Kear Wilsdorf Research Articles

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.

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.

Microstructure evolution and deformation mechanism of a [111]-oriented nickel-based single-crystal superalloy during high-temperature creep

Through creep measurement and microstructure observation, the microstructure evolution and deformation mechanism of a [111]-oriented nickel-based single-crystal superalloy at 980 °C/186 MPa were studied. The results show that the [111]-oriented single-crystal superalloy after full heat treatment featured cubic γ' phases coherently embedded in the γ phases and regularly arranged along the <100> direction. During the creep, the cubic γ' phases were interconnected, phase thickening and coarsening occurred, and the phase transformed into a lamellar rafted structure that grew staggered in three-dimensional space. The deformation characteristics of the [111]-oriented nickel-based single-crystal superalloy during creep under experimental conditions were that the dislocations slipped, propagated, and climbed over the γ' phases in the matrix, and then cut into the γ' phase and cross-slipped from the {111} plane to the {100} plane to form Kear–Wilsdorf (K–W) locks. Finally, the dislocations cut the γ' phase in the form of dislocation pairs, causing the γ' phase to lose its creep resistance, resulting in creep fracture.

Ultra-high-temperature creep behavior of a single-crystal nickel-based superalloy containing 6% Re/5% Ru

The creep behavior and influence factors of a nickel-based single-crystal superalloy containing 6% Re/5% Ru (mass fraction) at ultra-high-temperatures were studied through microstructural observations and ultra-high-temperature creep property measurements. The results indicated that the alloy's creep life at 1170 °C/110 MPa was 193 h. During steady-state creep, the alloy's deformation mechanism involved a large number of dislocations sliding in the γ matrix and climbing over the rafted γ' phases. With the increase in the content of refractory elements, the resistance of dislocation movement in the γ matrix increased. In the later stage of ultra-high-temperature creep, the dislocations shearing into the γ' phase could cross-slide from the {111} plane to the {100} plane to generate the Kear–Wilsdorf (K-W) dislocation locks. A greater number of K-W dislocation locks could inhibit the sliding and cross-sliding of dislocations to enhance the alloy resistance, which was one of the reasons for the smaller strain rate and better creep resistance. The larger effective stress in the necking area could initiate the dual-orientation slip of the dislocations, causing the rafted phase to twist. Crack initiation occurred in the kinking area. The initiated crack propagated along the direction perpendicular to the stress axis and expanded until fracture. This process was considered to be the alloy's deformation and damage mechanism during ultra-high-temperature creep. The Ru dissolved in γ' phase could replace Al atoms. The interactions of Ru with Re and W atoms in the high-concentration Re/Ru alloy caused more Re and W atoms to dissolve in the γ' phase, delaying elemental diffusion and depressing dislocation movement. This was the main reason for the retention of more K-W dislocation locks and the good resistance during creep at ultra-high temperatures.

Creep Damage and Deformation Mechanism of a Directionally Solidified Alloy during Moderate-Temperature Creep

Through creep performance tests, microstructural observations, and contrast analysis of the dislocation configuration, the deformation and damage mechanism of the directionally solidified nickel-based superalloy during creep at moderate temperatures was investigated. The findings suggested that the deformation of the alloy in the late stage of creep at moderate temperatures involved dislocations slipping in the γ matrix and shearing into the γ′ phase. The super-dislocations sheared into the γ′ phase could either be decomposed to form a &lt;112&gt; super-Shockley incomplete dislocation plus superlattice intrinsic stacking fault (SISF) configuration, or it could slip from the {111} plane to the {100} plane and decompose to form a dislocation configuration of the Kear–Wilsdorf (K-W) lock plus antiphase domain boundary (APB). The configurations of the dislocations could inhibit the slipping and cross-slipping of dislocations to enhance the alloy creep strength, which is thought to be one reason that the alloy displayed good creep resistance. In the late creep stage, the primary/secondary slipping systems were alternately activated, and the interaction of the slipping traces caused micro-holes to appear on the interface of the γ/γ′ phases at the intersection areas of the two slipping systems. The micro-holes gathered and grew to form micro-cracks, which extended along the grain boundary at 45° to the stress axis until creep rupture occurred. These were the damage and fracture characteristics of the alloy in the late stage of creep at moderate temperatures.

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.

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.

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.

Influence of element Re on deformation mechanism within γ′ phase of single crystal nickel-based superalloys during creep at elevated temperatures

By means of creep properties measurement and microstructure observation, combined with the contrast analysis of dislocation configurations, an investigation has been made into the effects of the element Re on the deformation mechanisms within the γ′ phase of single crystal nickel-based superalloys during creep at elevated temperatures. Results show that, compared to free-Re superalloy, 2% Re alloy at 980°C displays a lower strain rate and better creep resistance. In the later stage of creep at 980°C, the deformation feature of the free-Re superalloy is the 〈110〉 super-dislocation shearing into the rafted γ′ phase and slipping on {111} planes, and the Kear–Wilsdorf (KW) locks are formed within the γ′ phase of 2% Re single crystal nickel-based superalloy. And the dislocations in KW locks in Re-containing superalloy under thermal activation may be re-activated for slipping on {111} planes to release the KW locks as the creep temperature further increases to 1070°C.

Non-basal slip in Ni3(Ti, Nb) and Ni3(Ti, Al) single crystals with various long-period stacking ordered structures

Variations in operative non-basal slip systems in Ni3Ti-based long-period stacking ordered (LPSO) phases with the stacking sequence of the closely packed planes and the temperature dependence of the critical resolved shear stress (CRSS) were investigated. It was confirmed that the {1¯100} prism slip is operative in most of the LPSO phases with hexagonal unit cells, while the {1¯011} pyramidal slip is operative in 9R LPSO phase with rhombohedral unit cell. The mechanism controlling the deformation by non-basal slip at low temperatures is considered to be governed by the energy of the antiphase boundary (APB) formed between the a/6〈112¯0〉 superpartial dislocations in accordance with the kink pair nucleation mechanism proposed by Mitchell et al. The dependence of variations in CRSS for non-basal slip on the Nb content was found to be much smaller than that for basal slip. The present results suggest that the mobility of dislocations on the non-basal plane hardly affected the behavior of yield stress anomalies (YSAs) caused by basal slip in Ni-based LPSO phases, although the Kear–Wilsdorf locking process, which is responsible for the YSA by basal slip, accompanies microscopic cross-slip from the basal slip plane onto the non-basal planes.

Frank–Read source mechanism, mobile dislocation density exhaustion and flow stress anomaly in L12 alloys: a dislocation dynamics study

We present a dislocation dynamics simulation study on the effect of Kear–Wilsdorf locks formation on the Frank–Read mechanism at temperatures below the peak temperature of the flow stress in some L12 alloys. The simulation developed in this paper differs from previous ones by taking into account the detailed structure of the connection of the Kear–Wilsdorf lock with the glissile part of the dislocation, as well as by allowing glide in the cube plane. Assuming that the plastic strain is carried out by the motion and development of the kinks connecting the Kear–Wilsdorf locks, we analyse their behaviour under the effect of an applied stress at temperatures below the peak temperature. We show that the area swept by an expanding dislocation loop in the octahedral plane, starting from an edge dislocation segment, is the result of the competition between the locking rate and the velocity of its screw part, whatever the length of the initial segment. When the applied stress is smaller than a critical stress τc/ d , the length of a moving edge segment decreases until it eventually stops. For a stress larger than τc/ d , the length of the segment increases and Frank–Read sources may develop. The first case is referred to as convergence, the second as divergence. The simulation results are applied to the analysis of the micro- and macro-yield behaviour of these alloys as well as to some of their specific properties.

Plastic deformation behavior of Ni 3(Ti 0.7Nb 0.3) single crystals with D0 19 structure

We have investigated the plastic deformation behavior of Ni 3(Ti 0.7Nb 0.3) crystals with the D0 19 structure in the temperature range of −196 to 1200 °C. The ( 0001 ) < 2 1 ¯ 1 ¯ 0 > basal slip, { 1 1 ¯ 00 } < 11 2 ¯ 0 > prism slip, and { 2 ¯ 111 } < 2 ¯ 11 6 ¯ > pyramidal slip are operative depending on the temperature and loading orientation. A strong yield stress anomaly (YSA) is caused by slip on the basal plane similar to that observed in other hcp-derivative geometrically close-packed (GCP) compounds such as D0 a-Ni 3Nb and D0 24-Ni 3Ti, due to the Kear–Wilsdorf (K–W) locking of screw dislocations. The features of YSA by basal slip, such as the profile of the temperature vs. critical resolved shear stress (CRSS) curve are quite similar in those Ni 3(Ti 0.7Nb 0.3), Ni 3Nb, and Ni 3Ti crystals; nevertheless, the profiles of the temperature vs. CRSS curves observed in the case of prism slip are significantly different for each crystal. The results indicate that in the case of these hcp-derivative GCP compounds, although the K–W locking, which is the origin of YSA, accompanies the thermally activated cross-slip of the screw partial dislocations from the (0001) primary slip plane onto the { 01 1 ¯ 0 } prism plane, the mobility of dislocations on the prism plane does not directly influence the YSA behavior of basal slip.

Effects of the anisotropy of the anti-phase boundary energy on the yield-stress anomaly in Ni3 X compounds with close-packed crystal structures

This paper is aimed at clarifying the effects of the anisotropy of the anti-phase boundary (APB) energy on the yield-stress anomaly (YSA) of five distinct Ni3(Ti, Nb) single crystals with various long-period ordered structures, in which the APB energy on the non-close-packed (non-CP) plane varies with changes in the stacking sequence of the close-packed planes. The APB energies of interest are estimated, based on a pair-wise interaction parameter model to the second neighbour. The YSA can be categorized in two groups of two and three compounds, within which the mechanical properties are nearly independent of composition. The microstructures resulting from deformation at 700°C were compared. It is verified that the YSA is dominantly controlled by differences in the probability of locking by a Kear–Wilsdorf mechanism, which itself is governed by the anisotropy of APB energy in the non-CP planes relative to the primary slip plane.

An athermal deformation model of the yield stress anomaly in Ni 3Al

An athermal deformation model was proposed for the yield stress anomaly in Ni 3Al based on the concept that dislocation multiplication versus the immobilization by the Kear–Wilsdorf (KW) locking controls the plastic deformation. In the model dislocations, superkinks lying between two KW locks multiply through expansion in an athermal manner. Regarding the KW locking, the entire screw segment cross-slips onto (010) to form long KW locks in one thermal activation event. The model can explain the macroscopic characteristics of the deformation, the strain-rate independent of the yield stress and the compliance with Schmid's law, which were recently revealed in binary, stoichiometric Ni 3Al single crystals. Quantitative assessment proved that the model reproduces the temperature dependence of the yield stress with reasonable fitting parameters. The driving force for the KW locking, one of the most important parameters for the yield stress anomaly, was evaluated at 127 mJ m −2. This value is in good agreement with those calculated from the anti-phase boundary energies on {111} and {100} reported in the literature.

A mechanism of strong solid-solution hardening of intermetallics within the range of anomalous temperature behavior of the yield stress

Strong solid-solution hardening of intermetallic materials in the region where the yield stress increases anomalously with temperature is shown to be caused by the stimulating influence of solution atoms on dislocation locking via the Kear–Wilsdorf mechanism. The dependence of the yield stress on temperature, solution concentration, strain rate, and other parameters is described in terms of a simple model in which the thermally activated kinetics of cross slip of dislocations is modified by solution atoms. The proposed theory is compared with available experimental data for a (Ni 1− c Fe c ) 3Ge solid solution.