Onset Of Plastic Deformation Research Articles

Mechanical properties of MoW–HfN surrogate cermet fuel for nuclear thermal propulsion

AbstractThe mechanical performance of MoW‐HfN, a surrogate cermet for MoW‐UN Nuclear Thermal Propulsion (NTP) fuel, was characterized from room temperature to 1600°C. The modulus of elasticity and flexure strength were obtained from four‐point bend tests. Those tests revealed a loss of stiffness with increasing temperature and systematic increase in ultimate strength up to about 1400°C. This was followed by loss of ultimate strength and the onset of plastic deformation, attributed to the increased ductility of the MoW matrix above 1400°C. Chevron notch tests show that failure originates from features with a critical flaw size of ∼30 εm, which is comparable to the mean particle size of the ceramic phase. X‐ray diffraction (XRD) and Williamson–Hall (WH) analysis suggest that residual stress may contribute to the observed strength‐versus‐temperature behavior.

Determination of the onset of yielding and the Young’s modulus after a change in the loading direction

The onset of plastic deformation is an important parameter for an accurate description of the flow curve and the Young’s modulus. Determining the actual physical start of flow is already experimentally challenging for classic sheet metal materials. In addition to the experimental challenge, the onset of flow depends on numerous parameters such as strain rate, temperature and forming history. Non-proportional load paths in particular can significantly influence the onset of flow. Three different materials, a micro-alloyed steel HC340LA, a dual-phase steel CR330Y590-DP and an aluminium alloy AA6016-T4 are investigated in this publication. The physical onset of flow of the materials is determined at three different pre-strain levels as well as without and with a change in the load direction. Temperature-based approaches are used for this purpose. In-situ synchrotron diffraction is used to validate the results obtained. Those results can help to improve existing material models and springback prediction. Such models rely on material parameters that are as accurate as possible.

Origin of nucleation and growth of extension twins in grains unsuitably oriented for twinning during deformation of Mg-1%Al

A large number of anomalous extension twins, with low or even negative twinning Schmid factors, were found to nucleate and grow in a strongly textured Mg-1Al alloy during tensile deformation along the extruded direction. The deformation mechanisms responsible for this behaviour were investigated through in-situ electron back-scattered diffraction, grain reference orientation deviation, and slip trace-modified lattice rotation. It was found that anomalous extension twins nucleated mainly at the onset of plastic deformation at or near grain boundary triple junctions. They were associated with the severe strain incompatibility between neighbour grains as a result from the different <a> basal slip-induced lattice rotations. Moreover, the anomalous twins were able to grow with the applied strain due to the continuous activation of <a> basal slip in different neighbour grains, which enhanced the strain incompatibility. These results reveal the complexity of the deformation mechanisms in Mg alloys at the local level when deformed along hard orientations.

In-situ positron annihilation spectroscopy during tensile tests on Al alloys

We present a materials analysis method to enable in-situ Doppler broadening spectroscopy (DBS) of the 511 keV annihilation line in matter during tensile tests. This technique allows the correlation between the formation of lattice defects on an atomic scale and the macroscopic physical materials properties stress and strain. By implanting a monoenergetic positron beam into samples of AlMg3 (3.3535) and AlMgSi (3.3206) the onset of plastic deformation was clearly observed in the recorded annihilation spectra during the measurement of the corresponding stress–strain curves. The changes of the DBS spectra are attributed to positron annihilation in (open-volume) defects—predominantly vacancies elastically bound to dislocation lines—formed during plastic deformation. The elastic strain (Hook’s region), however, does not lead to changes in DBS spectra.

Grain-level mechanism of plastic deformation in harmonic structure materials revealed by high resolution X-ray diffraction

Materials with heterogeneous microstructures have been reported to have an attractive combination of strength and ductility. This is attributed to synergistic strengthening effects from the difference in strength of fine- and coarse-grained regions. Understanding the interaction of the regions is crucial for further optimization of the microstructures. In this work, we fabricated nickel of harmonic structure (HS) and a reference with homogenous coarse grains. The HS constitutes of an interconnected fine-grained network that surrounds regions of coarse grains. The interplay of the regions was studied by monitoring Bragg reflections from individual grains in situ during tensile deformation until approximately 2 % strain through synchrotron X-ray diffraction. The technique allows grain-level assessment of the degree of plastic deformation. Two grains were followed in the reference and two small grains (fine-grained region) and two large grains (coarse-grained region) in the HS. Three deformation regimes were identified: elastic deformation, onset of plastic deformation and significant plastic deformation. Our results reveal that the large grains in the harmonic structure onset plastic deformation during the macroscopic elastic stage. With increasing applied stress, the small grains yield plastically also and once a large fraction of the fine-grained network deforms plastically the large grains undergo significant plastic deformation. Notably, the onset of significant plastic deformation of large grains in the HS occurs at approximately 100 MPa higher applied stress than in the grains in the reference. This shows that fine grains constrain the large grains from deforming plastically in the HS.

Effects of lattice distortion and chemical short-range ordering on the incipient behavior of Ti-based multi-principal element alloys: MD simulations and DFT calculations

One of key factors in designing high-performance alloys is to understand their incipient behavior. To get detailed real-time atomic scale evolutions of multi-principal element alloys (MPEAs) during nanoindentation incipient yielding, molecular dynamics (MD) simulations were carried out. Emphases are given to the effects of lattice distortion (LD) and chemical short-range ordering (CSRO) on the initiation of plasticity in body-centered cubic (BCC) MPEAs. The initial plastic mechanism is embryo nucleation assisted and the incipient behavior is strongly affected by LD and CSRO. Specifically, the embryo nucleation load Pe is reduced by LD and CSRO and the incipient drop P0 is lowered by LDI that only comes from atomic size difference and further declined by CSRO. This is because LD and CSRO could boost embryo nucleation, which can act as precursors for dislocation nucleation. This is consistent with the free-end nudged elastic band observations. Although Pe is weakened by LDII generated by introducing extra elements, P0 is intensified because severe LD could dominate the following process. The dislocation starvation mechanism is found in an average-atom (AA) model. However, it is replaced by dislocation interactions due to LD and CSRO. Density functional theory (DFT) calculations suggest that charge density distribution and the amount of directional Al-Al bonding would play a critical role in the onset of plastic deformation. When a larger indenter radius was used in MD simulations, the dislocation starvation mechanism is absent in AA model due to the reduced maximum shear stress. Instead, the dislocation propagation prevails. With respect to the orientation effect, three investigated orientations demonstrate a distinct yielding behavior.

Deformation behavior and strengthening effects of an eutectic AlCoCrFeNi2.1 high entropy alloy probed by in-situ synchrotron X-ray diffraction and post-mortem EBSD

In this work, high energy synchrotron X-ray diffraction was used during tensile testing of an as-cast eutectic AlCoCrFeNi2.1 high entropy alloy. Aside, from determining for the first time the volume fractions of existing phases, we further detail their role on the alloy deformation behavior. The two major phases, a soft disordered FCC and a hard ordered B2 BCC, were observed to exhibit a stress partitioning effect which can be used to modulate the mechanical response of the material based on the relative volume fraction of each phase. Dislocation density analysis revealed that the soft FCC phase had a significantly higher dislocation density right after the onset of plastic deformation. This is attributed to the existence of strain gradients across the lamellar structure, where the hard B2 BCC prevents free deformation of the FCC phase. Nonetheless, despite the increase of the dislocation density in the soft FCC phase, calculations of the strengthening effects induced by generation of dislocations are more significant in the hard B2 BCC phases, as this phase is primarily responsible for the strength increase in the alloy. Besides, the evolutions in dislocation density of the soft FCC and hard B2 BCC phases during tensile deformation obtained from synchrotron X-ray diffraction data are consistent with the evolution of KAM determined by EBSD characterization. Also, lattice strain analysis across two principal directions (parallel and perpendicular to the loading axis) reveals that for these specific orientations there is a preferential deformation of the hard FCC planes which can be related to the deformation response of specific lattice planes at distinct orientations, as well as to the phase partitioning stress behavior.

Comparison of the Strain Rate Sensitivity in AZ31 and WE43 Magnesium Alloys under Different Loading Conditions

The initial texture effect of strain rate sensitivity response in magnesium alloys was studied using the quasi in situ electron backscatter diffraction technique. The strain accumulations, under uniaxial loading of both tensile and compression along the rolling direction, showed strain rate dependency. For the AZ31 magnesium sheets with strong basal texture (max intensity to 11.2), twinning first occurs at the onset of plastic deformation, resulting in macroscopic strain rate insensitivity. With further loading, the strain rate sensitivity significantly increases (with the strain rate sensitivity coefficient m increased from 0.0025 to 0.012), arising from the initiation of slip. For the WE43 alloy sheets with weak basal texture (max intensity to 2.6), the overall deformation is sensitive to strain rate under both tensile and compression. The twinning growth rate and twinned volume fraction have been used to identify whether the process is twin-dominated or slip-dominated during the plastic deformation of a magnesium alloy.

Stress partitioning in harmonic structure materials at the early stages of tensile loading studied in situ by synchrotron X-ray diffraction

In situ X-ray diffraction was employed to gain insight into the tensile deformation behavior of harmonic structure (HS) nickel. HS consists of a continuous network of fine grains encompassing islands of coarse grains. With the aid of a newly developed separation algorithm, the X-ray diffraction signals from coarse and fine grains were analyzed separately. Using line profile analysis, the evolution of local stresses and microstrains in the respective grain fractions were examined. Several stages were identified during yielding and the onset of plastic deformation in HS nickel. The mechanical behavior of the individual fractions was related to the macroscopic behavior of the HS and compared to that of homogeneous counterparts with similar average grain sizes as the grain fractions. The analysis reveals that the stress partitions between the grain fractions and that the plastic deformation of coarse grains within the HS are constrained by the network of fine grains.

Size effect on the mechanical responses of aluminum nanoparticles under indentation: A molecular dynamics study

Micro- and nanoparticles are significant raw materials for cold spray and aerosol deposition in which the plastic deformation of particles plays an important role. The mechanical properties of the micro- and nanoparticle are greatly influenced by size. Aluminum particles of various radii under various spherical indenters are simulated with molecular dynamics method to investigate the size effect on the mechanical responses at nanoscale. The particle radius varies in a wide range from 20 nm to 400 nm, and the indenter radius is about 4 nm to 40μm. As the onset of plastic deformation, the first yielding point of the force–displacement curve is influenced by both the sizes of the particle and the indenter, which indicates that the mechanical behavior of nanoparticles depends not only on their own sizes but also on their loading conditions. The ratio between the particle size and the indenter size is found to be an essential factor, which categorizes the indentation models. Under the same ratio, the smaller particle is easier to activate plastic deformation.

Strength of titanium-zirconium alloy implants with a conical connection after implantoplasty

Statement of problemPeri-implantitis occurs around dental implants, and implantoplasty has been used to address this ongoing disease; however, the changes to the physical properties of an implant after implantoplasty have not been well documented. PurposeThe purpose of this in vitro study was to determine the effect of implantoplasty on fracture strength and the load required for plastic deformation after cyclic fatigue on dental implants. Material and methodsTwenty-six titanium/zirconium (TiZr) alloy implants (Roxolid Bone Level Implant; 4.1×10 mm) were embedded with 50% thread exposure and divided into 4 groups based on whether they had implantoplasty treatment by using different diamond rotary instruments and/or cyclic loading at 250 N for 2 million cycles: C0 (control, no cyclic loading), T0 (test, no cyclic loading), CM (control, cyclic loading), and TM (test, cyclic loading). After implantoplasty and/or cyclic loading, all implants underwent a load-to-failure test. The maximum fracture strength (FS) and load required for the onset of plastic deformation (PD) were recorded in Newtons. One-way ANOVA and nonparametric comparisons with control by using the Dunn and Wilcoxon method for joint ranking were used for statistical analysis. ResultsThe mean ±standard deviation FS for C0, CM, T0, and TM was 1465.2 ±86.4 N, 1480.7 ±64.1 N, 1299.3 ±123.8 N, and 1252.1 ±85.7 N, respectively. The mean ±standard deviation load for onset of PD for C0, CM, T0, and TM was 860.2 ±88.1 N, 797.0 ±130.5 N, 776.5 ±181.8 N, and 631.3 ±84.5 N, respectively. The TM group had a significantly lower FS and PD than the C0, CM, and T0 groups (P<.05) ConclusionsBoth fracture strength (FS) and the onset of plastic deformation (PD) were significantly reduced after a TiZr alloy implant received implantoplasty and cyclic loading.

Analysis of the evolution of the Lankford coefficient in uniaxial tensile tests and validation with grain structure analysis

Modern materials mostly exhibit a low formability or a complex material behavior. For example, the common mild steel DC06 exhibits a strain state dependency in the yield stress and a strain dependency of the Lankford coefficients. Thus it is necessary to characterize the material properties from the onset of plastic deformation up to fracture. For example in the uniaxial tensile test, the flow stress and the r-values can be determined. According to the standards, the characterization is only applicable up to uniform elongation. In the case of DC06, the uniform elongation is with 24% comparatively low in contrast to the ultimate elongation with 60%. Research has proven, that the true stress can be characterized up to high strain in the uniaxial tensile test. For the strain dependent Lankford coefficient, this validation is missing. The aim in this investigation is the analysis of the strain dependent anisotropy up to high strain levels in uniaxial tensile tests and the validation. This is done with a comparison of optically measured r-values with the grain structure evolution of the pre stretched parts. These results prove the applicability of the standardized evaluation of Lankford coefficients out of DIC data above the uniform elongation.

Note on the Early Thermoelastic Stage Preceding Rayleigh–Bénard Convection in Soft Materials

In this paper, we focus on the first stage of transition to Rayleigh–Bénard convection in soft-jammed systems (yield stress fluids) confined in a parallelepiped box heated from the bottom. Up to yielding, the material is in a solid-state with a constant elastic modulus. By means of a linear thermoelastic model, an analytical solution for stresses and strains induced by the gravity and the temperature gradient is derived. The analytical solution allows us to emphasize the appropriate dimensionless parameters. The onset of plastic deformation is then investigated using the classical yield criteria (Tresca, von Mises and Drucker–Prager). This analysis is subsequently applied to experimental data of the literature dealing with Rayleigh–Bénard convection in Carbopol micro gels.

Challenges related to tomographic reconstruction of 3D intragranular orientation fields in the presence of orientation relationships

X-ray diffraction contrast tomography (DCT) is a near-field diffraction imaging technique to characterize the 3D shape and crystallographic orientation of grains within polycrystalline samples. The presence of orientation relationships as such encountered between Σ3n annealing twins found in Cu and Ni lead to systematic diffraction spot overlap, since a significant fraction of the lattice planes is shared between the parent and twin crystal lattices. If not correctly addressed, these overlaps will lead to artifacts in the individual grain reconstruction. In this paper we introduce a strategy for joint, tomographic reconstruction of Twin Related Domains (TRD) and cost function weights into DCT to improve the grain reconstruction quality. A large-grained polycrystalline sample made from pure Ni is used for testing this approach and its ability to reveal intragranular orientation gradients related to plastic strain localization at the onset of plastic deformation in this type of materials.

Fatigue improvement and residual stress relaxation of shot-peened alloy steel DIN 34CrNiMo6 under axial loading

Shot-peening treatment was applied to a quenched and tempered DIN 34CrNiMo6 steel to improve its high-cycle R: −1 axial fatigue strength. Compared with the machined condition, the increase in the fatigue limit was 21.8%. S-N curves for shot-peened and the as machined condition were presented and compared with those obtained in previous research for rotating bending fatigue, including curves for mirror-polished specimens.The applied shot-peening treatment in this work (Isp: 8A and 200% coverage) for quenched and tempered (Q + T) DIN 34CrNiMo6 steel introduced a compressive residual stress field and an increase in surface roughness, as well as minor variations in microstructure, hardness and the FWHM (full width of the diffraction peak at half maximum intensity) parameter.The introduced compressive residual stress field tended to reduce when an external stress is applied. This was due to the onset of plastic strain. In this paper, two types of quasi-static tests were conducted by applying an axial stress with six different magnitudes and in the two directions (compressive or tensile). This was in order to assess their influence on the relaxation of surface residual stresses. Due to the introduced compressive residual stresses, if the applied stress was compressive, the onset of plastic deformations was achieved with a lower stress magnitude.In addition, surface residual stress relaxation under cyclic applied stress was evaluated at four different stress magnitudes. Due to the cyclic-softening behaviour of this Q + T steel, its cyclic mechanical properties must be considered to assess the onset of plastic strains. With the experimental data, a logarithmic model to predict the evolution of surface residual stresses with the number of cycles for different applied stress magnitudes was presented.

Length effect on the plastic deformation of SiO2 microcantilevers

SiO2 films are of much interest for the realization of suspended micromechanical structures such as microcantilevers and micro-bridges owing to their superior characteristics when compared to conventional c-Si. However, presence of residual stress induced bending and structural plastic deformation in these devices especially at elevated temperatures hinder their potential for real-time field applications. Moreover, the impact of micro-scale dimensional effects on the magnitude of deformation in these devices is not well understood. In this work, we have investigated the effect of post-release high temperature annealing on the bending and resonance frequency characteristics of SiO2 microcantilevers (MCs) with a specific focus on the length effect. For this purpose, SiO2 MCs of various lengths (Length: 50–190 μm, Width: 37 μm and thickness: 0.98 μm) were fabricated using direct laser writer and wet chemical etching methods and were subjected to high temperature annealing under N2 and O2 environments. We demonstrate through our experiments that the curvature of SiO2 MCs increases with both annealing temperature and length square of MCs. Our experiments revealed that, apart from onset of plastic deformation, stress accumulation at the fixed end of SiO2 MCs is also responsible for the observed curvature increase. From the measured curvature, mean (σ0) and gradient (σ1) residual stress values were estimated and found to increase with annealing temperature. Interestingly, there exists a critical length of MCs (90 μm) below which both σ0 and σ1 were found to increase further with reducing MC length and is shown to be due to the existence of large transverse curvature at these dimensions. High temperature annealing was also found to increase the resonance frequency of MCs and the increase was maximum ∼ 8% for 130 μm long MC. Various possible reasons for the frequency shift is discussed in detail.

Asymmetry evolutions in microstructure and strain hardening behavior between tension and compression for AZ31 magnesium alloy

This work investigated the asymmetry evolutions in the microstructure and the plastic deformation behavior between the entire tension and compression for AZ31 magnesium alloy by combining the experiment and visco-plastic self-consistent (VPSC) model. Large yield stress asymmetry between the tension and the compression existed at the onset of plastic deformation, but such stress asymmetry slightly increased first and then gradually decreased to 1.01 with the increase of logarithmic plastic strain. This kind of asymmetry variation was ascribed to the microstructure evolution asymmetry. Compared to the tension deformation, tension twin boundaries migrated more easily in the compression deformation owing to the appropriate crystal orientation, producing more obvious fraction variation of tension twin boundary and the intersection of different tension twin variants. The VPSC model clarified the texture evolution asymmetry between the tension and the compression by the contribution of each deformation mechanism to the macroscopic strain, revealing that prismatic slip activation in the tension triggered the development of ⟨101‾0⟩//LD (LD: loading direction) type texture while abundant tension twinning in the compression laid the foundation of ⟨0002⟩//LD type texture. Additionally, owing to the deformation mechanism transformation from basal slip to prismatic slip in tension deformation, the attenuation degree in the strain hardening rate was gradually weakened, while in compression deformation, the participation of elastic deformation contributed to the increasing stage of strain hardening rate after the exhaustion of tension twinning. In addition, the geometrical orientation factor and deformation mechanism factor together contributed to the decline of flow stress asymmetry.

Origin of {112} &lt; 111 &gt; antitwinning in a Ti-24Nb-4Zr-8Sn superelastic single crystal

$${\left\{112\right\}\langle {111}\rangle }_{\beta }$$ twins are observed in a superelastic β Ti-24Nb-4Zr-8Sn (wt.%) single crystal after tensile test. A careful Schmid factor analysis shows that these twins are formed in the antitwinning sense regarding the classical $${\left\{112\right\}\langle {111}\rangle }_{\beta }$$ twinning system of bcc structures. These are then $${\left\{112\right\}\langle {111}\rangle }_{\beta }$$ antitwins. Moreover, a full stress-induced martensitic (SIM) transformation of β phase into α" martensite is evidenced from in situ synchrotron X-ray diffraction. This transformation is fully accomplished before the onset of plastic deformation and, in turn, the formation of twins. From crystallographic reconstruction, $${\left\{112\right\}\langle {111}\rangle }_{\beta }$$ antitwins are shown to be passively formed from the reversion, during the reverse SIM transformation when the stress is released, of $$ \left\{ {110} \right\}\left\langle {110} \right\rangle _{{\alpha ^{{\prime \prime }} }}$$ twins actually formed in α" martensite. The martensitic transformation occurring before twinning plays a key role in the activation of antitwinning systems by reducing both shear and shuffle magnitudes of twinning. Variant selection of stress-induced martensite and Schmid factor analysis show that the classical $${\left\{112\right\}\langle {111}\rangle }_{\beta }$$ twins can never be activated in β titanium alloys involving SIM transformation, while the non-classical $${\left\{112\right\}\langle {111}\rangle }_{\beta }$$ twinning system in the antitwinning sense is always favored.

ВЛИЯНИЕ ХИМИЧЕСКОГО СОСТАВА НА ТВЕРДОРАСТВОРНОЕ И ДЕФОРМАЦИОННОЕ УПРОЧНЕНИЕ МОНОКРИСТАЛЛОВ ГЦК ВЫСОКОЭНТРОПИЙНЫХ СПЛАВОВ

A characteristic feature of high-entropy alloys is high strength at maintaining plasticity, wear and corrosion resistance, and fracture toughness at cryogenic temperatures. Currently, CoCrFeNiMn is the best-investigated high-entropy compound. However, its application is limited in the high-temperature region due to the low values of the deforming stress level at the plasticity breaking point at T&gt;296 K. One of the common ways to improve the material durability is the addition of substitution atoms of larger atomic radius, and Al, Ti, and Mo are some of these atoms. The paper presents the analysis of the mechanical behavior of single crystals of CoCrFeNiMn and CoCrFeNiMо FCC high-entropy alloys (at. %) oriented along the [001] direction: the author studied the temperature dependence of critical shear stresses τcr(T) within the temperature range of T=77–973K, the type of dislocation structure, strain hardening coefficient θII, plasticity and fracture at Т=296 K under tension. The study shows that the alloying with Mo atoms 4 at. % of the CoCrFeNi system (at. %) causes the solid solution hardening, and critical shear stresses τcr increase within the entire studied temperature range. The onset of plastic deformation is associated with slip at all temperature tests. At T=296 K, the author identified a planar dislocation structure with flat dislocation pile-ups and dislocation networks in CoCrFeNiMo while in equiatomic CoCrFeNiMn, at such test temperature, a uniform distribution of dislocations was observed in several systems without flat pile-ups. Work hardening coefficient, plasticity, and the level of stresses before fracture turn out to be similar in [001]-crystals of CoCrFeNiMo and CoCrFeNiMn high-entropy alloys, which are determined by the development of slip deformation simultaneously in several systems. Crystals are destroyed viscously at 296 K at the same level of stress.

Atomistic simulations to study shock and ultrashort pulse response of high entropy alloy

This article aimed to study the shock compression and ultrashort pulse shock compression behavior of multi-elemental alloy, high entropy alloy (HEA), and single-crystal Ni. Non-equilibrium molecular dynamics (NEMD) based simulations were utilized in this article with the help of EAM potential to capture the deformation under dynamic and complex loading scenarios in HEA (Co-Cr-Cu-Fe-Ni) and single-crystal Ni. Simulations were performed with continuous shock compression and ultra-short shock pulse traversing the atomic configuration at velocity 0.75 km/s. Simulation helps capture the distribution of pressure and velocities in the simulation box as a function of the time from the onset of the shock front. Simulation helps capture the distribution of pressure and velocities in the simulation box as a function of the time from the beginning of the shock front. It was concluded from the MD simulations that the insertion of nano-voids in the path of the shock front helps in dispersing the energy and reducing the speed of the shock front. Nano-voids significantly affect the shock deformation of crystals and help in inducing plastic deformation in the HEA and Ni, even at shock velocities below Hugoniot elastic limit (HEL). Nucleation of dislocation from the nano-voids is more significant in HEA crystal as compared to Ni. The onset of plastic deformation in HEA and higher dislocation densities helps in dispersing the energy of the shock front, and the performance of HEA improved at higher shock velocities. The results in this article will help develop high entropy alloys for applications containing complex loading scenarios such as high strain rate loading experienced in space structures and armor vehicles.