Anomalous Hardening Research Articles

Anomalous hardening of two-component disordered crystals

The nature of increasing the strength of disordered two-component solid solutions in comparison with materials consisting of atoms of one component is studied. For this purpose, the contribution of extreme fluctuations in the distribution of solution atoms, which create obstacles for the movement of dislocation kinks, is calculated. It is shown that a slow - power - decrease in the probability of large delays on such obstacles leads to anomalous kinetics of kinks. It is accompanied by a slowdown in the movement of dislocations. This may be the reason for the hardening of the material.

Void and helium bubble interactions with dislocations in an FCC stainless steel alloy: anomalous hardening and cavity cross-slip locking

The critical stress for cutting of a void and He bubble (generically referred to as a cavity) by edge and screw dislocations has been determined for FCC Fe0.70Cr0.20Ni0.10—close to 300-series stainless steel—over a range of cavity spacings, diameters, pressures, and glide plane positions. The results exhibit anomalous trends with spacing, diameter, and pressure when compared with classical theories for obstacle hardening. These anomalies are attributed to elastic anisotropy and the wide extended dislocation core in low stacking fault energy metals, indicating that caution must be exercised when using perfect dislocations in isotropic solids to study void and bubble hardening. In many simulations with screw dislocations, cross-slip was observed at the void/bubble surface, leading to an additional contribution to strengthening. We refer to this phenomenon as cavity cross-slip locking, and argue that it may be an important contributor to void and bubble hardening.

Core and surface structure and magnetic properties of mechano-synthesized LaFeO3 nanoparticles and their Eu3+-doped and Eu3+/Cr3+-co-doped variants

The core and surface structure and magnetic properties of mechano synthesized LaFeO3 nanoparticles (30–40 nm), their Eu3+-doped (La0.70Eu0.30FeO3), and Eu3+/Cr3+ co-doped (La0.70Eu0.30Fe0.95Cr0.05O3) variants are reported. Doping results in a transition from the O′-type to the O-type distorted structure. Traces of reactants, intermediate phases, and a small amount of Eu2+ ions were detected on the surfaces of the nanoparticles. The nanoparticles consist of antiferromagnetic cores flanked by ferromagnetic shells. The Eu3+ dopant ions enhance the magnetization values relative to those of the pristine nanoparticles and result in magnetic susceptibilities compatible with the presence of Eu3+ van Vleck paramagnetism of spin–orbit coupling constant (λ = 363 cm−1) and a low temperature Curie–Weiss like behavior associated with the minority Eu2+ ions. Anomalous temperature-dependent magnetic hardening due to competing magnetic anisotropy and magnetoelectric coupling effects together with a temperature-dependent dopant-sensitive exchange bias, caused by thermally activated spin reversals at the core of the nanoparticles, were observed.

Unveiling the correlation between anomalous hardening and grain boundary diffusional transformation in ω single-phase nano-grained Ti-Fe alloy

A metastable ω single-phase nanograined (NG) Ti-Fe alloy was synthesized using laser inert-gas condensation (IGC). Upon being annealed at 360 °C, the NG Ti-Fe alloy exhibited a remarkable ultra-hardening, increasing the hardness from 4.7 to 8.6 GPa. Subsequent findings revealed an unexpected hardness enhancement, rising from 6.6 GPa at 420 °C to 7.6 GPa at 460 °C, despite the occurrence of grain growth. In-depth investigations into the strengthening mechanisms of the NG Ti-Fe alloy were conducted using in-situ synchrotron high-energy X-ray diffraction (XRD) and transmission electron microscopy (TEM). The comprehensive analysis unveiled that the diffusion-controlled structure evolution during annealing played a pivotal role in enhancing the alloy’s mechanical properties. This study not only presents the synthesis of a novel metastable alloy but also provides valuable insights into the intricate relationship between diffusion-controlled structure evolution and the resulting superior mechanical properties.

The anomalous annealing hardening behaviors in commercial pure Tantalum foil

Annealing softening phenomenon is usually observed in pure metals. However, in this study, the anomalous annealing hardening behavior is found in commercial pure Tantalum (Ta) foil. When the cold-rolled Ta foils are annealed at a temperature lower than 700 °C, both the yield strength and ductility gradually increase with the rise in annealing temperature, resulting in an anomalous "low-temperature annealing hardening" phenomenon. The yield strength of Ta foils annealed at 700 °C reaches the maximum value of about 750 MPa. This phenomenon in Ta foil is closely related with the shear bands and the pinning behavior between interstitial atoms and dislocations. When annealing temperature is higher than 700 °C, the yield strength decreases. When annealing temperature exceeds 900 °C, recrystallization occurs in Ta foil. Anomalous yield point phenomenon appears in the recrystallization foils during stretching, which is also related with the pinning of dislocations by interstitial atoms. As annealing temperature further increases, the volume fraction of surface grains gets larger. And {112}<111> slip systems are preferred over {110}<111> slip systems in the surface grains.

The electroplastic effect in titanium alloys under tension

The electroplastic effect (EPE) is a phenomenon which consists in a decrease in the strain resistance and enhancing of the plasticity of metals under the effect of the electric current of a sufficiently high density [1]. The goal of the study is to compare the deformation behavior of single-phase commercially pure tita­nium Grade 4 and two-phase VT6 alloy under tension and external heating with introduction of a pulsed current. Current of various pulse ratio and density was supplied to the grips of the tensile testing machine from a pulse generator. To estimate the relative contribution of the electroplastic effect during passage of current to the reduction of flow stresses, the materials were also exposed to external heating. The microstructure of the samples in the sample head and in the vicinity of the fracture region in the longitu­dinal section was studied using optical microscopy. The electroplastic effect in the studied materials is manifested on the tensile curve through individual jumps in the downward flow stress at a high pulse ra­tio, whereas at a low current pulse ratio a decrease in the flow stress and strain hardening and increase in the plasticity are observed. It is shown that tension of the sample under the effect of current results in a greater decrease in the flow stresses than that observed under external heating at the same temperature for both materials. This confirms the athermal nature of the pulsed current effect. The critical density of the high pulse ratio current (q - 5000) capable of providing manifestation of the electroplastic effect is two times lower for a VT6 alloy than for pure titanium Grade 4. Under the same pulsed current modes, the flow stresses for VT6 decrease more than for Grade 4. Pulsed current of high pulse ratio caused an anomalous hardening effect in a VT6 alloy, but the physical nature of this effect requires an additional study. The pulsed current modes used in the study did not lead to any structural changes noticeable under optical magnification in the samples under tension, except for the disappearance of twins and separation of the impurity particles in Grade 4 and spheroidization of grains in VT6.

Temperature-dependent formation of gradient structures with anomalous hardening in an Al–Si alloy

The temperature effect on forming gradient structure in an Al–Si alloy during surface severe deformation is studied in this report. The intermediate temperature (473 K) produces the steepest gradient structure by an anomalous hardening on the top layer compared to lower (300 K) and higher temperature (673 K) counterparts. Our analysis shows profuse aluminum oxide particles in the top layer of the gradient structure under 473 K lead to anomalous hardening by oxide-dispersion strengthening. The counterintuitive enhancement of strengthening at the intermediate temperature is explained by the dynamic interplay between thermal-driven, mechanical-aided oxidation and the wear-induced loss of surface materials as a function of temperature, which yields a critical processing temperature to achieve the steepest gradient structure.

Vacancy Hardening and Ordering in Rhenium Tungsten Carbides

Vacancies generally reduce the number of chemical bonds and hence cause structural softening. It is puzzling, however, that substoichiometric Re0.5W0.5C0.4 with 60% carbon vacancies was identified as a superhard material. Here, we report the underlying mechanism responsible for such anomalous vacancy-induced hardening in the Re0.5W0.5C1–x system via first-principles calculations. The shear stiffness and hardness increase consistently with rising carbon vacancy concentration in Re0.5W0.5C1–x and reach the maximum at about x = 40%. Such an unexpected hardening phenomenon originates from a gradual relief of the shear-unstable dd bonding and unfavorable pd antibonding interactions owing to the formation of C-vacancies. We further predict that the simultaneous ordered vacancies of W and C atoms can produce a cubic crystalline Re2/3W1/3C that is isomorphic with the NbO type. The calculations on vibrational, mechanical, and electronic properties reveal that this phase has considerable structural stability and is a hard metallic material. This work not only elucidates the intriguing mechanism responsible for the vacancy hardening in this class of systems but also provides a new principle for the structural stability of other transition-metal compounds.

Spin-induced strongly correlated magnetodielectricity, magnetostriction effect, and spin-phonon coupling in helical magnet Fe3(PO4)O3

Helical magnets are extremely promising, as they have fascinating magnetic, electric, and phononic properties. Here, we report a spectrum of simultaneously occurring and highly entangled intriguing phenomena induced by helical spin ordering in a noncentrosymmetric and spin-frustrated system ${\mathrm{Fe}}_{3}(\mathrm{P}{\mathrm{O}}_{4}){\mathrm{O}}_{3}$. These phenomena include magnetodielectric effect in the form of a frequency-independent pronounced dielectric peak, clear magnetostriction effect manifested as a dramatic downturn in the thermal variation of lattice parameters, and strong spin-phonon coupling (which displays a unique anomalous hardening and softening of various phonon modes) at temperatures as high as ${T}_{N}=163\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The observed dielectric peak is seemingly associated with a structural distortion via the strong magnetostriction effect.

Microstructural Evolution of a Selective Laser Melted FeCoCrNiMn–(N,Si) High‐Entropy Alloy Subject to Cold‐Rolling and Subsequent Annealing

Herein this article, selective laser melting (SLM) technique is used to manufacture N and Si contained FeCoCrNiMn high‐entropy alloy (HEA). The SLMed FeCoCrNiMn–(N,Si) alloy is then treated by cold‐rolling and isochronous annealing. The influences of Si and N additions on the microstructural evolution and tensile properties of the HEA are investigated. It is shown that adding Si and N elements increases the recrystallization temperature and postpones the whole recrystallization processing of the alloy. The annealing‐induced anomalous hardening is seen for the alloy at both 500 and 600 °C. When the annealing treatment temperature is 800 °C and above, the alloy is fully recrystallized in 1 h. The tensile properties evolution of the FeCoCrNiMn–(N,Si) alloy with annealing temperature has the similar tendency to the equiatomic FeCoCrNiMn alloy. In addition, uniformly distributed Cr2N particles form during the annealing processing. The dispersion and small size of the Cr2N particles significantly improve the tensile mechanical properties of the alloy by inhibiting the movements of both dislocations and grain boundaries.

Anomalous hardening of Al-8%Ca eutectic alloy due to a non-equilibrium phase state transition under laser irradiation

The relationship between the change in the structure and the hardening effect of an Al-8%Ca eutectic aluminum alloy under laser irradiation was studied. Laser surface processing of the as-cast alloy was carried out at a laser power of 370 W and a beam velocity of 750 mm/s. It is shown that laser irradiation leads to a 4-fold increase in the microhardness of the alloy (up to 3.6 GPa) due to the creation of a supersaturated solid solution of calcium in aluminum and a multiple increase in the dislocation density.

Unvailing the spin–phonon coupling in nanocrystalline BiFeO3 by resonant two-phonon Raman active modes

We report on temperature dependence of two-phonon Raman spectra in BiFeO3 nanocrystals, above and below the Néel temperature TN using a resonant laser excitation line (λ=532 nm). Two-phonon modes exhibited anomalous frequency hardening and deviation from the anharmonic decay below TN. Such behavior strongly supported the existence of spin-two-phonon interaction, because these modes are known to be very sensitive to the antiferromagnetic ordering. Within the mean-field theory for the nearest-neighbor interaction, the linear relationship between spin–spin correlation function and observed two-phonon frequency shift below TN was obtained. This approach enabled to quantify the spin–phonon interaction by spin–phonon coupling strength for both two-phonon modes and justified the application of mean-field approach. Magnetic measurements revealed the coexistence of antiferromagnetic and weak ferromagnetic phases below TN, which were found non competitive, additionally supporting the mean-field approach from which we deduced that the two-phonon modes in BiFeO3 are correlated with antiferromagnetic ordering below TN.

Ultrafast photo-induced phonon hardening due to Pauli blocking in MAPbI3 single-crystal and polycrystalline perovskites

Metal-halide perovskite semiconductors have attracted intense interest over the past decade, particularly for applications in photovoltaics. Low-energy optical phonons combined with significant crystal anharmonicity play an important role in charge-carrier cooling and scattering in these materials, strongly affecting their optoelectronic properties. We have observed optical phonons associated with Pb–I stretching in both MAPbI3 single crystals and polycrystalline thin films as a function of temperature by measuring their terahertz conductivity spectra with and without photoexcitation. An anomalous bond hardening was observed under above-bandgap illumination for both single-crystal and polycrystalline MAPbI3. First-principles calculations reproduced this photo-induced bond hardening and identified a related lattice contraction (photostriction), with the mechanism revealed as Pauli blocking. For single-crystal MAPbI3, phonon lifetimes were significantly longer and phonon frequencies shifted less with temperature, compared with polycrystalline MAPbI3. We attribute these differences to increased crystalline disorder, associated with grain boundaries and strain in the polycrystalline MAPbI3. Thus we provide fundamental insight into the photoexcitation and electron–phonon coupling in MAPbI3.

Modeling Gamma-Ray SEDs and Angular Extensions of Extreme TeV Blazars from Intergalactic Proton-Initiated Cascades in Contemporary Astrophysical EGMF Models

The intrinsic spectra of some distant blazars known as “extreme TeV blazars” have shown a hint at an anomalous hardening in the TeV energy region. Several extragalactic propagation models have been proposed to explain this possible excess transparency of the Universe to gamma-rays starting from a model which assumes the existence of so-called axion-like particles (ALPs) and the new process of gamma-ALP oscillations. Alternative models suppose that some of the observable gamma-rays are produced in the intergalactic cascades. This work focuses on investigating the spectral and angular features of one of the cascade models, the Intergalactic Hadronic Cascade Model (IHCM) in the contemporary astrophysical models of Extragalactic Magnetic Field (EGMF). For IHCM, EGMF largely determines the deflection of primary cosmic rays and electrons of intergalactic cascades and, thus, is of vital importance. Contemporary Hackstein models are considered in this paper and compared to the model of Dolag. The models assumed are based on simulations of the local part of large-scale structure of the Universe and differ in the assumptions for the seed field. This work provides spectral energy distributions (SEDs) and angular extensions of two extreme TeV blazars, 1ES 0229+200 and 1ES 0414+009. It is demonstrated that observable SEDs inside a typical point spread function of imaging atmospheric Cherenkov telescopes (IACTs) for IHCM would exhibit a characteristic high-energy attenuation compared to the ones obtained in hadronic models that do not consider EGMF, which makes it possible to distinguish among these models. At the same time, the spectra for IHCM models would have longer high energy tails than some available spectra for the ALP models and the universal spectra for the Electromagnetic Cascade Model (ECM). The analysis of the IHCM observable angular extensions shows that the sources would likely be identified by most IACTs not as point sources but rather as extended ones. These spectra could later be compared with future observation data of such instruments as Cherenkov Telescope Array (CTA) and LHAASO.

A systematic study of interatomic potentials for mechanical behaviours of Ti-Al alloys

Intermetallic Ti-Al alloys exhibit attractive physical and mechanical properties at high temperatures relevant to turbine machinery applications. However, TiAl and Ti3Al alloys have low ductility and low fracture toughness, as well as anomalous hardening behaviour at high temperatures. Molecular dynamics (MD) simulations can directly model critical atomistic mechanisms responsible for these intriguing properties, but the validity of the underlying empirical/semi-empirical interatomic potentials for such complex alloys is not clear. Here, we examine four interatomic potentials (two EAM and two MEAM) developed for the Ti-Al material system and identify their respective strength and weakness in modeling the plastic and fracture properties of the L10γ-TiAl and D019α2-Ti3Al crystal structures. We compare the lattice constants, elastic constants and their temperature-dependence, cohesive energies, stacking fault energies, surface decohesion and thermal expansion coefficients against available experimental and density functional theory data. In addition, we test the deformation behaviours of these potentials under uniaxial tensile loading conditions at a wide range of temperatures. Our results show that these interatomic potentials can accurately reproduce some but not the full scope of the material properties of the γ and α2 crystal structures. In particular, one MEAM potential reproduces some of the properties (some of the negative Cauchy pressure and stacking fault energies) comparable to DFT-calculated data. The results here provide guidance to select the appropriate interatomic potentials for specific applications. Our study also indicates that further optimization of the parameters in the MEAM formalism may lead to better interatomic potentials for the Ti-Al system.

Scale dependence of thermal hardening of fcc metals under shock loading

A dislocation-based constitutive model is applied to revisit the anomalous thermal hardening behavior of fcc metals under shock loading. Two scale-dependent dislocation motion mechanisms are found to dominate the thermal hardening behavior of fcc metals under shock loading. In particular, because of the shear stress decay with the propagation of the shock wave, the relativistic effect of dislocation motion is only significant near the impact surface, whereas the phonon drag mechanism dominates dislocation motion in a macroscopic fcc metal. Furthermore, we provide a detailed picture of the thermal hardening behavior on the continuum scale, in which the role of the newly generated stress wave from plastic deformation in the dynamic deformation process is highlighted. We show that the mechanical response at the elastic precursor is mainly controlled by the stress wave emanating from the plastic front.

Uniaxial ratcheting deformation of 316LN stainless steel with dynamic strain aging: Experiments and simulation

Dynamic strain aging (DSA) widely exists in austenitic stainless steel at elevated temperature. The materials will be drastically strengthened by this phenomenon. A series of symmetric strain-controlled and asymmetric stress-controlled cyclic experiments were conducted at different temperatures from 293 K to 823 K for 316LN stainless steel. The DSA phenomenon was observed in all the tests under elevated temperatures and it became more significant with the increase of temperature until reached the maximum at 823 K. There were a series of manifestations of DSA such as the anomalous cyclic hardening with temperature increasing in the strain-controlled tests and the quick-shakedown of ratcheting strain in the stress-controlled tests. An appropriate constitutive model to describe the cyclic behavior at different temperatures is necessary. The proposed kinematic hardening model is based on Ohno-Wang model and modified by introducing a hardening index into dynamic recovery term. The index accounts for DSA hardening degree of material under high temperature, therefore the performance mentioned above can be simulated well by this proposed model. By relating the parameters with temperature and plastic strain amplitude, a unified visco-plastic constitutive model was proposed. Simulation results of the proposed model agree much better with experimental values.

Oxygen solutes induced anomalous hardening, toughening and embrittlement in body-centered cubic vanadium

Vanadium (V) is sensitive to minute quantity of oxygen interstitials, which induce pronounced hardening and embrittlement. Here, we utilize oxygen to synthesize V solid solutions in order to reveal the mechanism of oxygen solutes induced hardening. With increasing of oxygen solute concentrations, the fracture modes of V samples transform from dimple, to a mixture of dimple and cleavage, and to a fully transgranular cleavage. High density of dislocations and dislocation debris are produced in strained samples. The mobility of screw dislocations is reduced and the dislocation cross-slip events are promoted by oxygen solutes. In addition to oxygen solution hardening, the generation of high density of oxygen-vacancy complexes plays a dominant role in the strengthening. High quantity of loop-shaped dislocation debris are direct evidence for the formation of oxygen-vacancy complexes. Profuse oxygen-vacancy complexes trap dislocations, promote cross-slips and assist dislocation storage, thus give rise to a superior combination of strengthening, strain hardening, and ductility in V with 1.0 at% of oxygen. Once beyond a critical oxygen concentration (>1.6 at%), V shows catastrophic brittle failure due to the exceptional high density of oxygen-vacancy complexes. These findings provide insight to design high performance refractory metals utilizing oxygen solutes.

Lattice hardening due to vacancy diffusion in (GeTe)mSb2Te3 alloys

GeTe-Sb2Te3 alloys have been widely studied for use in rewritable media, and in recent years, they have emerged as excellent thermoelectric materials, with reports of zT&amp;gt;2 for Ge-rich compositions. GeTe-Sb2Te3 alloys exhibit a solid-state phase transition from a layered structure with rhombohedral symmetry to a cubic rocksalt structure, which plays an important role in their thermoelectric behavior. Here, we investigate the impact of the phase transition on the thermal expansion and elastic moduli of (GeTe)17Sb2Te3 using high-temperature X-ray diffraction and resonant ultrasound spectroscopy. The high-temperature elastic moduli of GeTe, Sb2Te3, and Bi2Te3 were also measured for comparison. While it is typical for materials to soften with increasing temperature due to thermal expansion, our study reveals anomalous hardening of the elastic moduli in (GeTe)17Sb2Te3 at temperatures below the phase transition, followed by further hardening at the transition temperature. In contrast, the elastic moduli of GeTe, Sb2Te3, and Bi2Te3 soften with increasing temperature. We attribute the anomalous hardening of (GeTe)17Sb2Te3 to the gradual vacancy diffusion accompanying the transition from a layered to a cubic structure. The stiffening elastic moduli lead to increasing speed of sound, which impacts the lattice thermal conductivity by flattening the temperature dependence.

Effect of Recovery and Recrystallization on the Hall–Petch Relation Parameters in Submicrocrystalline Metals: III. Model for the Effect of Recovery and Recrystallization on the Hall–Petch Relation Parameters

A theoretical model is developed to describe the influence of recovery and recrystallization on the macroelastic limit, the yield strength, and the grain-boundary hardening coefficient of the submicrocrystalline (SMC) metals fabricated by severe plastic deformation. The anomalous hardening and the increase in the grain-boundary hardening coefficient upon annealing of the SMC metals are shown to be related to defect accumulation in migrating grain boundaries in them. Equations are derived to relate the Hall–Petch relation parameters to the grain-boundary migration velocity, the nonequilibrium state of grain boundaries, the lattice dislocation density, and the annealing temperature and time. The results of the numerical calculations performed using the developed model are compared with the experimental results obtained in part I of this work.