Kear-Wilsdorf Locks Research Articles

Origin of the yield stress anomaly in L1[formula omitted] intermetallics unveiled with physically informed machine-learning potentials

The yield stress anomaly of L12 intermetallics such as Ni3Al, Ni3Ga, Co3(Al,W) is controlled by the so-called Kear–Wilsdorf lock (KWL), of which the formation and unlocking are governed by dislocation cross-slip. Despite the importance of this anomalous behavior in L12-strengthened alloys, microscopic understanding of the KWL is limited. Here, molecular dynamics simulations are conducted by employing a dedicated machine-learning interatomic potential derived via physically informed active learning. The potential facilitates modeling of the dislocation behavior in Ni3Al with near ab initio accuracy. KWL formation and unlocking are observed and analyzed. The unlocking stress demonstrates a pronounced temperature dependence, contradicting the assumptions of existing analytical models. A phenomenological model is proposed to effectively describe the atomistic unlocking stresses and extrapolate them to the macroscopic scale. The model is general and applicable to other L12 intermetallics. The acquired knowledge of KWLs provides a deeper understanding on the origin of the yield stress anomaly.

Temperature-dependent deformation mechanisms of γ′ phases in a newly developed NiCoCr-based superalloy

The γ′-strengthened NiCoCr-based superalloys are extensively used in aerospace, energy, and chemical industries. This work focuses on tensile properties and evolution of deformation mechanism in a newly developed NiCoCr-based superalloy, designated K439B, at temperatures ranging from 25 °C to 1000 °C. The results demonstrate that the deformation mechanisms of this alloy are temperature-dependent. Slip bands and strongly-coupled dislocation pairs shear γ′ precipitates at 25 °C, resulting in high yield strength and work hardening rate. At 600 °C and 700 °C, the Lomer-Cottrell (L-C) locks are observed, and stacking faults shearing γ′ precipitates become the primary deformation mechanism. At temperatures reaching 800 °C, the yield strength exhibits an anomalous increase originating from the formation of Kear-Wilsdorf (K-W) locks. When the temperature exceeds 800 °C, the primary deformation mechanism is transformed into dislocations bypassing γ′ through the Orowan mechanism. The present study elucidates the deformation mechanism of this novel designed superalloy, thereby furnishing a theoretical foundation for the further development of the alloy system.

Tensile Deformation Behaviors and Microstructure Evolution Under Various Temperatures for MAR‐M247 Nickel‐Based Superalloy

The study aims to analyze the tensile behavior and microstructural changes in MAR‐M247 nickel‐based superalloy across different temperatures. Tensile behavior is examined under a constant strain rate of 2.5 × 10−4 s−1 at varying temperatures. Results indicate a temperature‐dependent nature of the alloy's tensile properties. At 550 °C, a Portevin–Le Chatelier effect is observed, attributed to twinning nucleation and carbon atom diffusion. Dynamic recrystallization occurs at 950 °C, manifesting in a sinusoidal stress–strain curve. At room temperature, the primary fracture mechanism involves dislocation shearing in the γ matrix, with minor dislocation shearing in the γ′ phase. At 550 °C, carbides along grain boundaries and the γ′ phase impede dislocation motion. Concurrently, dislocations shear the γ′ phase, leading to the formation of superlattice intrinsic stacking faults and Kear–Wilsdorf locks, thereby enhancing tensile strength. However, at 950 °C, dislocation motion primarily involves climb, carbides decompose, the γ′ phase softens and enlarges, resulting in a notable decrease in tensile strength.

Yield stress anomaly and creep of single crystal Ni-base superalloys – Role of particle size

In the present work we subject the single crystal Ni-base superalloy ERBO1 (CMSX 4 type) to constant strain rate (CSR) and creep testing at temperatures between 1023 and 1223 K. Three material states are considered which have similar particle volume fractions >60% but differ in γ′-particle sizes (material states S, M and L of particle sizes: 240, 390 and 540 nm). In constant strain rate testing, a yield stress anomaly is observed for all three material states, with a yield stress maximum at 1073 K. This increase of strength with increasing temperature is not observed during creep testing at significantly lower deformation rates in this low temperature high stress creep regime, where different elementary deformation mechanisms govern CSR and creep behavior. In contrast, in the low stress high temperature creep regime, stress/strain rate data pairs from CSR creep tests both show decreasing strength with increasing temperature. It is found that in both types of tests the material state M shows the highest strength (highest yield stress and lowest creep rate). This can be rationalized based on a scenario where both, γ-channel and γ′-particle dislocation activities are important. Diffraction contrast transmission electron microscopy is used to study the relevant elementary deformation processes. Details of dislocation arrangements are discussed with a special focus on the role of Kear Wilsdorf (KW) locks, γ′-particle shearing by superlattice stacking faults (extrinsic and intrinsic) and dislocation climb.

Effect of microstructure and temperature on the bulk and small-scale deformation behavior of 2nd and 3rd generation advanced intermetallic TiAl alloys

Advanced intermetallic TiAl alloys have attracted considerable attention as a high-temperature structural material in aerospace and automobile sectors owing to their low density, elevated temperature strength, superior creep, and oxidation resistance. The present study is focused on assessing the effect of alloy composition, heat treatment and microstructural variation on the bulk and small-scale deformation behavior of commercial grade 2nd generation and newly developed 3rd generation TiAl alloys at 25 °C (298 K), 300 °C (573 K), and 500 °C (773 K). Twinning is noted to be the dominant deformation mechanism irrespective of the temperature. The highest yield strength and hardness are noted for the as-cast 3rd generation TiAl alloy owing to the finer microstructure and compositional variation. Heat treatment however modifies the TiAl microstructure significantly resulting to unique bilamellar-biglobular (BLBG) microstructure, devoid of the brittle β0 phase. Consequently, nano twins- and micro twins-induced enhanced deformability is noted for such microstructure, as compared to the as-cast counterparts. Interestingly, an anomalous increase in strength with an increase in temperature is observed for the studied alloys owing to the presence of high-density twins and Kear-Wilsdorf locks. Most importantly, a temperature dependant hardness anomaly is noted in the TiAl alloys that opens new avenues for further research in nanoscale deformation. This piece of research potentially strengthens the understanding of the role of constituent phases and temperature in the deformation behavior of complex TiAl alloys. Twin-induced deformation mechanism noted for the novel BLBG microstructure, can also be valid for different generations of TiAl alloys to achieve enhanced deformability that paves the way for the development of next-generation material.

Creep Behavior and Deformation Mechanism of a Third-Generation Single Crystal Ni-Based Superalloy at 980 °C

As the primary choice for aero-engine turbine blades, creep resistance is an important mechanical property for the developing third-generation single crystal Ni-based superalloys. The creep behavior of the superalloy in the [001] orientation was studied at 980 °C under a series of stress levels, accompanied with scanning electron microscope (SEM) and transmission electron microscope (TEM) observation to investigate the microstructure and deformation mechanism. The deformation mechanism of the alloy is found to be dislocation gliding, propagating and forming a dislocation network in the γ/γ′ interface. Dislocation networks could hinder the movement of dislocation and decrease the creep rate to a constant during the steady-creep stage. The formation of dislocation networks was analyzed due to the interaction of <110> {111} dislocations. Then dislocations cut into γ′ phases as individual <110> super-dislocations, anti-phase boundary dislocation pairs, and stacking faults. The <110> super-dislocation in the γ′ phase may cross-slip into the {001} plane from the {111} plane to form Kear–Wilsdorf locks, which could inhibit dislocations from gliding or cross-slipping and then enhance the creep resistance.

Investigation on tensile deformation and fracture behavior of a Ni-based superalloy specially designed for additive manufacturing

A nickel-based superalloy special developed for additive manufacturing (AM) were prepared by laser directed energy deposition (LDED) technology. Through tensile test at different temperatures, the ultimate tensile strength (UTS), yield strength (YS), and elongation were obtained; The corresponding microstructure and fracture morphologies were characterized as well as the properties. The results show that the abnormal peak values of UTS and YS were found at 700 °C, accompanying with poor ductility. Furthermore, the value of flow stress is little affected by strain rate in the temperature range of 23–700 °C, while it is very sensitive for that of 800–1100 °C. The dominant deformation mechanism of the former is anti-phase boundary (APB) shearing, and the fracture is featured by shear fracture without obvious necking; while for the latter, the dislocation climbing, cross-slip and Orowan looping is dominant, and the fracture mechanism is mainly characterized by the aggregation and growth of micropores and crack propagation, and postmortem specimen exhibit distinct necking phenomena. The abnormal peak values of UTS and YS at 700 °C could be attributed to the formation of Kear–Wilsdorf locks (K-W locks). The experimental results will support the application of the investigated alloy as well as design new alloy for AM.

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.

Effect of temperature on tensile behavior, fracture morphology and deformation mechanisms of Nickel-based single crystal CMSX-4

The effect of temperature on the tensile behavior and deformation mechanisms in a single crystal superalloys CMSX-4 is addressed and deduced by transmission electron microscopy in the temperature range from room temperature to 1100 °C. It is found that the tensile yield strength reaches a peak at 800 °C. And then, the yield strength decreases with increasing temperature. At room temperature, anti-phase boundary shearing dominates the plastic deformation. From 800 °C to 850 °C, the plastic deformation mechanism is mainly controlled by stacking fault shearing. The Kear-Wilsdorf locks have also appeared. When the temperature reaches at 950 °C, dislocation loops with anti-phase boundary shearing of the γ ′ precipitates are presented. Above 950 °C, the plastic deformation mechanism is processed by the rafted structure of the γ ′ precipitates by-passing, i.e., Orowan by-passing and dislocation climb. Finally, according to the experimental results, the variety of stacking faults with temperatures and the relationship between the yield strength and plastic deformation mechanism are discussed. • The tensile deformation mechanism of CMSX-4 is dependent on the temperatures. • The Kear-Wilsdorf locks are responsible for higher tensile yield strength at 800 and 850 °C. • The temperature for SSFs formation under tensile tests is lower than that at compression tests. • The deformation mechanism at 950 °C is controlled by dislocation loops with APB cutting of γ ′ precipitates. • Above 950 °C, the deformation mechanism is addressed by rafted γ ′ phase by-passing.

Micropillar compression deformation of single crystals of Fe3Ge with the L12 structure

The plastic deformation behavior of single crystals of Fe3Ge with the L12 structure has been investigated at room temperature as a function of crystal orientation by micropillar compression tests. In addition to slip on (010), slip on (111) is observed to occur in Fe3Ge for the first time. The CRSS (critical resolved shear stress) for (111)[101¯] slip, estimated by extrapolating the size-dependent strength variation to the ‘bulk’ size, is ~240 MPa, which is almost 6 times that (~40 MPa) for (010)[101¯] slip similarly estimated. The dissociation scheme for the superlattice dislocation with b=[101¯] is confirmed to be of the APB (anti-phase boundary)-type both on (010) and on (111), in contrast to the previous prediction for the SISF (superlattice intrinsic stacking fault) scheme on (111) because of the expected APB instability. While superlattice dislocations do not have any preferential directions to align when gliding on (010) (indicative of low frictional stress at room temperature), the alignment of superlattice dislocations along their screw orientation is observed when gliding on (111). This is proved to be due to thermally-activated cross-slip to form Kear-Wilsdorf locks, indicative of the occurrence of yield stress anomaly that is observed in many other L12 compounds such as Ni3Al. Some important deformation characteristics expected to occur in Fe3Ge (such as the absence of SISF-couple dissociation and the occurrence of yield stress anomaly) will be discussed in the light of the experimental results obtained (APB energies on (111) and (010) and CRSS values for slip on (111) and (010)).

Dependence on temperature of compression behavior and deformation mechanisms of nickel-based single crystal CMSX-4

The compression behavior under a temperature range from room temperature to 1100 °C were conducted on the single crystal superalloys CMSX-4. The objective of this study is to identify the effect of the temperature on the compression behavior and understand the deformation mechanism by transmission electron microscopy after yielding. Experimental results show that with increasing temperature, the yield strength increases until 850 °C. After that, there is a opposite trend in yield strength.At room temperature, plastic deformation is dominated by anti-phase boundary shearing. From 700 °C to 850 °C, the major anti-phase boundary and minor stacking fault shearing is observed. Besides, Kear–Wilsdorf locks are frequently observed especially at 800 °C and 850 °C, resulting in the best yield strength at these temperatures. At 950 °C, the deformation mechanism is mainly controlled by stacking fault shearing. With temperature increasing to 1100 °C, the dislocation network and rafted structure are formed, where the deformation mechanism is Orowan by-passing and dislocation climb. Finally, the formation of stacking faults with temperatures and the relationship between yield strength and deformation mechanism under compression tests are deduced.

Micropillar Compression Deformation of Single Crystals of Fe <sub>3</sub>Ge with the L1 <sub>2</sub> Structure

The plastic deformation behavior of single crystals of Fe3Ge with the L12 structure has been investigated at room temperature as a function of crystal orientation by micropillar compression tests. In addition to slip on (010), slip on (111) is observed to occur in Fe3Ge for the first time. The CRSS (critical resolved shear stress) for (111)[10‾1] slip, estimated by extrapolating the size-dependent strength variation to the ‘bulk’ size, is ~240 MPa, which is almost 6 times that (~40 MPa) for (010)[101] slip similarly estimated. The dissociation scheme for the superlattice dislocation with b=[10‾1] is confirmed to be of the APB (anti-phase boundary)-type both on (010) and on (111), in contrast to the previous prediction for the SISF (superlattice intrinsic stacking fault) scheme on (111) because of the expected APB instability. While superlattice dislocations do not have any preferential directions to align when gliding on (010) (indicative of low frictional stress at room temperature), the alignment of superlattice dislocations along their screw orientation is observed when gliding on (111). This is proved to be due to thermally-activated cross-slip to form Kear-Wilsdorf locks, indicative of the occurrence of yield stress anomaly that is observed in many other L12 compounds such as Ni3Al. Some important deformation characteristics expected to occur in Fe3Ge (such as the absence of SISF-couple dissociation and the occurrence of yield stress anomaly) will be discussed in the light of the experimental results obtained (APB energies on (111) and (010) and CRSS values for slip on (111) and (010)).

Mechanistic Modeling of Strain Hardening in Ni-Based Superalloys

This article presents a mechanistic approach for modeling the strain hardening response of polycrystalline Ni-based superalloys such as ME3, RR 1000, Alloy 720 Li, and IN 100. The mechanistic approach considers strain hardening in Ni-based superalloys in two stages: (a) self-hardening of individual {111} slip systems in the low plastic strain regime and (2) latent hardening of multiple {111} slip systems in the high plastic strain regime. Both strain hardening regimes have been modeled on the basis of interactions of superkinks with Kear–Wilsdorf locks and related to pertinent microstructural parameters such as the volume fractions of γ′ precipitates, grain orientation, and dislocation substructure. The mechanistic strain hardening model predicts that the strain hardening exponents in both the low plastic strain (n1) and the high plastic strain (n2) regimes increase with increasing values of the sum of the squares of the volume fractions of the primary and secondary γ′ precipitates, the number of {111} and {010} slip systems activated, and the critical height of the superkinks. A comparison of model predictions against experimental strain hardening exponents indicates good agreement between model predictions and experimental data. Implications of the operative strain hardening mechanisms during low-cycle fatigue and high-cycle fatigue are elucidated.

Influence of Ru on creep behaviour and concentration distribution of Re-containing Ni-based single crystal superalloy at high temperature

The influence of Ru on the creep behaviour of a Re-containing Ni-based single crystal alloy at high temperature is investigated by creep capabilities measurement and microstructure evaluation. The creep life of the Re alloy at 1100 °C/137 MPa obtained is 164 h, after added 3.0%Ru, the creep lifetime of 4.5Re/3Ru alloy at the same conditions is 321 h. The deforming characteristic of the alloys during creep is dislocations gliding on γ matrix and climbing over γ′ phase. And a few dislocations of shearing γ′ phase may cross-glide for forming Kear-Wilsdorf (KW) locks in Re-containing alloy. While more KW locks reserve in the γ′ phase of 4.5Re/3Ru alloy. The reason of 4.5Re/3Ru alloy displaying an outstanding properties is put down to more Re, W atoms being distributed in the γ′/γ interface to delay dislocations shearing γ′ phase and depress the sliding and cross-sliding of dislocations. Wherein, the atoms Ru may replace the Al atoms in γ′ phase, the stronger bonding force between Ru atoms and Re, W, Mo promotes much more ones dissolving into γ′ phase to delay the diffusion of them, this is deem as an important reason of the Re/Ru alloy displaying an outstanding creep resistance at high temperature.

An atomic-scale insight into Ni3Al slip traces

We examined at the atomic scale using in situ scanning tunnelling microscopy under ultra-high vacuum environment the slip traces left by dislocation movements at the free surface of Ni3(Al,Ta) single crystals deformed in compression at different temperatures, namely room temperature, 400 K and 600 K. The purpose is to extract from the slip traces, considered as direct signatures of dislocation movements in the bulk, the elementary dislocation mechanisms that could be at the origin of the so-called yield stress anomaly in this ordered crystallographic structure. The atomic surface structure has never been concomitantly observed with slip traces, but atomic scale resolution has been currently achieved allowing the imaging of a single lattice spacing deviation along a slip trace. Our atomic scale observations of slip traces do not permit to draw decisive conclusions regarding the various models that have been proposed to explain the yield stress anomaly, but they definitively bring new and unique insights in the understanding of this phenomenon. In particular, they show the formation and breaking of incomplete Kear–Wilsdorf configurations, which unambiguously supports interpretations of in situ TEM observations. They also prove that the so-called Kear–Wilsdorf locks are indeed early mobile on the cube cross slip plane, which is usually not accounted for in the models.

Deformation features and affecting factors of a Re/Ru-containing single crystal nickel-based superalloy during creep at elevated temperature

By means of creep properties measurement and contrast analysis of dislocation configuration, the deformation features and affecting factors of a Re/Ru-containing nickel-based single crystal alloy during creep is studied. Results show that the creep life of alloy at 1040 °C/160 MPa is measured to be 725 h to display a good creep resistance. During creep, the W, Re atoms in γ′ phase may be excluded for enriching in the γ phase near the interface, which may bring the lattice aberrance to delay the shearing of γ′ phase. The creep feature of alloy during steady state is dislocations climbing over γ′ phase. While the deformation feature of alloy in the latter period of creep is dislocations shearing γ′ phase, and the dislocations in γ′ phase may form the Kear-Wilsdorf locks by cross -gliding. Wherein, the interaction of the Ru and Re, W atoms make some Re, W atoms reserving in γ′ phase to delay the diffusing of elements, which may maintain the K-W locks from being released at high temperature in the later period of creep. This is considered to be one of the reasons for the alloy displaying a better creep resistance.

MicroROM: A Physics-Based Constitutive Model for Ni-Based Superalloys

In this article, a physics-based constitutive model is developed for representing the stress–plastic strain response of Ni-based superalloys by considering the hardening mechanisms that contribute to the macroscopic yield stress and the subsequent strain-hardening response in order to derive the entire tensile stress–strain curve. It is demonstrated that the log stress vs log plastic strain curves of Ni-based superalloys such as 702 Li and ME3 exhibit a bilinear behavior with a lower strain-hardening exponent in the low plastic strain regime and a much higher strain-hardening exponent in the high plastic strain regime. These two hardening regimes can be modeled on the basis of self-hardening of individual slip planes and latent hardening of five operative independent slip systems due to cross-slip from {111} planes to {001} planes, leading to the formation of incomplete and complete Kear-Wilsdorf locks. The proposed physics-based constitutive model, dubbed as MicroROM, exhibits a form that is similar to the Ramberg–Osgood (RO) constitutive model, which is widely used in structural analyses of engineering designs and components. MicroROM is applied to predict the tensile stress–strain curves of 720 Li and ME3 with either a subsolvus or supersolvus microstructure for temperatures ranging from 24 °C to 815 °C. The agreement is good between model predictions and experiment data from the literature. The sensitivity of the predicted stress–strain response to individual microstructural parameters is highlighted and the relation to individual hardening mechanisms is elucidated.

A phenomenological creep model for nickel-base single crystal superalloys at intermediate temperatures

For the purpose of good reproduction and prediction of creep deformation of nickel-base single crystal superalloys at intermediate temperatures, a phenomenological creep model is developed, which accounts for the typical γ/γ′ microstructure and the individual thermally activated elementary deformation processes in different phases. The internal stresses from γ/γ′ lattice mismatch and deformation heterogeneity are introduced through an efficient method. The strain hardening, the Orowan stress, the softening effect due to dislocation climb along γ/γ′ interfaces and the formation of dislocation ribbons, and the Kear–Wilsdorf-lock effect as key factors in the main flow rules are formulated properly. By taking the cube slip in slip systems and twinning mechanisms into account, the creep behavior for [110] and [111] loading directions are well captured. Without specific interaction and evolution of dislocations, the simulations of this model achieve a good agreement with experimental creep results and reproduce temperature, stress and crystallographic orientation dependences. It can also be used as the constitutive relation at material points in finite element calculations with complex boundary conditions in various components of superalloys to predict creep behavior and local stress distributions.

The tension/compression asymmetry of a high γ′ volume fraction Nickel-based single-crystal superalloy

A high γ′ volume fraction single-crystal superalloy specimens with various crystallographic orientations were tested under uniaxial tension/compression. The results show that there is apparent tension/compression asymmetry at the initial yielding (plastic strain < 0.2%, where no Kear-Wilsdorf lock and superlattice stacking fault generates during the deformation, so the existing models such as LCP and PPV cannot apply to two-phase nickel-based single-crystal superalloys). In this paper, the effect of elastic deformation on tension/compression asymmetry is discussed. A new constitutive model of two-phase nickel-based single-crystal superalloys is proposed to simulate the tension/compression asymmetry of yield strength. The model could well explain tension/compression asymmetry of two-phase alloy in [0 0 1] and [0 1 1] orientations, however it is not suitable for [1 1 1] orientation mainly due to the different slip mode

Modeling of abnormal mechanical properties of nickel-based single crystal superalloy by three-dimensional discrete dislocation dynamics

Unlike common single crystals, the nickel-based single crystal superalloy shows surprisingly anomalous flow strength (i.e. with the increase of temperature, the yield strength first increases to a peak value and then decreases) and tension–compression (TC) asymmetry. A comprehensive three-dimensional discrete dislocation dynamics (3D-DDD) procedure was developed to model these abnormal mechanical properties. For this purpose, a series of complicated dynamic evolution details of Kear–Wilsdorf (KW) locks, which are closely related to the flow strength anomaly and TC asymmetry, were incorporated into this 3D-DDD framework. Moreover, the activation of the cubic slip system, which is the origin of the decrease in yield strength with increasing temperature at relatively high temperatures, was especially taken into account by introducing a competition criterion between the unlocking of the KW locks and the activation of the cubic slip system. To test our framework, a series of 3D-DDD simulations were performed on a representative volume cell model with a cuboidal Ni3Al precipitate phase embedded in a nickel matrix. Results show that the present 3D-DDD procedure can successfully capture the dynamic evolution of KW locks, the flow strength anomaly and TC asymmetry. Then, the underlying dislocation mechanisms leading to these abnormal mechanical responses were investigated and discussed in detail. Finally, a cyclic deformation of the nickel-based single crystal superalloy was modeled by using the present DDD model, with a special focus on the influence of KW locks on the Bauschinger effect and cyclic softening.