Large Compressive Stress Research Articles

Atomistic simulations of the face-centered-cubic-to-hexagonal-close-packed phase transformation in the equiatomic CoCrFeMnNi high entropy alloy under high compression

We performed the modified-embedded-atom-method (MEAM) based molecular dynamics (MD) simulations to investigate the plastic deformation and phase transformation behaviors in the CoCrFeMnNi HEA under high compression at room temperature. Our MD simulations revealed that the stress-induced phase transformations in the CoCrFeMnNi HEA are strongly crystal orientation-dependent. The [0 0 1] uniaxial compression can induce the significant face-centered-cubic (fcc) -to-hexagonal-close-packed (hcp) phase transformation via successive emissions of partial dislocations from the extended stacking faults, twin boundaries and hcp-lamellas created during the early stage of deformation. As for the [1 1 0] and [1 1 1] uniaxial compressions, however, the transformed hcp atoms can simply form the intrinsic/extrinsic stacking faults. Although the [0 0 1] uniaxial compression produced a much lower dislocation density than the other two systems, it induced much more constituents transformed into the hcp atoms at the end of phase transformation. Our results clearly indicated that the deformation twin boundaries and extended hcp-lamellas play a critical role in facilitating the stress-induced fcc-to-hcp phase transformation in the CoCrFeMnNi HEA. Furthermore, it was found that the phase transformation in the CoCrFeMnNi HEA can be effectively facilitated by a large deviatoric compressive stress while it may tend to be significantly retarded by a hydrostatic compression. Our results also showed that the plastic deformation behaviors in Ni under high compression are very similar to those occurred in the CoCrFeMnNi HEA though nearly all the hcp atoms can simply constitute the intrinsic/extrinsic stacking faults without the formation of any bulk hcp phase in the fcc lattice. The main discrepancy in the phase transformation behaviors between the Ni and CoCrFeMnNi HEA can be largely attributed to the much lower stacking fault energy of the CoCrFeMnNi HEA than other fcc metals.

Mechanical properties and enhancement mechanisms of titanium-graphene nanocomposites

Molecular dynamics (MD) simulations of the titanium-graphene nanocomposites (TiGNCs) under uniaxial tension are carried out to investigate the mechanical properties and reinforcement mechanism of graphene in composites. It is found that introduction of mechanically robust graphene limits the strain-induced dislocation and amorphization and thereby highly improves the mechanical properties of metallic titanium that are greatly affected by the crystal stacking orientation of graphene and titanium layers. The thickness of titanium layers, interface interaction and working temperature play an important role in the mechanical strength and elastic moduli of composites. The results show the mechanical properties of TiGNCs are monotonically enhanced with reduction of the titanium layer thickness and working temperature, and the Young’s modulus obtained by MD simulation are higher than that predicted by the rule of mixture (ROM) due to consideration of interfacial interaction in computational calculation. In addition, once the critical thickness of titanium layer is reached, graphene wrinkles are induced in composites because of Poisson’s effect induced large lateral compression stress in the interface region. This study provides helpful insights into fundamental understanding reinforcing mechanism of graphene and ultimately contribute to the optimal design and performance of mechanically robust graphene-based metallic composites.

Anisotropic crystallization and magnetic properties of Fe-based amorphous alloys induced by ion inclined bombardment

The structures and magnetic properties of Fe73.5Si13.5B9Nb3Cu1 amorphous alloys after ion bombardment were investigated. The needle-like structure and granular structure nanocrystalline was respectively formed for the samples bombarded by ion beam at the incident angle of 75° and 90°, which was explained according to the crystallization and crystal growth under anisotropic stress produced by ion beam inclined bombardment. The ion beam inclined bombardment produced a compressive and parallel stress on the sample, the compressive stress promoted the nucleation by decreasing the critical nucleation free energy and the parallel stress caused the directional migration of atoms. Low incident angle ion bombardment only led to the structure relaxation, and high incident angle ion bombardment produced a large compressive stress on the sample, which then induced the crystallization of amorphous alloys. The anisotropic crystallization caused by ion beam inclined bombardment resulted in the increase of magnetic anisotropy and then improved the high-frequency properties.

Oxidation Behavior of Ta-W Alloy

The oxidation behaviors of tantalum-tungsten alloy with 10-20% W was investigated between temperature range of 700 to 900 °C exposed in air. The kinetics of Ta-W alloy was determined by TG-DTA, the characteristics of oxides were analyzed by SEM, EDS and XRD. The oxidation tests revealed that the alloys obeyed parabolic kinetic in the initial stage, then translated in linear law. The addition of W has a good effect on the oxidation resistance of Ta-W alloys at experimental temperature. Solid solution of Ta2O5form in case of oxidation product of Ta-10W, Ta-15W alloys, while the complex oxide Ta22W4O67form after Ta-20W alloy oxidized. The formation of solid solution and complex oxide impeded the volatilization. The compact oxide film protects the penetration of oxygen in the initial oxidation stage. The large compressive stresses and mismatch of the coefficient of thermal expansion between oxide scale and matrix alloys make the oxides layer be broken, which cause kinetic of oxidization obeying linear law.

Modeling the Interfacial Chemistry of Graphene-Based Composite Electrode Materials for Lithium Ion Battery

High volumetric expansions, large compressive stresses, and subsequent interfacial debonding are largely associated with high energy density materials such as Silicon (Si). Incorporation of additional materials such as Graphene provides Si anode a porous skeleton, allowing Si to expand/contract easily with low mechanical stresses and continuous electrical contact with added advantages of flexibility, good conductivity, lightweight, and high surface Li diffusivity. The interface between Si and Graphene has been extensively studied computationally, and effects of its attributes on battery performance have been determined via experimental investigation of Si-Graphene based composite electrodes. Graphene significantly lowers the stresses in expanding Si, thus increasing its cycle life. In this study, specific interfacial attributes, which directly or indirectly contribute towards the strain accommodation in expanding electrodes have been analyzed, with the aim to create a model that will help in understanding and predicting the performance of graphene provided-interface in Lithium ion batteries. For this purpose, First Principle calculations are carried out using VASP to examine the interfacial chemistry of Si-Graphene system. Produced results are then further compared with additional electrode models: Tin(Sn) and Selenium(Se). While Sn is another high energy density material, which holds promise as anode for ion batteries beyond LIB, Se is a cathode material with superior electrical conductivity and lithiation rates. Both materials undergo strenuous phase changes during battery performance. Changes in interface characteristics post phase change of electrode have also been emphasized. It was understood that interfacial cohesive energy is not merely dependent upon the atomic attributes and type of bonding with the Graphene surface as mostly highlighted, but also on unaccounted for lattice mismatch between the crystalline electrode and Graphene. With changing phases and deviation in lattice constants, interface adhesion changes even though bonding between electrode and Graphene remains unchanged.

Surface finishing and enhancement of Ni-based alloy using sliding burnishing with active rotary tool

The burnished surface quality of a Ni-based alloy was investigated. For this, we developed and utilized a sliding burnishing process with an active rotary diamond-like carbon-coated carbide tool. Two kinds of Ni-based alloys subjected to solution heat and aging heat treatments were used as workpiece materials. The burnished surface quality was evaluated based on the roughness and profile of the surface, hardness, and the microstructure of the subsurface, residual stress, and full-width at half maximum based on X-ray analysis, and bending property of the testpiece. The preliminary surface, which was finished by filing, was also evaluated for comparison. A smooth-finished surface of approximately Ra = 0.1 μm or less was obtained by the burnishing process in both materials. The subsurface hardness of the solution heat-treated material was increased by the burnishing process, and it was also observed to increase with the thrust force as a burnishing condition, while the influence of the burnishing process on the subsurface hardness at a depth of 20 μm or more from the surface was hardly observed in the aging heat-treated sample. A large compressive residual stress and the full-width at half maximum can be obtained for the burnished surface. Based on the three-point bending test using the solution heat-treated material, the yield load transitioned from the elastic to the plastic region increased and the bending strength also clearly increased due to the burnishing process for a thrust force of 100 N. The developed sliding burnishing process with diamond-like carbon-coated carbide tool can yield a high-quality surface with low roughness, high hardness, and large compressive residual stress of Ni-based alloys based on solution heat treatment and aging heat treatment, respectively.

High toughness carbon-nanotube-reinforced ceramics via ion-beam engineering of interfaces

In nanocomposites, improved mechanical performance is critically linked to understanding and controlling interfacial properties. In the work reported here, ion implantation is introduced as a new method for tailoring the complex nanoscale interfaces between multiwall carbon nanotubes (MWCNTs) and a ceramic matrix. The results show that surface layers of the nanocomposite with high toughness can be created with C2+ ions. This enhanced toughening due to ion implantation is associated with large compressive stresses in the MWCNTs and with significant changes in the carbon structure. These observations are consistent with molecular dynamics (MD) simulations, which indicate that partial amorphization of the MWCNTs is enhanced by the compressive stress and confinement within the ceramic matrix. This work opens up new opportunities for using ion implantation to create a new class of exceptionally tough ceramic nanocomposites.

Finite Element Calculation and Analysis of Stress and Deformation Characteristics of Arch Dam

The arch dam is a high-order and ultra-static overall space shell structure on the consolidation and bedrock, and its geometry, boundary conditions and stress state are complex. The three-dimensional finite element solid model of arch dam is established, and the stress and deformation characteristics of the actual dam project are calculated. It is concluded that there is a large tensile stress at the dam at the arch crown section and a large compressive stress at the dam site. The transverse displacement of the body occurs at the abutment of the upstream and downstream faces. The calculation results are in accordance with the stress and deformation characteristics of the arch dam, and the maximum stress and maximum displacement meet the design requirements of the arch dam.

Surface Integrity and Oxidation of a Powder Metallurgy Ni-Based Superalloy Treated by Laser Shock Peening

Laser shock peening (LSP) is a mechanical surface treatment which can induce large compressive residual stresses and microstructural changes in a material by using repetitive shocks from laser pulses. In this work, the surface integrity (surface microstructure, topography, hardness and residual stress) of a LSP-treated powder metallurgy Ni-based superalloy was investigated for the first time. LSP treatment introduced large plastic deformation especially at a depth of about 100 μm from the surface, which increased the local hardness. The residual stress from the surface to the interior of the sample was investigated by synchrotron x-ray diffraction. The maximum compressive stress reached 400 MPa at the surface, while the depth of the compressive stress is about 0.7 mm. Lastly, the oxidation behavior of the treated and untreated samples was evaluated by thermal exposure at 700°C for 312 h. The LSP treatment decreased the thickness of the oxide layer, thereby showing improved oxidation resistance.

Correlation of Microstructure and Properties of Cold Gas Sprayed INCONEL 718 Coatings

In the cold gas spray process, deposition of particles takes place through intensive plastic deformation upon impact in a solid state at temperatures well below their melting point. The high particle impact velocities and corresponding peening effects can lead to high compressive residual stresses in cold spray coatings. This can be advantageous with regard to mechanical properties as fatigue life and hence, cold spray is an ideal process for repair applications. In this study, INCONEL 718 particles were cold sprayed by using nitrogen as propellant gas. The deposited coatings with different thicknesses were characterized using electron microscopy techniques to study grain refinement and precipitates in the coating. In addition, depth-resolved residual stress measurements have been performed by the incremental hole drilling method. The residual stress depth profiles in the coatings indicate compressive residual stresses of several hundred MPa which are hardly influenced by the coating thickness. In addition, large compressive stress levels are found in surface-near regions of the substrate due to the grit blasting process. Furthermore, a post-heat treatment analysis was performed to investigate its influence on residual stresses and bonding strength. These findings are used to develop a consistent explanation of the dependence of strength values on thickness.

Influence of unintentionally incorporated Ar atoms on the crystalline polarity of magnetron-sputtered Al-doped ZnO polycrystalline films on glass and sapphire substrates

The authors investigated the relationship between the content of incorporated Ar atoms and the crystalline polarity of polycrystalline Al-doped ZnO (AZO) films and examined the influence of this relationship on the resulting growth rate, crystallographic texture, and electrical properties. They deposited AZO films on glass and c-plane sapphire substrates via radio-frequency magnetron sputtering at substrate temperatures (Ts) of 100 or 200 °C using sintered AZO targets with an Al2O3 content of 2.0 wt. %. The incorporation of a large amount of Ar atoms induced a change in the crystalline polarity from Zn-polar to O-polar, the latter of which is associated with numerous obstacles, such as a lower deposition rate, large residual compressive stress, and increased electrical resistivity. The authors demonstrated that increasing the Ts led to a reduction in the amount of unintentionally retained Ar atoms, thereby affording Zn-polar AZO films with their associated advantages.

Measurement of Swelling-Induced Residual Stress in Ion Implanted Sic, and its Effect on Micromechanical Properties

Ion implantation is widely used as a surrogate for neutron irradiation in the investigation of radiation damage on the properties of materials. Due to the small depth of damage, micromechanical methods must be used to extract material properties. In this work, nanoindentation has been applied to ion irradiated silicon carbide to extract radiation-induced hardening. Residual stress is evaluated using HR-EBSD, AFM swelling measurements, and a novel microcantilever relaxation technique coupled with finite element modelling. Large compressive residual stresses of several GPa are found in the irradiated material, which contribute to the significant hardening observed in nanoindentation measurements. The origin of these residual stresses and the associated hardening is the unirradiated substrate which constrains radiation swelling. Comparisons with other materials susceptible to irradiation swelling show that this effect should not be neglected in studying the effects of ion irradiation damage on mechanical properties. This constraint may also be influencing fundamental radiation defects. This has significant implications for the suitability of ion implantation as a surrogate for neutron irradiations. These results demonstrate the significance of swelling-induced residual stresses in nuclear reactor components, and the impact on structural integrity of reactor components.

Effect of ultrasonic longitudinal–torsional composite milling of the residual stress on the surface of titanium alloy

The present study envisages the study of the residual stress characteristics of titanium alloy surface, based on the ultrasonic longitudinal–torsional composite milling process. Initially, the tool path analysis of common milling and ultrasonic longitudinal–torsional composite milling was performed. When compared with the common milling, it was found that the tool–chip cycle separation phenomenon occurred in the torsional vibration direction in the ultrasonic milling tool path. Subsequently, an ultrasonic longitudinal–torsional composite milling experimental platform was built and the effects of the ultrasonic amplitude (A), cutting speed ( vf), axial depth of cut ( ap), and feed per tooth ( fz) on the residual stress of titanium alloy were studied using an orthogonal experiment. The results indicated that the residual stress in ultrasonic longitudinal–torsional composite milling comprised a combination of the “plastic bulge effect” caused by cutting force–cutting heat and the “extrusion effect” of the tool flank on the workpiece. The “extrusion effect” was dominant and presented a compressive stress on the machined surface. The ultrasonic amplitude (A) showed the most significant influence on the residual stress. With an increase of the amplitude, the extrusion and collision effect of the tool on the workpiece surface became more obvious, along with a stronger “extrusion effect” between the tool and the workpiece surface. Experimental results provide a good processing method for the difficult-to-machine materials such as titanium alloy, and offers an effective method for obtaining large surface residual compressive stress.

Mechanical properties of thermally grown submicron oxide layers on a nickel-based superalloy

Nickel-based superalloys are frequently used under operating conditions that lead to the formation of an oxide layer. In this study, the mechanical properties of such a submicron oxide scale formed on a nickel-based superalloy after oxidation at 650 °C were evaluated. Poisson’s ratio and Young’s modulus of the layered oxide structure were determined to be respectively 0.29, and 259 GPa. Depth-resolved residual stress profiling with nanoscale resolution showed large compressive residual stresses up to 3000 MPa in the oxide, with magnitude correlated to the oxide scale thickness. The relevance of the results to the crack initiation and growth behaviour is discussed.

Effect of laser shock peening on microstructure and residual stress of Ti<sub>2</sub>AlNb alloy

Ti2AlNb-based alloys are developed from conventional titanium aluminides (TiAl-based alloys and Ti3Al-based alloys). Comparing with TiAl and Ti3Al-based alloys, Ti2AlNb-based alloys have better mechanical properties of strength density ratio, high temperature fracture toughness and creep resistance. It is expected to replace traditional high temperature materials such as nickel-based alloys for the manufacture of aero-engine components. Laser shock peening (LSP) is an advanced surface modification technology which induces high amplitude, large depth residual compressive stress on the surface of materials. It can effectively enhance mechanical properties of material such as micro-hardness, fatigue resistance, high temperature oxidation performance. Compared to other surface modification technologies such as shot peening and rolling, LSP can not only induce deeper compress residual stress (1–2 mm), which is about five to ten times deeper than that of shot peening, but it can refine the grain or even form nanocrystals which plays a significant role in enhancing properties of material. For these reasons, this surface treatment technology has been widely used in the aircraft and nuclear energy industries. In this paper, the surface modification of Ti2AlNb alloy was carried out by laser shock peening, and the microstructure evolution, residual stress and the performance at high temperature were investigated. The results show that laser shock peening can significantly reduce the grain size of the surface of Ti2AlNb alloy. Dislocation lines, dislocation tangles and dislocation walls were generated after LSP. The micro-hardness is increased from 350 HV to 395 HV and the effect layer is about 1.2 mm. The residual compressive stress of −337 MPa is generated on the surface of the sample. In addition, residual stress is released obviously at high temperature. The residual stress on the surface of sample reduces to −65 MPa after heat treatment at 720°C for 4 h. The residual stress is almost completely released after heat treatment at 720°C for 10 h. However, the release rate of residual stress is slowed down at 600°C. The effect of laser shock peening is still presence after heat treatment for 100 h.

Определение качества поверхностного слоя маложестких цилиндрических изделий после правки поперечной обкаткой гладкими плитами

To restore the shape of low-stiff cylindrical parts such as shafts and axles, straightening by transverse bending with the subsequent processing of workpieces by the method of surface plastic deformation based on the transverse burnishing with smooth plates is proposed. The experimental and calculated results are presented to determine the effect of absolute compression on the main characteristics of the quality of the surface layer of parts such as surface roughness and residual stresses. The analysis of experimental data for the evaluation of the parts after straightening by transverse burnishing showed the following positive changes: a sharp decrease in the initial roughness, the formation of equilibrium residual compressive stresses in the surface layers and ensuring stabilization of the accuracy of the processed part in size and shape. Depending on the magnitude of the absolute reduction, the surface quality increases by 2–3 classes, and rather large compressive residual stresses are formed (up to 375 MPa). The results of the work justify recommending the proposed method of straightening by transverse burnishing with smooth plates for implementation into the technology of machine part restoration.

Insights From Micromechanical Modeling of Intact Rock Failure: Event Characteristics, Stress Drops, and Force Networks

AbstractWe use a bonded‐particle method to investigate event characteristics, b values, internal force distributions, and stress drops in triaxial deformation tests. We simulate brittle through ductile deformation regimes. We find the following: (1) Event rates are proportional to anelastic axial strain: (i) significant and accelerating events rates only occur near peak stress (brittle deformation); (ii) event rates gradually increase until the anelastic strain plateaus (ductile deformation). (2) b value patterns show a systematic decrease when approaching peak stress, after which (i) they increase again (brittle case) or (ii) reach a constant minimum (ductile case). A decrease in b values is indicative of progressive internal damage, with an increasing event rate. (3) Weak‐ and strong‐force networks exist within the sample. Macrofailure of the sample occurs due to collapse of the strong‐force network. Acoustic emissions predominantly occur (i) within the weak‐force network allowing for tensile crack opening and closing despite the large compressive external stresses, and (ii) in areas with the largest spatial force gradients, close to the strong‐force networks. All internal compressive and extensional normal forces display exponential distributions despite uniform boundary stresses. (4) Stress drops of the largest events are inversely proportional to peak stress; the largest stress drops occur for brittle failure, progressively approaching a zero magnitude for ductile deformation. The systemic correlations between b values, the number of acoustic emissions, and stress drops with the maximum principal stress may offer opportunities to invert for changes in the stress state from these remote observables during earthquake cycles.

Residual stress measurement of {112}-oriented CrN layers in CrN/Cr multilayer films

In this work, the authors propose and verify a method of measuring the residual stress of {112}-oriented chromium nitride (CrN) layers in CrN/Cr multilayer thin films. The CrN layers of a CrN/Cr multilayer film deposited on a Ti6Al4V substrate by arc ion plating form both a randomly oriented mixed crystal structure and a {112}-oriented structure. Therefore, accurate stress measurement of the CrN layers cannot be performed by applying the sin2 ψ x-ray method assuming an isotropic homogeneous material. To overcome this obstacle, the proposed method to measure the residual stress uses four CrN-422 diffractions: at ψ = 0°, 33.56°, 48.19°, and 60.00°. Next, the authors vary the density of Cr droplets on the film surface to evaluate how it affects the residual stress in the CrN/Cr multilayer film. The results indicate that the Cr layer has a residual compressive stress of −350 to −530 MPa and that the two CrN layers have a very large residual compressive stress of −3.5 to −8.2 GPa. In addition, both residual compressive stresses decrease with increasing droplet density.

Measurement of the residual stress in chromium nitride coatings deposited on an aluminum alloy substrate using arc ion plating method

Chromium nitride (CrN) coatings were deposited on Al alloy substrates using the arc ion plating method with different bias voltages and different thicknesses. The residual stresses of these samples were measured via x-ray diffraction using the sin2 ψ method because the CrN crystals in the coatings were nonoriented. The stress gradient across the CrN coating was calculated from the curved 2θ-sin2 ψ diagram. In the case of CrN coatings deposited at low bias voltage, the compressive residual stress that formed at the substrate interface was larger than the stress at the surface of the CrN coating. Conversely, in the case of CrN coatings deposited at high bias voltage, the compressive residual stress on the surface of the CrN coating was larger than the stress on the interface with the substrate. In CrN coatings deposited at high bias voltage, very large compressive residual stress on the CrN coating surface decreased with increasing coating thickness.

Experimental and numerical study of granite blocks containing two side flaws and a tunnel-shaped opening

In this study, uniaxial compression tests were conducted on granite specimens containing two side flaws. Based on the experimental results, the numerical models containing two side flaws and a tunnel-shaped opening were created to investigate the peak stress, cracking process and stress distribution under uniaxial compression. It is found that the flaw inclination angle strongly affects the strength, cracking process and stress distribution, while the slight change of the opening shape has little influence. The cracks initiate from the flaw tips and the top and bottom of openings. Unlike the case of a circular opening, in addition to arch waists, large compressive stress occurs at the arch foots of the tunnel-shaped opening.