 GPa as the layer thickness is 3.01 nm. However, the yield strength of the (111) one seems irrelevant to layer thickness, which fluctuates between 10.89 and 11.81 GPa. These findings can provide new insights into a deep understanding of the evolution and deformation of the IMD network of Ni/Ni3Al multilayer structures.

Enhancing microstructural stability and creep properties by Ta addition in Ni-based single crystal superalloys

To investigate influences of Ta content on microstructural stability during thermal exposure at 900 °C (up to 3000 h) and creep properties at 980 °C/163 MPa in Ni-based SX superalloy, alloys with 7Ta, 8Ta and 9Ta (wt.%) were studied utilizing DSC, SEM, TEM, HR-XRD and APT. The results shows that Ta addition enhanced the partitioning behavior of alloying elements. Besides, lattice misfit was controlled by both alloy composition and temperature. Ta addition increased absolute misfit both room temperature and 980 °C. The difference was that the γ/γ′ lattice misfit changed from positive to negative at room temperature. The γ′ coarsening indicated that Ta addition reduced the effective diffusion coefficient and exacerbated W segregation at γ/γ′ interface, which was beneficial to improve the microstructural stability. Merging of several adjacent γ′ precipitates in preferred direction was observed to be delayed with increasing Ta content, which occurred at 500 h for 7Ta alloy, 800 h for 8Ta alloy and 1000 h for 9Ta alloy, respectively. Ta addition affected the diffusion kinetics and greatly prolonged the steady creep duration, and then resulted in longer creep rupture life. It was discussed based on the γ matrix channels, γ′ raft thickness, the perfection degree of γ′ rafts, residual γ′ precipitates, interfacial dislocation networks and solid solution strengthening.

Effect of heat treatment on the microstructure and recrystallization of an SLM nickel-based superalloy

Heat treatment is essential for selective laser melting (SLM) of nickel-based superalloys. However, the conventional solution heat treatment method of cast superalloys usually leads to serious recrystallization in the samples. The recrystallized grain boundaries under high-temperature conditions reduce the strength of the alloy, which hinders their widespread applications in aerospace. In this work, a nickel-based superalloy with high γ′ content fabricated by SLM was heat-treated at different solution temperatures. The microstructure of the alloy in different heat-treated states was also characterised. A sub-solvus heat treatment was proposed, which can effectively reduce residual stress and inhibit recrystallization. The microstructure showed that the dislocation density was significantly reduced and recrystallized after the solution heat treatment at 1285 ℃/2 h+1290 ℃/2 h+1295 ℃/4 h and 1250 ℃/0.5 h, and the proportion of recrystallization were 60.0 % and 42.9 %, respectively. However, after sub-solvus heat treatment at 1240 °C/0.5 h, the sample dislocation density decreased and the volume fraction of recrystallization (3.2 %) was slightly higher than that of the as-SLM sample (2.5 %). The sample maintained a strong cubic texture in the build direction. After sub-solvus + aging heat treatment, the microstructure maintained a columnar grain structure. The γ/γ′ interfacial dislocation networks and dislocations pile-up were the main dislocation configurations.

Degradation and dislocation evolution of the nickel-based single crystal superalloy DD6 under the coupling high-speed rotating and high temperature environments

To model the centrifugal loads and complex geometries experienced by turbine blades during service, the nickel-based single crystal superalloy DD6 samples with a groove structure are designed and tested under the coupling conditions of high-speed rotating of 20,000 r/min and high temperature of 980 ℃. Scanning electron microscopy and transmission electron microscopy are used to analyze the microstructural evolution of γ-γ' phases and dislocation morphologies of 400 h and 1000 h samples. The results indicate that microstructural degradation of the γ-γ' phase attributes to the larger centrifugal tensile stress S11 in the applied physical stress compositions until deformation occurs. Our observations suggest that a/2 < 101 > -type dislocations initiate plastic deformation by slipping within the γ phase. However, their movement is constrained at the interface between the γ and γ' phases, resulting in their tendency to aggregate at this boundary. After the formation of stable interfacial dislocation networks, these dislocations then react to generate either a< 010 > -type dislocations or a/2 < 112 > -type dislocations, which cut into the γ' phase. However, under high stress conditions, the a/2 < 101 > -type dislocations are primarily cut directly in the form of dislocation pairs.

Influence of interfacial dislocation network on strain-rate sensitivity in Ni-based single crystal superalloys

The excellent mechanical properties of Ni-based superalloys rely on their unique two-phase microstructure. Here we probe the influence of interfacial dislocation network on strain-rate sensitivity in Ni-based single crystal superalloys. The interfacial dislocation could generate a weak long-range interaction to the matrix dislocations at low stress, and react with the incoming dislocation to form junctions at high stress. The weak interaction results in the dislocation pile-up around the interphase boundary, while the strong dislocation reaction makes the gradual loss of interface coherency. With the activation energies, we predict the dependence of strain-rate sensitivity on temperature in a regime with two bounds correlated with these two kinds of interactions. The predicted values match well with that measured in experiments at elevated temperatures.

Role of microstructural stability and superdislocation shearing on creep behavior of two low-cost Ni-based single crystal superalloys at 1100 °C/130 MPa

In order to satisfy the pursuit of advanced aeroengines with low cost and high temperature capacity, the novel Re-free and Re-low single crystal superalloys were designed based on the strategy of Re-reduction. The creep lives of the two experimental alloys at 1100 °C/130 MPa both exceed that of René N5, and the Re-low alloy displays better creep performance and microstructural stability than the Re-free alloy. At the end of steady creep stage, due to the strong hindrance of interfacial dislocation networks, only a few superdislocations with Burgers vectors a<110> and a<001> enter into γ′ phase in the Re-low alloy. In the Re-free alloy, non-uniform plastic deformation leads to the severe γ/γ′ topological inversion, resulting in high-density a<110> superdislocation arrays in γ′ rafts, which can further react to form superdislocation networks. Moreover, the increasing μ phase precipitated with the prolonging creep time also promotes earlier failure of the Re-free alloy. In summary, the Re addition can improve creep resistance through reducing rafting rate of γ′ particles, stabilizing interfacial dislocation networks and increasing the APB energy of ordered γ′ phase. The above results provide favorable experimental basis for the development of low-cost single crystal superalloys.

The role of Mo on the creep deformation of a 4th generation Ni-based single crystal superalloy at 1100 °C

The effects of Mo content on the creep property of a 4th generation Ni-based single crystal superalloy were investigated systematically at high temperature and low stress. Under the condition of 1100 °C and 137 MPa, the performance of SC containing 2 wt% Mo and 3 wt% Mo was close, revealing that low Mo addition could meet the requirements of creep properties at 1100 °C and 137 MPa. However, the massive precipitation of TCP phases significantly degraded the creep performance when Mo content was more than 3 wt%. Mo addition also accelerated the broadening of the γ channel, decreasing the resistance of the dislocation slip. With regard to the positive impacts, Mo addition increased the magnitude of lattice misfit, leading to dense interfacial dislocation networks with high creep resistance. Mo also retarded the diffusion of alloying elements, thereby suppressing the formation of creep pores and the elemental inhomogeneity during creep. The comprehensive Mo-effect was summarized. Moreover, the optimum Mo content in the 4th generation single crystal superalloys was determined for the guidance of alloy design based on the creep results and comparison with other alloys.

Microstructure Evolution and Dislocation Mechanism of a Third-Generation Single-Crystal Ni-Based Superalloy during Creep at 1170 °C.

The present study investigates the creep behavior and deformation mechanism of a third-generation single-crystal Ni-based superalloy at 1170 °C under a range of stress levels. Scanning electron microscopes (SEM) and transmission electron microscopes (TEM) were employed to observe the formation of a rafted γ' phase, which exhibits a topologically close-packed (TCP) structure. The orientation relationship and elemental composition of the TCP phase and matrix were analyzed to discern their impact on the creep properties of the alloy. The primary deformation mechanism of the examined alloy was identified as dislocation slipping within the γ matrix, accompanied by the climbing of dislocations over the rafted γ' phase during the initial stage of creep. In the later stages of creep, super-dislocations with Burgers vectors of a<010> and a/2<110> were observed to shear into the γ' phase, originating from interfacial dislocation networks. Up to the fracture, the sequential activation of dislocation shearing in the primary and secondary slipping systems of the γ' phase occurs. As a consequence of this alternating dislocation shearing, a twist deformation of the rafted γ' phase ensued, ultimately contributing to the fracture mechanism observed in the alloy during creep.

Tensile properties and deformation mechanisms of two low-cost second-generation single crystal superalloys designed by optimization of Re and W compositions at various temperatures

In this work, two novel low-cost second-generation single crystal superalloys were designed by optimizing compositions of W and Re elements. The tensile properties and deformation mechanisms were systematically investigated from room temperature to 1070 ℃. It was found that yield strength of both alloys reached the maximum value (1099 MPa for 1.0Re alloy and 1086 MPa for 1.5Re alloy) at 760 ℃. At room temperature and 760 ℃, dislocations sheared into γ′ phase and dissociated into partial dislocations and stacking faults, which controlled the whole deformation process. At 760 ℃, the interactions between isolated stacking faults caused by the successive initiation of multiple slip systems led to the formation of Lomer-Cottrell locks, which effectively improved yield strength of 1.0Re alloy. 1.5Re alloy was strengthened effectively by isolated stacking faults, Kear-Wilsdorf locks and dislocation loops at 760 ℃. At high temperatures (980 ℃ and 1070 ℃), dislocation pairs with anti-phase boundary shearing into γ′ phase became the main deformation mechanism, and dislocations climbing and bypassing mechanisms could also be activated under higher thermal activation. Meanwhile, compared with that in 1.0Re alloy, interfacial dislocation networks in 1.5Re alloy is more regular and denser. Generally, the two novel self-designed single crystal superalloys characterized by outstanding tensile properties and cost reduction displayed great potentials for application of advanced aero-engines in the future.

Influence of Strain Amplitude on Low-Cycle Fatigue Behaviors of a Fourth-Generation Ni-Based Single-Crystal Superalloy at 980 °C

Total strain-control, low-cycle fatigue experiments of a fourth-generation Ni-based single-crystal superalloy were performed at 980 °C. Scanning electron microscopy and transmission electron microscopy are employed to determine fracture morphologies and dislocation characteristics of the samples. As the strain amplitude increased from 0.6 to 1.0%, the cyclic stress and plastic strain per cycle increased, the cyclic lifetime decreased, more interfacial dislocation networks were formed, and the formation rate accelerated. Cyclic hardening is associated with the reaction of accumulated dislocations and dislocation networks, which hinder the movement of dislocations. The presence of interfacial dislocations reduces the lattice mismatch between the γ and γ′ phases, and the presence of dislocation networks that absorb mobile dislocations results in cyclic softening. At a strain amplitude of 1.0%, the reaction of a high density of dislocations results in initial cyclic hardening, and the dislocation cutting into the γ′ phase is one of the reasons for cyclic softening. The crack initiation site changed from a near-surface defect to a surface defect when the strain amplitude increased from 0.6 to 0.8 to 1.0%. The number of secondary cracks initiated from the micropores decreased during the growth stage as the strain amplitude increased.

Stress-rupture behavior of a Re-containing Ni-base single crystal superalloy at high temperatures

A Re-containing Ni-base single crystal superalloy was used to investigate the elementary processes associated with stress-rupture behavior at different temperatures where the γʹ rafting occurs. At 900 °C, the rupture behavior is mainly determined by the multiplication of dislocations within the widening γ channels, which is closely linked with the propagation of microcracks along the inherent γ/γʹ interfaces. The rapid formation of lamella γ/γʹ raft structure, along with the developed-well interfacial dislocation networks, and its elastic instability are primarily responsible for the rupture behavior at 1100 °C. There is a clear curvature tendency in the Larson-Miller plot of stress-rupture lifetime in relation to stress at high temperatures. It indicates that the influence extent of γʹ rafting on stress-rupture behavior is sensitive to the acting conditions of temperature and stress.

Deformation mechanisms of low cycle fatigue of a fourth generation Ni-based single crystal superalloy

Ni-based single-crystal superalloys are extensively employed in turbine blades for aircraft and industrial gas engines. The blades are subjected to complex thermal and stress impacts during service, which may cause sudden low-cycle fatigue damage and subsequent flight accidents. To investigate the relationship between the microstructure and mechanical properties, fatigue-damaged microstructures are compared under cyclic deformation at 800 and 900 °C using combined electron microscope characterizations. With an increase in temperature, the deformation is more concentrated in the γ matrix channels and the cyclic deformation behaviors stabilizes, whereas cutting the γ′ phase becomes difficult with decreasing slip bands. In addition, the significance of interfacial dislocation networks in enhancing cyclic stability and hindering cyclic tension–compression asymmetry is discussed.

Evolution of phase interface microstructure and its effects on stress rupture property of a 4th generation nickel-based single crystal superalloy after thermal exposure

Lattice constants, phase interface microstructure and stress rupture properties of nickel-based single crystal superalloy after thermally exposed at 1100 °C for different time were investigated using synchrotron radiation diffraction and transmission electron microscopy techniques. The results show that the lattice constants of γ and γ' phase at room temperature decrease and the misfit at room temperature becomes more negative after 200 h exposure, and these parameters remain unchanged after exposure for 500 h. The release of misfit stress during thermal exposure is completed by the transformation from coherent γ/γ' interface to semi-coherent interface as well as the formation of misfit dislocation. The misfit dislocation network generated during thermal exposure is mainly rectangular shape, and its formation process is analyzed in detail. Additionally, it is found that the stress rupture lives decrease obviously after thermal exposure, while the morphology and size of interfacial dislocation network of thermally exposed alloys after rupture are very similar with that of unexposed alloy. The relation of γ' phase morphology and interfacial dislocation network to the stress rupture property deterioration has been discussed.

High-Temperature Creep Behavior of a Nickel-Based Single-Crystal Superalloy with Small-Angle Deviation from [111] Orientation

By means of tensile creep tests and microstructure analysis, the creep behavior and deformation mechanism of a nickel-based single-crystal alloy with small-angle deviations from the [111] orientation at 1040°C/137 MPa were investigated. The results suggested that in the early stage of creep, proliferating dislocations generated by different slip systems get accumulated in the γ phase. Dislocations mainly slipped in the γ phase and climbed over the γ′ phase, and they began to form a dislocation network at the interface. γ′ phases were connected to each other, and the coherence relationship was destroyed. In the steady-state stage of creep, the solid interfacial dislocation network was formed, and the γ′ phase transformed into a lamellar raft structure, which hinders the slip and climb of dislocations in the matrix channel. In the accelerated creep stage, the dislocation network was destroyed, and dislocations sheared into the γ′ phase in the way of dislocation pairs, which could react to form a superdislocation node and decompose to form APBs on the &lt;111&gt; plane or cross slip from the &lt;111&gt; plane to the &lt;100&gt; plane to form K-W locks.

Transient dislocation emission from the nanosized interface of Cu–Ag composite under the coupled thermal-mechanical shock: Molecular dynamics simulations study

In this study, we use molecular dynamics to simulate the deformation of Cu–Ag nanocomposite under the coupled thermal-mechanical shock. We compare the dislocation evolution under different loading conditions by changing the strain rate and the heating rate applied to the nanocomposite. The result indicates that for a given strain rate with increasing heating rates, the dislocation density decreases, and so does the interaction between dislocations. The resistance encountered by the dislocation movement decreases as the dislocation movement mode shifts from dislocation slip to climb. The plastic deformation mechanism transforms from dislocation movement to the amorphization of the whole composite, resulting in the disruption of the interfacial dislocation network. These factors conspire to cause a reduction in yield stress. For a given heating rate with increasing strain rates, owing to the strain rate effect and the temperature difference during the deformation process, the yield stress increase. We can conclude that due to the difference in dislocation density, dislocation movement mode, and the plastic deformation mechanism, the dislocation nucleation stress and the yield stress of composite decrease with the strain rate decreases and the heating rate increases (which involves the effect of temperature difference).