Interfacial Dislocation Networks Research Articles

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).

On dislocation networks and superdislocations in Re-containing nickel-based SX superalloy under different creep conditions

In this paper, the creep behaviors and deformation mechanisms of a 3 wt% Re-containing single crystal superalloy, are investigated under two tough conditions of 1120 °C/137 MPa and 850 °C/690 MPa. The detailed TEM observation on interfacial dislocation networks and superdislocations are analyzed to interpret the creep resistance. Results show that at 1120 °C/137 MPa, dislocation networks degrade into unstable structures and give rise to the universal shearing of superdislocations. At 850 °C/690 MPa, three mechanisms including superdislocation pairs, stacking faults, and bypassing loops jointly contribute to the plastic deformation. In addition, the comparison of superdislocations and creep performance of superalloys with different Re contents are concluded. In general, the deficiency of interfacial dislocation networks and γ′ phases in resisting dislocation climbing and shearing causes the poor creep performance, which could be improved by stabilizing interfacial dislocation networks with additional W contents. This paper guides the composition optimization of low-cost third-generation superalloys in the future.

Mechanical properties and [formula omitted] interfacial misfit network evolution: A study towards the creep behavior of Ni-based single crystal superalloys

The aim of this study is to investigate the role of the temperature, stress, and rhenium (Re) on the γ/γ' interfacial misfit dislocation network and mechanical response of Ni-based single crystal superalloys. After aging at elevated temperatures, mismatch between the two phases results in an interfacial dislocation network to relieve the coherency stress. Molecular dynamics (MD) simulations have been performed to study the properties of the (100), (110), and (111) phase interface crystallographic directions. Increasing temperature disperses the atomic potential energy at the interface diminishing the strength and stability of the networks. In the case of loading, when a constant strain rate of 2×108(s−1) is applied at 0 K, the (100) and (111) phase interface models lose their co-coordinating role of maintaining the dynamic equilibrium. Dislocation propagation in the γ phase is the dominant deformation mechanism in these two interfacial models, resulting in dislocations pile-up in the damaged area, and the network is no longer able to fortify the interface. For the (110) phase interface model, the dominant deformational mechanism is precipitate shearing. As temperature increases, the elastic modulus, initial mismatch stress, and yield strength decrease. Also, the pinning effect of Re atoms is evaluated in the γ phase at 1600 K. The dislocation hampering property of Re is more perceptible when enough dislocations in the γ phase are moving at elevated temperatures. In addition, Re manages to relieve the interfacial stress field and does not affect the network morphology. Finally, an investigation of the creep behavior of the superalloy is provided. It is observed that the escalated damage to the interfacial network due to the increased temperature leads to the domination of the softening mechanisms (cross-slip and dislocation climb) on the deformation and shortens the steady-state creep. Moreover, Re atoms act as an extra hardening factor to improve the tertiary creep.

Role of interfacial dislocation networks during secondary creep at elevated temperatures in a single crystal Ni-based superalloy

Generally, the denser dislocation networks in the γ/γʹ interface should result in a lower minimum creep rate in single crystal Ni-based superalloy at high temperatures. However, in this study, with rising temperature from 1100 °C to 1150 °C, the spacing of dislocation networks decreases almost linearly, while the minimum creep rate increases nonlinearly. This is attributed to the enhanced dislocation-climb in the γ/γʹ interface and γʹ-cutting by a<010> superdislocations. Further, dislocation networks can act as a superdislocation source during the secondary creep stage besides the well-known strengthening effect.

Effect of Ru on γ/γ' microstructural evolutions and precipitation of TCP phases during thermal exposure at 1100 °C in a single crystal superalloy

The effect of 2 wt% Ru on γ/γ' microstructural evolutions and precipitation of TCP phases were investigated in two experimental alloys. The two alloys were subjected to unstressed thermal exposure at temperatures of 1100 °C for periods of 100, 500 and 1000 h. The microstructures of the samples before and after thermal exposures were characterized. Directional coarsening of γ' phases and the precipitation of TCP phases were observed in the both alloys after long-term exposure. The experimental results show that Ru has a very significant effect on the suppression of TCP phases by influences the partitioning of elements. Detailed analysis showed that Ru increases the lattice misfit strain, resulting in more dense and stable interfacial dislocation networks, which can promote the stability of γ/γ' microstructures by delaying topological inversion. Ledges and grooves at γ/γ' interfaces associated with the dislocations and strong segregations of Cr, Co and Re along the dislocation networks were both observed, under which a detailed explanation of the coarsening process during thermal exposure was given. • Ru increases the microstructural stability. • Ru increases the density and stability of dislocation networks. • Ru promotes the stability of γ/γ' microstructures by delaying topological inversion. • Ru reduces the nucleation of TCP precipitates.

Thermal cycling creep properties of a directionally solidified superalloy DZ125

Aero-engine turbine blades may suffer overheating during service, which can result in severe microstructural and mechanical degradation within tens of seconds. In this study, the thermal cycling creep under (950 °C/15 min+1100 °C/1 min)-100 MPa was performed on a directionally solidified superalloy, DZ125. The effects of overheating and thermal cycling on the creep properties were evaluated in terms of creep behavior and microstructural evolution against isothermally crept specimens under 950 °C/100 MPa, 950 °C/270 MPa, and 1100 °C/100 MPa. The results indicated that the thermal cycling creep life was reduced dramatically compared to the isothermal creep under 950 °C/100 MPa. The plastic creep deformation mainly occurred during the overheating stage during the thermal cycling creep. The thermal cycling creep curve exhibited three stages, similar to the 1100 °C isothermal creep, but its minimum creep rate occurred at a lower creep strain. The overheating events caused severe microstructural degradation, such as substantial dissolution of γ' phase, earlier formation of rafted γ' microstructure, widening of the γ channels, and instability of the interfacial dislocation networks. This microstructural degradation was the main reason for the dramatic decrease in thermal cycling creep life, as the thermal cycling promoted more dislocations to cut into γ' phase and more cracks to initiate at grain boundaries, carbides, and residual eutectic pools. This study underlines the importance of evaluating the thermal cycling creep properties of superalloys to be used as turbine blades and provides insights into the effect of thermal cycling on directionally solidified superalloys for component design.

Creep failure and deformation mechanism investigation on a novel single crystal superalloy with various primary ageing temperatures

The heat treatment design and optimization of Re-free low-cost single crystal (SX) superalloys, which are commonly regarded as the most practical and economic alloys to manufacture guide vanes and high-pressure turbine blades of aero-engines, are in great demand. In this study, significant adjustments to the primary ageing temperature (PAT) were made, and the synthetic impact of PAT variations from 1050 °C to 1100 °C on the microstructure and creep behaviours in novel Re-free SX alloys was investigated in detail. The results showed that the γ′ size and γ/γ′ lattice misfit increased with increasing PAT, resulting in a reduction in the γ/γ′ interface area and a shift in the morphology of γ' from spherical to cubical. Notably, element diffusion and the resultant coarsening of γ/γ′ structures were facilitated by increasing the PAT. The alloy with 1080 °C PAT showed the longest creep life at 1100 °C/137 MPa, which was approximately 1.5 and 1.2 times that of alloys with PATs of 1050 °C and 1100 °C, respectively. Moreover, as PAT increased, the number of microvoids decreased due to increasing the diffusion rate of elements and hindering the formation of vacancies. When the PAT was too low or too high, the “inland shapes” of the γ′ phase and topological inversion phenomenon occurred during creep, which were detrimental to the creep properties. Furthermore, increasing the PAT resulted in the morphology of γ/γ′ interfacial dislocation networks shifting from sparse waves to fine hexagons and then to coarse hexagons. The dense interfacial dislocation networks in the sample with 1080 °C PAT remarkably impeded superdislocations cutting into γ′ phases. Ultimately, the thermodynamics and kinetics of γ′ formation and growth were also discussed, and the optimum PAT was ascertained to provide further guidance to design and develop Re-free low-cost SX superalloys.