Nickel-based Single Crystal Alloy Research Articles

A microdamage model for FCC single crystals considering a mixed failure mechanism of slip and cleavage

In this study, a microdamage model is newly proposed to predict the failure process of FCC single crystals. Two micro damage mechanisms, slip and cleavage, are both involved in the model. A microscopic damage variable is first defined at the slip plane scale, and a damage coupled crystal plasticity constitutive equation in a co-rotational tensor framework is then established. A damage driving force consisting of generalized energies related to the resolved shear stress and the positive resolved normal stress is proposed to reflect the mixed failure mechanism of FCC single crystals induced by slip and cleavage. The damage evolution equation is then derived in the framework of thermodynamics. An explicit algorithm for the damage coupled crystal plasticity finite element method is given. The proposed microdamage model is utilized to predict the slip-dominant fracture behavior of a single crystal copper and the cleavage-dominant fracture behavior of a nickel-based single crystal alloy. The damage evolution process of slip planes is given and the influence mechanism is revealed. The calculated results agree well with the experimental data, which verifies the good capability of the proposed model in describing and predicting the multi failure processes of FCC metals. Finally, a 2D fracture simulation of a single crystal notched specimen under shear loading is conducted, and the effect of damage parameters on the fracture modes is also discussed.

Atomic-scale study of linear reciprocating friction of TCP/γ phase in nickel-based single crystal alloy

In order to systematically investigate the role of TCP (topologically close-packed) phases in the fretting wear process of nickel-based single crystal alloys (NBSC), this study employed molecular dynamics to conduct comparative analyses of mechanical properties, atomic displacements, wear depth, defects, dislocation density, and the influence of temperature under constant load on the friction process in material wear. The research revealed that during the repetitive friction process, the friction force exhibited a peak at the extreme positions of reciprocating friction on the workpieces, and this peak increased with the number of friction cycles. The dislocation density in the worn area increased, resulting in hardening, and the removal rate of material decreased. At the initial stages of friction, the presence of interfaces notably hindered the transfer of temperature, defects, and atomic displacements in the workpiece, and this inhibitory effect weakened with an increasing number of friction cycles. The TCP phases experienced stratification due to the overall deformation they underwent. Furthermore, as the relaxation temperature increased, the workpiece exhibited enhanced plastic deformation capacity, an increase in dislocation density, and adhesion between abrasive particles and the grinding ball occurred.

The micro-structure and mechanical performance of Nickel-based single crystal alloy

Because of the increasing temperature in front of turbine in aero-engine field, Ni-base single crystal alloys are of high demand, which achieve high strength at high temperature. Therefore, a variety of methods have been developed to detect or describe nickel-base single crystal alloys. This article will take stock of the alloy valve and the mainstream description of valve methods. This paper will describe the basic micro-mechanism of rafting. Then the channel width method, which is the most widely used method to describe the degree of rafting, is introduced. After that, the updated version of the channel width method, the method to describe the rate of rafting and the effect on its strength, and the related methods of image processing are described. It is hoped that this paper can help readers to have a preliminary understanding of the principle and mathematical description method of nickel-base alloy rafting. There are many branches of mathematical description, but the main ideas are very similar. It is hoped that the researchers concerned will be able to develop some mathematical models that require fewer measurement parameters but not reducing the overall accuracy of the measuremen.

Effect of Re/Ru on constituents distribution and creep performance of nickel-based single crystal alloys

ABSTRACT The effects of Re/Re on the element distribution and creep performance of nickel-based single crystal alloy by using atom probe tomography (APT) and creeping property testing has been studied. Results show that the elements Re, Ru, W, Mo, Cr, and Co are the former of γ phase; the elements are distributed in γ and γ‘ phases of two alloys according to various ratios. Some of the Ru atoms in Re-containing alloy make more Al, Ta atoms are dissolved in γ matrix and make more Mo, W, and Re atoms dissolving in γ‘ phase, which increases the alloying degree of γ“, γ phases to enhance the strength and creep resistance of alloy. Particularly, the Re and W atoms dissolved in γ” phase are excluded, during creep, for the enrichmrnt in γ phase near the interface to form their peak content; the lattice distortion coming from the enriched Re and W atoms restrains dislocations gliding to delay the γ’ phase from being sheared. The deformed mechanisms of alloy in the later stage of creep are the dislocations gliding in γ phase and shearing γ’ phase. Wherein, the dislocations of shearing γ’ phase are both glided on {111} planes and cross-glided from {111} to {001} planes to form the KW locks, the ones that restrain the gliding and cross-gliding of dislocations for improving the creep resistance of alloy. While the interaction of the Ru with Re and W atoms make some Reand W atoms reserved in the γ′ phase to delay the diffusion of other elements, which prevents the KW locks from being released, to keep the good resistance of alloy.

Molecular dynamics study of the effect of rolling process on subsurface strengthening of nickel-based superalloy GH4169 plastic deformation

In this paper, the molecular dynamics (MD) method is used to establish a rolling model at the nanoscale, the same depth of rolling is rolled, and three rolling methods are used, namely, one time, two times and three times respectively. The effects of three rolling methods on the workpiece are studied, and the evolution of dislocation and defect atoms, subsurface damage, surface quality and subsurface micro-strengthening mechanism of the rolled workpiece are mainly analyzed. The results show that the dislocation density increases most significantly by one rolling, and the difficulty of material deformation is reduced by multiple rolling, with the increase of rolling times, the surface roughness increases. We found that the rolling process has a directional effect on the micro-strengthening of nickel-based single crystal alloys, and the strengthening effect is most significant in the normal direction. The subsurface strengthening effect of rolled workpiece is positively correlated with dislocation density and rolling force. Compared with the previous macro rolling test research, we observed the strengthening effect of dislocation density on rolling plastic deformation of nickel-based single crystal alloy from the microscopic level, which can provide a reference for understanding the strengthening mechanism of rolling process on nickel-based single crystal alloy and the optimization of rolling parameters.

In-situ SEM study on fatigue crack behavior of a nickel-based single crystal alloy at 950 °C and 1050 °C

As the blade material of engine, nickel-base single crystal superalloy has been subjected to complex service environment such as high temperature and alternating load for a long time. In this work, the mechanism of fatigue microcrack initiation and propagation in a nickel-based single crystal at 950 °C and 1050 °C was studied by using the in-situ scanning electron microscope high temperature fatigue test system. At 950 °C, the alloy deformation was controlled by slip and the crack shear γ/γ’ phase cracking, showing Stage I mode. At 1050 °C, the alloy crack tearing occurred at the γ phase parallel to the loading direction and γ/γ' interface perpendicular to the loading direction and presents a Stage II mode perpendicular to the loading axis, then changes into Stage I in the accelerating stage. The deformation mechanism of fatigue cracking at two temperatures was compared. Besides, the influence of the microstructure defects on the fatigue crack behavior was discussed. The results show that the propagation of the microcrack below 100 μm shows evident fluctuations, which is closely related to the microstructure and the development stage of the fatigue crack.

Atomic-scale study of the repeated friction processes of γ/γ' phase nickel-based single crystal alloys

The molecular dynamics method was used to simulate the micro-motion friction process of nickel-based single-crystal alloy. With the repeated friction processes increases, the peak friction force gradually increases, the friction coefficient changes more drastically, the transmission of atomic displacement gradually changes from having discontinuity to having continuity, the increment of abrasion depth gradually decreases, the dislocation entanglement is more likely to appear near the two-phase interface, and the friction hardening effect is weakened when the position of the grinding ball is closer to the two-phase interface. The transfer of temperature is relatively linear, which is different from the effect of the amorphous interface layer. Under constant load, the faster the friction speed, the smaller the wear of the workpiece.

Creep Behavior Characterization of Nickel-Based Single-Crystal Superalloy DD6 Thin-Walled Specimens Based on a 3D-DIC Method.

The thickness debit effect of creep behavior has been a focal point of nickel-based single-crystal superalloy research, and there is a need for an advanced creep deformation measurement method. This study developed a novel high-temperature creep test system based on a single-camera stereo digital image correlation (DIC) method with four plane mirrors to conduct creep tests on thin-walled specimens of a nickel-based single-crystal alloy, DD6, with thicknesses of 0.6 mm and 1.2 mm under experimental conditions of 980 °C/250 MPa. The reliability of the single-camera stereo DIC method in measuring long-term deformation at a high temperature was experimentally verified. The experimental results show that the creep life of the thinner specimen was significantly shorter. It was found the lack of coordination in the creep deformation process of the edge and middle section of the thin-walled specimens may be an important factor in the thickness debit effect according to the full-field strain contour. By comparing the local strain curve at the rupture point with the average creep strain curve, it was found that the creep rate at the rupture point was less affected by the specimen thickness during the secondary creep stage, while the average creep rate in the working section significantly increased as the wall thickness decreased. The thicker specimen usually had a higher average rupture strain and higher damage tolerance, which prolonged the rupture time.

Molecular dynamics study on defect evolution during the plastic deformation of nickel-based superalloy GH4169 single crystal under different rolling temperatures.

Nickel-based superalloy GH4169 is widely used as an important material in the aviation field. The rolling forming process can improve its surface quality and performance. Therefore, conducting an extensive investigation into the microscopic plastic deformation defect evolution process of nickel-based single crystal alloys during the rolling process is crucial. This study can offer valuable insights for optimizing rolling parameters. In this paper, a nickel-based superalloy GH4169 single crystal alloy was rolled at different temperatures from the atomic scale using the molecular dynamics (MD) method. The crystal plastic deformation law, dislocation evolution and defect atomic phase transition under different temperature rolling were studied. The results show that the dislocation density of nickel-based single crystal alloys increases as the temperature increases. When the temperature continues to increase, it is accompanied by an increase in vacancy clusters. When the rolling temperature is below 500 K, the atomic phase transition of the subsurface defects of the workpiece is mainly a Close-Packed Hexagonal (HCP) structure; when the temperature continues to increase, the amorphous structure begins to increase, and when the temperature reaches 900 K, the amorphous structure increases significantly. This calculation result is expected to provide a theoretical reference for the optimization of rolling parameters in actual production.

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 <111> plane or cross slip from the <111> plane to the <100> plane to form K-W locks.

Determination of elastoplastic properties in anisotropic materials with cubic symmetry by instrumented indentation

Multi-dimensional finite element modelings of instrumented sharp indentation on materials with cubic symmetry are conducted. A novel reverse analysis algorithm for identifying elastoplastic properties is established for the cubic material, while the anisotropy of the material is determined by indentations on distinct orientations. Influences of elastic anisotropy and anisotropic yielding behavior are considered with the help of three-dimensional computations. Sensitivity analysis confirms the accuracy and robustness of the new method. The obtained results are compared and verified with experimental data of a nickel-base single crystal alloy.

Microstructure characterization and damage coupled constitutive modeling of nickel-based single-crystal alloy with different orientations

The mechanical properties of nickel-based single-crystal superalloys are sensitive to temperature and orientation. The present work focuses on the stress and strain responses of a second-generation nickel-based superalloy at different temperatures and with different orientations. Microstructure and dislocation arrangements are observed by scanning and transmission electron microscopy of samples after tensile testing. The results reveal the tensile fracture failure modes and dislocation morphologies (braids, cells, stacking faults, etc.) for different temperatures and orientations. Although the [001] sample shows higher yield strength than the other two orientations, the strain hardening was more significant at [111] orientation and stress softening at [011] orientation due to their different dislocation mechanisms. In addition, a new hardening model and damage evolution law are proposed and coupled with a crystal plastic constitutive model. Numerical analysis results show that the proposed constitutive model can accurately fit the full stress-strain response from tensile experiments for different temperatures and orientations.

Molecular dynamics study of nano-cutting mechanical properties and microstructural evolution behavior of Ni/Ni3Al phase structure

Ni/Ni3Al (γ/γ′) phase interface structure in nickel-based single crystal alloys determines its excellent mechanical properties, which is widely used in aeroengine turbine blades. Because the deformation behavior of the γ/γ′ phase interface under external force is very different from that of a single crystal without an interface, it is necessary to study the influence of the γ/γ′ phase interface structure on nano-cutting performance. In this paper, molecular dynamics simulation is used to study the nano-cutting performance and mechanism of the γ/γ′ phase interface structure. We found that the γ/γ′ phase interface has a significant effect on the magnitude of cutting force and cutting stability. The cutting force of the γ phase is greater than that of the γ′ phase, but the cutting stability is reversed. Surprisingly, the cutting force gradually decreases when the tool moves from the γ phase to the γ′ phase; and when the tool moves from the γ′ phase to the γ phase, the cutting force gradually increases. When cutting processes are performed entirely in γ phase and alternately in γ and γ′ phases, respectively, nano-cutting deformation mechanisms are dominated by dislocation slip at lower speeds. The nano-cutting deformation mechanism transitions from dislocation slip to disordered atoms leading to unstable cutting forces when the cutting process is completely performed in the γ phase at a cutting speed of 200 m/s. However, when the cutting process occurs alternately in the γ and γ′ phases, the cutting deformation mechanisms of the γ′ and γ phases are twins, point dislocations and stacking faults, respectively. Moreover, we found that the stacking defects of the γ phase are absorbed by the γ/γ′ interface when the tool moves from the γ phase into the γ′ phase. But the dislocations are activated again when the tool moves from the γ phase into the γ′ phase. Finally, we found that the surface roughness of the workpiece is the smallest at cutting speed of 100 m/s, which is manifested as pits and surface steps. This study will provide theoretical guidance for the nano-cutting scheme of nickel-based single crystal alloy.

Study on the evolution mechanism of subsurface defects in nickel-based single crystal alloy during atomic and close-to-atomic scale cutting

Nickel-based single crystal alloys are widely used in aerospace industry. From the point of view of defect evolution and energy change, the evolution mechanism of subsurface defects in the atomic and close-to-atomic scale (ACS) cutting single crystal nickel-based alloy with silicon nitride tool was studied, combining with the molecular dynamics method. The mixed potential function was used to describe the force interaction between atoms in the ACS cutting system. The process of nucleation, propagation and transformation of subsurface defects were analyzed. Based on the dislocation emission model, the relationship between dislocation slip length and change of energy was described. The variety, quantity and area of subsurface defect changes caused by the change of cutting speed, crystal orientation and cutting depth were discussed. The result shows that increasing cutting speed can effectively reduce the number and area of subsurface defects and improve the quality of machined parts. The larger the cutting speed, the larger the chip volume. With the increase of cutting depth, the number of stacking fault tetrahedrons in the subsurface area of workpiece increases. When crystal orientation 010 00 1 ¯ is used for cutting, more defects will be produced in the subsurface region. When crystal orientation are 111 10 1 ¯ and 011 01 1 ¯ , the dislocation emission angle is π /2, it is the most favorable for the emission of partial dislocations and easy to remove materials. The research content provides important references for the optimization of machining parameters and improvement of processing quality for the ultraprecision cutting of nickel-based alloy.

Creep Fracture Mechanism of a Single Crystal Nickel Base Alloy Under High Temperature and Low Stress

This paper reports the creep behavior of a nickel-based single crystal alloy and the creep fracture mechanism under high temperature and low stress was discussed. The creep curves were analyzed with the Kelvin model. The retardation spectra suggest that the involved atoms during creep need more relaxation time to achieve the viscous flow, nevertheless, the creep under the stress of 120 MPa may be caused by the mismatch of dislocation motion and visco-plastic deformation. The fracture morphologies of crept alloys indicate that the nickel base single crystal alloy presents micro-pore aggregation fracture mechanism under the condition of high temperature and low stress creep.

Investigation on efficiency and quality for ultrashort pulsed laser ablation of nickel-based single crystal alloy DD6

The high processing efficiency and quality are constant pursuits for modern manufacturing industry. This paper investigated the ablation efficiency and quality of ultrashort pulsed laser ablation of DD6 single crystal alloy based on experiments and theoretical analysis. The experimental results showed that the ablation rate increases with the increase of laser fluence and with the decrease of scanning speed and scanning width, while the ablation efficiency decreases with the increase of laser fluence. A relatively flat and low melted zone ablation surface could be obtained by employing a low laser fluence and high scanning speed. The influence of laser parameters on the ablation diameter, equivalent energy density, and heat accumulation effect was analyzed based on the theory of laser ablation and heat conduction. The theoretical analysis revealed the material removal transforms from plasma or vaporization removal to melt ejection with the pulse energy increases and the scanning speed decreases, which can explain the formation mechanism of surface morphology very well. In addition, the scanning strategy of high efficiency and quality was proposed based on the theoretical analysis and experimental results.

Study on staged work hardening mechanism of nickel-based single crystal alloy during atomic and close-to-atomic scale cutting

Nickel based single crystal alloys have excellent properties such as heat resistance, corrosion resistance and creep resistance, which are widely used in aerospace and other national defense fields. Severe work hardening occurs in the process of cutting nickel based single crystal alloy. How to improve the machining quality and grasp the cutting deformation mechanism has become the research focus. In this paper, the effect of work hardening on the surface of workpiece during the atomic and close-to-atomic scale (ACS) cutting process is studied. The model of Si3N4 ceramic tool cutting the nickel based single crystal alloy was established, and the ACS cutting process was simulated by the molecular dynamics method. The existing strain rate conversion model was modified to make it suitable for the process of ACS cutting into nano compression with the same strain rate. The results show that the dislocation density of Ni-based single crystal alloy workpiece changes greatly with the change of cutting distance. According to the change of microstructure in the workpiece, a new staged work hardening mechanism is proposed. The development of work hardening in the cutting process is divided into three stages, and the transition node of each hardening stage is defined. An important sign of the transformation from the first stage to the second stage of work hardening is the occurrence of a large number of dislocation pile-up group, dislocation tangles and the appearance of non-basal slip lines. The distinctive feature of the transformation from the second stage to the third stage of work hardening is that a large number of screw dislocations are cross-slip and the dislocation pile-up group is destroyed. At the same time, the different hardening mechanisms in each stage and the reasons for the change of work hardening mechanism in different stages are summarized. The research content is believed to be helpful to understand the mechanism of significant work hardening effect in nickel-based alloys.

Study on phase transformation in cutting Ni-base superalloy based on molecular dynamics method

Nickel-based single crystal alloys are widely used in aerospace and other important fields of national defense due to their excellent properties. Phase transformation occurs during high-speed cutting of nickel-based single crystal alloy, which seriously affects the surface quality. It is of great significance to carry out theoretical research on phase transformation for improving the machining quality of nickel-based alloy. In this paper, molecular dynamics method is used to study the nano-cutting of single crystal nickel-based alloy with silicon nitride ceramic tool. The mechanism of phase transformation and the effect of cutting speed on phase transformation in workpieces are studied in detail. The nano-cutting model is established. Morse potential functions for molecular dynamics simulation are calculated, and EAM and Tersoff potential functions are selected. The effect of cutting speed on phase transformation was studied by using radial distribution function, coordination number analysis, common neighbor analysis, and the deep reasons for the sharp change of lattice structure were analyzed from many aspects. Finally, in order to verify the universality of the research results and explore the new properties of compression, nano compression (the same strain rate as the nano cutting process) was simulated. The results show that the increase of cutting speed leads to the increase of hydrostatic stress, the increase of energy in crystal and the rise of cutting temperature. As a result, the change of lattice structure becomes more and more intense, and the conversion rate of different crystal structures increases greatly.

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.

Effect of mismatch degree on mechanical properties of Ni-based single crystal alloy under force-temperature coupling

Mismatch degree has an important effect on the mechanical properties of Ni-based single crystal alloys. However, the effect of mismatch degree on mechanical properties under force-temperature coupling is unclear. Thus, we used molecular dynamics method to study mechanical properties and microstructural evolution of Ni-based single crystal alloy. The interface dislocation network and mechanical properties under different positive mismatches are discussed respectively. We found that the lower the mismatch degree, the more regular the dislocation network is, which can effectively prevent the dislocation motion and increase the strength. Meanwhile, when the mismatch degree is less than 1.43, the yield strength gradually decreases with the increase of the mismatch degree. When the mismatch degree is 1.43, the yield strength reaches the minimum value. Moreover, the lower interface energy can increase the yield strength of nickel-based single crystal alloy, and the yield strength decreases with the increase of interface energy. To compare the influence of positive and negative mismatch on mechanical properties and microscopic evolution, the yield stress of Ni-based single crystal alloy increases with the increase of mismatch, but the change is not significant. At the negative mismatch degree, the yield stress decreases as the mismatch decreases. When the absolute value of the mismatch is the same, the yield stress at a positive mismatch of 0.33% is greater than the yield stress at a negative mismatch of −0.33%.