Octahedral Slip Systems Research Articles

Near-threshold fatigue crack propagation in a Ni-based single crystal superalloy affected by crystallographic anisotropy

Fatigue crack propagation in the near-threshold regime was investigated experimentally and numerically in a Ni-based single crystal superalloy, namely, CMSX-4. Fatigue crack propagation tests were conducted at room temperature using four types of specimens with different primary and secondary crystallographic orientations. The results reveal that the crystallographic orientations strongly affect the fatigue crack propagation behavior, including the crack paths, fatigue crack propagation rates, and fatigue threshold. Crystal plasticity finite element analysis was conducted to quantify the slip activity of an octahedral slip system in front of the crack tip, considering the actual 3D geometry of the crack plane and elastic–plastic anisotropy. The fatigue damage parameter, considering the slip activities of the individual octahedral slip systems, provides reasonable explanation for the fatigue crack propagation rates and crack paths in the near-threshold regime, regardless of the crystallographic orientation. Furthermore, these damage parameters are equivalent at the fatigue thresholds for all specimens with different crystallographic orientation.

Effects of thermal exposure on microstructure deformation and evolution of shot peened nickel-based single crystal superalloy

Microstructure deformation and thermally activated microstructural evolution of nickel-based single crystal (NBSC) superalloy subjected to shot peening (SP) were investigated. SP induced high-density dislocations and activated the octahedral slip system {111} 〈110〉 of NBSC superalloy. Under 0.25 mmA SP intensity, there appeared a severely deformed layer with a depth of 2.5–3.0 μm beneath the surface. High SP intensity resulted in rougher surface morphology, more dense slip bands, deeper cold work layer and larger mean geometrically necessary dislocation (GND) density. However, the deformed microstructure was thermodynamically unstable. Dislocation annihilation and rearrangement were the main thermally activated mechanism of microstructural evolution, and high temperature promoted dislocation movement and consumption. Under 0.25 mmA SP intensity after 24 h thermal exposure, GND density decreased from 4.86 × 1014 m−2 to 2.72 × 1014 m−2, and the depth of cold work layer decreased from 67 μm to 62 μm. In addition, SP treatment provided rapid diffusion channels for alloy elements, while thermal exposure further aggravated element diffusion.

Physically based modelling of orientation deviation effect on mechanical behavior for dual‐phase single‐crystal superalloy

AbstractThis work systematically investigates the orientation deviation effect on the elastoplastic deformation of a dual‐phase, nickel‐based single‐crystal superalloy through a combined experimental study and crystal plasticity finite element modelling method (CPFEM). Physically based, dual‐phase microstructural model was developed based on scanning electron microscopy (SEM), which was implemented by finite element (FE) modelling using a representative volume element (RVE) with periodic boundary conditions. An extended equivalent yield criterion coupled with CPFEM was adopted to describe the non‐uniform yield behavior induced by octahedral and cubic slip systems. The predicted results have shown that both the bulk behavior and localized stress–strain nature are orientation deviation dependent and that the first Euler angle plays a more important role in elastoplastic behavior than the second Euler angle. This study has thus advanced the basic understanding of the relationship between orientation deviation and the bulk deformation behavior of the dual‐phase nickel‐based single crystal.

High-temperature thermal fatigue of Ni3Al-based single crystal alloy affected by wall thickness and peak temperature

Complex thin-walled structures are commonly employed in turbine components to improve cooling efficiency. However, thin-walled blades are prone to thermal fatigue failure due to the high thermal stress induced by frequent and abrupt temperature fluctuations during duty cycles. The thermal fatigue behavior of Ni3Al-based single crystal alloy with thin-walled structure during 25 °C ↔ 1000 °C/1100 °C was investigated by combining thermal fatigue tests and finite element simulation. The thermal fatigue demonstrated a significant thickness debit effect, with thermal fatigue property dropping as peak temperature and wall thickness increased. Wall thickness had a more pronounced effect than peak temperature. Visible slip traces were found on the wall thickness surface, and their density was positively correlated with wall thickness. Further investigation revealed that thermal fatigue is primarily driven by the octahedral slip system ({111}<110>), with significant contributions from (111)[101‾] and (1‾1‾1)[101]. Simulations of thermal stress distribution and J-integral at the crack tip during crack initiation and propagation stages indicated that wall thickness influences thermal fatigue properties by affecting thermal stress concentration at the notch and crack tip. Higher peak temperatures increased thermal stress and reduced yield strength, thus diminishing thermal fatigue performance. A thermal fatigue crack propagation prediction model was constructed incorporating Paris' law and the J-integral. The model revealed an unfavorable correlation between material Cj/ nj and both peak temperature/wall thickness, indicating that both factors adversely affect thermal fatigue resistance.

Dynamic evolution of the T1 phase and its effect on continuous dynamic recrystallization in Al–Cu–Li alloys

The T1 phase is the highest density and most prominent strengthening effect precipitate in Al–Cu–Li alloys, and it undergoes a complex dynamic evolution during hot deformation, which has significant effects on microstructure development and hot working. This study comprehensively characterizes and analyzes the dynamic evolution of the T1 phase at 400 °C/0.01 s−1 and its influence on continuous dynamic recrystallization (CDRX). The results indicate that the T1 phase successively undergoes coarsening, fracture, dissolution, dynamic precipitation, recoarsening, and spheroidization during deformation. A shear-coupled diffusion mechanism is proposed to explain the ultrafast coarsening rate of the T1 phase in early deformation. During the dynamic precipitation of the T1 phase, an anomalous inhomogeneous size and spatial distribution of the T1 phase are observed, and a high number density of fine T1 phases form in the matrix (with a denser, thinner and shorter size at the dislocation wall). The dynamically precipitated T1 phase is affected by the octahedral slip system during the coarsening process and exhibits a selective ripening phenomenon. The T1 phase formed by aging increases the inhomogeneity of the deformation and induces many substructures intragranularly, decreasing the percentage of CDRX grains but increasing the CDRX potential. Conversely, the dynamically precipitated T1 phase and its evolution accelerate CDRX development by promoting the transformation of LAGBs to HAGBs. In addition, the effects of dynamically evolving precipitates on the dislocations and formation modes of LAGBs at various deformation stages are elucidated. The results can provide valuable insights into the regulation of microstructure, and the development of high-performance Al–Cu–Li alloys, and also offer a theoretical and experimental basis for microstructure modeling.

High temperature uniaxial deformation characteristics in Al-Li alloy: Insights into the perspective of microstructure, microtexture and slip system activity

The present study investigates the uniaxial deformation response of Al-Li alloy at elevated temperatures (673 K and 773 K) and nominal strain rate of 0.0001 /s. The alloy exhibits significant softening following deformation at 673 K. On the contrary, a steady state followed by marginal softening is observed in the flow curve and the alloy experiences elongation close to superplastic regime during deformation at 773 K. The microstructure evolution in both the deformed specimens reveals the prevalence of dynamic recrystallization (DRX) via continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) mechanisms which resulted in flow softening. The evolution of Copper {112}〈111〉 as well as S texture {123}〈634〉 components and weakening of Cube texture {001}〈100〉 are observed in both the deformed specimens. Visco-plastic self-consistent (VPSC) simulation reveals that the activation of non-octahedral (NOC) slip systems ({100}〈110〉 and {110}〈110〉) in conjunction with octahedral (OC) slip system ({111}〈110〉) predicts the microtexture evolution accurately for both the deformation conditions. The higher activation of NOC slip systems and the simultaneous occurrence of dislocation slip as well as grain boundary sliding accommodated by dislocation slip could be ascribed to the significant improvement in elongation close to superplastic regime at 773 K.

Low cycle fatigue behavior and crack initiation mechanism of Ni-based single crystal curved thin-walled blade simulator specimen with film cooling holes

A novel curved thin-walled blade simulator (CTWBS) specimens was used to investigate low cycle fatigue (LCF) behavior and crack initiation mechanism nearby film cooling holes (FCHs) in Ni-based single crystal superalloy. LCF tests were conducted under two loading stress levels at 700 °C and 1000 °C. Experimental results demonstrated that LCF lifetime reduced as the fatigue load increased at both temperatures. Crack modes were characterized on macroscopic fracture paths, and the analysis of fatigue crack early propagation revealed its microscopic mechanism. The fracture morphology and microstructure evolution also showed temperature dependence, and oxidation played a role at high temperature. The finite element calculation was based on the crystal plasticity theory considering the back stress and slip damage. The resolved shear stress (RSS) distribution law under two temperatures led to the identification of the octahedral slip system activation type respectively. In addition, the resulting fatigue damage increased in direct proportion to the slip systems activated numbers locally surrounding the FCH. The damage distribution around the FCH was in good agreement with the experimental observation. Finally, the temperature dependence of LCF crack initiation mechanism was discussed from three perspectives: crack nucleation around the FCHs, microstructure evolution and dislocation motion near crack.

An arc-tenon attachment for nickel-based single crystal turbine blade with improved high-temperature fretting fatigue performance

Nickel-based single crystal (NBSC) turbine blade tenon is prone to fretting fatigue failure. In this study, an arc-tenon attachment was proposed and its fretting fatigue life increased by up to 408.18 % at 600 ℃ compared with the plane-tenon structure under the applied peak stress of 65 MPa. The arc-tenon attachment could effectively reduce the maximum von-Mises stresses and resolved shear stresses near the contact region, regulating the stress distribution and reducing the stress gradient, which was beneficial to alleviating the activation of octahedral slip systems and delaying crack initiation, thereby improving the fretting fatigue life at high temperature. Meanwhile, high temperature promoted the formation of oxide layer on the contact surface, providing low coefficient of friction and further reducing the contact stress. The large resolved shear stress near the contact leading edge region facilitated the activation of octahedral slip systems, then the resultant multiple slip lines sheared both the γ matrix phase and γ′ precipitates. The further growth of the slip lines accelerated the fretting fatigue crack initiation and propagation. The arc-tenon attachment could provide long fretting fatigue life and showed great potential in engineering application.

Effect of orientation deviation on random vibration fatigue behavior of nickel based single crystal superalloy

Vibration fatigue refers to the failure of a structure that is repeatedly subjected to loads causing resonance. The vibration fatigue performance of DD6 single crystal superalloy, an anisotropic material, is significantly affected by its various crystal orientations. This study investigates the impact of different vibration signal intensities and orientation deviation angles on the vibration fatigue behavior of DD6 single crystal superalloy. The fracture surface of the specimen that failed due to vibration fatigue was examined using scanning electron microscopy. The findings reveal that the primary failure mode for vibration fatigue of DD6 single crystal superalloy is the dislocation motion on the {111} plane with the greatest resolved shear stress in the octahedral slip system. Moreover, various methods, including time domain and frequency domain methods, were employed to predict the random vibration fatigue life of DD6 single crystal superalloy with different orientation deviation angles. Finally, a New Model was established to consider the vibration signal intensity and crystal orientation deviation angle, which improved the sensitivity of the frequency domain method to the crystal orientation deviation angle under different vibration signal intensities. Compared to other methods, the New Model is more suitable for predicting the random vibration fatigue life of DD6 single crystal superalloy with different orientation deviation angles.

Research on low cycle fatigue damage and macroscopic anisotropic constitutive model of Ni-based single crystal superalloy at different temperatures

Ni-based single crystal superalloys are extensively utilized in the turbine blades of aeroengines and gas turbines. Although there has been considerable research on the low cycle fatigue (LCF) behavior of single crystal at various temperatures, there is currently no universally damage coupled model for characterizing damage and failure modes at different temperatures. In this study, LCF behavior of a second-generation Ni-based single crystal superalloy was researched at room temperature, 700 °C, 800 °C, 900 °C and 1000 °C. The experimental results illustrated remarkable differences in the LCF behavior at different temperatures. Fracture modes exhibited distinct ductility characteristics at room temperature and high temperatures (900 °C and 1000 °C), whereas at medium temperatures (700 °C and 800 °C), quasi-brittle fracture along the octahedral slip system became dominant. A cumulative damage coupled constitutive model was developed to simulate the damage evolution of single crystal superalloys during LCF loading. The simulation results show that the proposed model can effectively predict the damage evolution at different temperatures, especially the damage index ζ corresponding to the same damage mode.

Modeling of the elastoplastic deformation process of single crystal superalloys

The aim of the research is the development and verification of a micromechanically motivated model of elastoplastic deformation of two-phase single-crystal nickel-based alloys, predicting behavior under high-temperature thermomechanical actionswith taking into account the presence of γ and γ' phases. The model is relevant for computations of the stress-strain state of cooled single crystal blades of gas turbine units. The constitutive equations for each of the phases took into account the anisotropy of elastic and plastic properties, the presence of octahedral slip systems, features of the cubic system, and various hardening mechanisms, including kinematic, isotropic and latent ones. The identification of the elastic and plastic constants of the material for the γ and γ 'phases was carried out on the basis of the known stress-strain curves for each phase. The determination of the effective properties and deformation diagrams of a two-phase single-crystal alloy, taking into account the presence of γ-γ'phases, was carried out both on the basis of finite element homogenization for the representative volume element, and using the simplest rheological (structural) models of the material, considering serial and parallel connection of phases. The dependences of the elastoplastic properties of two-phase single-crystal nickel-based alloys on the volume fraction of the γ'phase are determined by computational experiments and analytical estimates. In order to determine the optimal strategy for solving the class of problems under consideration, multivariant computational experiments were carried out for various types of boundary conditions of the homogenization problem, the number of periodicity cells, forms of inclusion of the γ'phase, volume fractions of the γ' phase, types of hardening, variants of rheological models and appropriate recommendations were given. The simulation results using the proposed two-level microstructural model of the material demonstrate a good agreement with the experimental data for the single-crystal superalloy CMSX-4.

MODELING OF CREEP PROCESSES OF SINGLE-CRYSTAL ALLOYS WITH TAKING INTO ACCOUNT OF RAFTING

The research is devoted to the development and verification of a two-level micromechani-cally motivated model of viscoelastic deformation of two-phase nickel-based single-crystal al-loys, predicting behavior under high thermomechanical loading with taking into account the presence of γ and γ' phases. The model is relevant for computations of the stress-strain state of cooled single crystal blades of gas turbine units. The formulation of the constitutive equations for each of the phases considered the anisot-ropy of elastic and viscous properties, the presence of octahedral slip systems, the features of the cubic system, and the presence of viscous properties both below and above the yield stress. Model parameters for γ and γ' phases were identified based on known creep curves for each phase. The effective properties of a single-crystal alloy, considering the presence of γ and γ' phas-es, were determined both based on finite element homogenization for a representative volume and using the simplest rheological (structural) models of the material, considering the series and parallel connection of phases. Based on multivariant computational experiments and analytical estimates, the dependences of the viscoelastic properties of nickel-based single-crystal alloys on the volume fraction of the γ' phase are determined. Phenomenological creep models that take into account the change in the volume fraction and the morphology of γ' inclusions have been proposed. The simulation results using the proposed two-level microstructural model of the material demonstrate a good agreement with the experimental data for the ZhS32 single-crystal heat-resistant alloy.

Crystal plasticity phase-field simulation of slip system anisotropy during creep of Co-Al-V monocrystal alloy under multidirectional strain

The preferable thermodynamic stability of L12 γ'-Co3(Al, V) phase and wide γ+γ' two-phase region endow the Co-Al-V alloy with great application potential. By introducing the digitized image of SEM morphology into phase-field simulation, utilizing the sublattice free energy model and the crystal plasticity strain model to simulate the anisotropic slip system, the rafting behaviors of the L12 γ'-Co3(Al, V) phase and the creep properties of Co-5Al-14V (at.%) alloy under tensile, compressive, shear, and monoclinic strains are studied. The partitioning ratio of element and orientation degree of γ' phase indicate that the uniaxial compressive strain improves the microstructural stability and the creep resistance at early creep stage. The creep resistance of alloy in state of equilibrium volume fraction of γ' phase is superior to that in the nonequilibrium volume fraction, and the creep resistance increases in order of tensile strain, compressive strain, and shear strain. Additionally, the dominant octahedral slip systems in fcc structure are (1¯1¯1)[101] under tensile strain, (1¯11)[01¯1] under compressive and 15° monoclinic strains, and (1¯1¯1)[1¯10] under 45° monoclinic strain. The hardening effect is prominent in γ matrix, and the high average critical resolved shear stress leads to the low creep strain. This study reveals the correlations between the slip system anisotropy and creep behaviors of Co-Al-V alloy under external strain, and also provides a method to combine the phase-field simulation with the experimental image for predicting the morphology evolution.

Fatigue crack initiation and propagation behavior of nickel-based single crystal DD6 under different drilling processes

The fatigue crack initiation and propagation behavior of film cooling holes (FCHs) under different drilling processes are of great significance to key structural parts. In this paper, it mainly comprehensively and systematically summarized the difference in surface condition caused by different drilling processes of nickel-based superalloys combining several characterization devices and described whole process from crack initiation to fracture in detail. The in-situ fatigue crack growth test at room temperature on the plate specimens with a different drilling FCHs (EDM and LDM) made of [001] nickel-based single crystal was carried out, and proposed a comprehensive evaluation method for the crack initiation mechanism from the three dimensions of geometry, metallurgy and mechanics. The four main octahedral slip systems are proved by finite element simulation. Then, we clarified the difference of fracture surface, crack path and fracture morphology of different drillings, and the competition mechanism between crack propagation path and slip surface was investigated. Finally, a simple unified model of fatigue crack growth rate was proposed, which provides a useful crack propagation law of different drilling hole.

Crack initiation anisotropy of Ni-based SX superalloys in the very high cycle fatigue regime

The anisotropic fatigue properties of three Ni-based SX superalloys (AM1, CMSX-4 and MC-NG) were investigated in the very high cycle fatigue regime at 20 kHz, R = −1 and 750 °C. The fatigue testing shows that the [111] orientation has higher fatigue lives at high stresses and lower fatigue lives at very low stresses compared to the standard [001] orientation. At high stresses, the resolved shear stress on the octahedral slip systems explains the anisotropy effect. At low stresses, pseudo-cube slip in the matrix becomes the prominent deformation mechanism over γ′-shearing in the [111] orientation, resulting in comparatively lower fatigue properties.

Fracture analysis of a Ni-based single crystal superalloy

Based on the tensile stress-dominated low cycle fatigue (LCF) tests at 760°C, the fracture morphology of a second-generation Ni-based single crystal superalloy (SC) was studied in this paper. It was found that under LCF loading the specimen cracked with one fatigue source. The angle of fatigue bands in crack propagation region was related to the secondary orientation deviation from crystal orientation [100]. In addition, evidence showed that the specimens fractured along the slip surface of {111}<110> octahedral slip systems.

Estimate of theoretical shear strength of C60 single crystal by nanoindentation

The onset of plasticity in a single crystal C60 fullerite was investigated by nanoindentation on the (111) crystallographic plane. The transition from elastic to plastic deformation in a contact was observed as pop-in events on loading curves. The respective resolved shear stresses were computed for the octahedral slip systems langle{01}overline{1}rangleleft{ {{111}} right}, supposing that their activation resulted in the onset of plasticity. A finite element analysis was applied, which reproduced the elastic loading until the first pop-in, using a realistic geometry of the Berkovich indenter blunt tip. The obtained estimate of the C60 theoretical shear strength was about {1}/{11} of the shear modulus on {111} planes.Graphical abstract

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

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

Experimental and numerical investigation on fretting fatigue behavior of Nickel-based single crystal superalloy at high temperature

The fretting fatigue behaviors of a nickel-based single crystal superalloy in contact with a powder metallurgy alloy at 600 °C are investigated. Fretting fatigue tests are conducted by using a novel high temperature fretting fatigue test apparatus that is developed, and the crystal plasticity finite element method is used to analyze the contact stress and activations of slip systems. The results show that the fretting conditions are partial slip regime for all loading conditions, and severe wear damage occurs across the slip region, which is accompanied by surface delamination and micro crack. Fretting fatigue cracks mainly initiate at the contact leading edge area, i.e., the stress concentration zone. The cracks grow along the (100) plane when multiple octahedral slip systems are activated simultaneously, or could be eliminated by wear. Considering the effects of both crystallographic slip and wear on the fretting fatigue damage, an improved fretting fatigue damage parameter, RA , is proposed to predict the fretting fatigue life. The predicted results agree well with the test results. • A novel high temperature fretting fatigue test apparatus has been developed. • Crystal plasticity finite element method is applied to calculate the contact stress and strain. • Fretting fatigue damage mechanism is proposed considering wear and cracking. • A damage parameter, RA , is proposed to describe surface damage and predict fretting fatigue life.

Transition behavior from Mode I cracking to crystallographic cracking in a Ni-base single crystal superalloy

The fatigue crack propagation of Ni-base single crystal superalloy strongly depends on the crystallographic orientation and usually shows transition behavior from Mode I cracking to crystallographic cracking at low temperature. To clarify these behaviors, fatigue crack propagation test at 450 °C and crystal plasticity analysis are conducted for a Ni-base single crystal superalloy. The Mode I cracking, the transition, and the crystallographic cracking are observed to be dependent on the level of stress intensity factor range, and to be affected by the crystallographic orientations in a compact specimen. Crystal plasticity finite element analysis for the Mode I cracking, the transition, and the crystallographic cracking are all conducted to investigate the slip activity of an octahedral slip system in front of the crack tip by carefully considering the actual 3D geometry of the crack tip. By taking account of the fatigue damage on the individual slip plane, a damage parameter provides reasonable explanations for the fatigue crack propagation rates during Mode I cracking and crystallographic cracking regardless of crystallographic orientation. This damage parameter has also quantified the critical condition that induces the transition from Mode I cracking to crystallographic cracking in a single crystal superalloy.