Temperature-dependent Softening Research Articles

Dynamic mechanical behaviors of pearlitic U71MnG rail steel: Deformation mechanisms and constitutive model

Pearlitic rail steels with excellent mechanical and wear properties are widely employed for high-speed railways. During the service period, the presence of track irregularities, switches, as well as crossings can cause intense dynamic loadings, which accumulate the severe deformation of rails, and sometimes even facilitate crack initiation and failure. Knowledge of the dynamic mechanical response and damage behaviors of the rail steels is therefore important for the service performance estimation of railways. In this study, the pearlite U71MnG rail steel was deformed using a split Hopkinson pressure bar (SHPB) apparatus to reach a wide range of strain rates and temperatures. Microstructures including the grain morphology, deformation characteristic of cementite lamellae, dislocation substructures in ferrite lamellae, and phase transformation at pearlite colony boundaries were characterized. Based on the microstructural observations, the rate-controlling strengthening mechanism and temperature-dependent softening mechanism of the steel were revealed when the process for the dynamic loading-induced cementite decomposition at boundaries was discussed. In consideration of the athermal deformation mechanism, thermally activated mechanism, and viscous-drag effect, a physically based constitutive model was proposed to describe the dynamic mechanical response of the pearlite rail steel. The predicted flow stresses were found to be consistent with the experimental one for all strain rates and temperatures.

Suppression of intervalley scattering and enhanced phonon anharmonic interactions in 2D Bi2TeSe2: Crystal-field symmetry and band convergence

The electron-phonon (el-ph) and phonon-phonon interactions play a key role in determining electronic and thermal transport properties, respectively, in promising two-dimensional (2D) semiconductor devices. In this study, we investigated el-ph interactions using Wannier-Fourier interpolation method and renormalized phonon scattering considering finite-temperature effects in Bi2TeSe2 monolayer. The results show that the optical phonon modes dominate the carrier scattering, where level repulsion induced by crystal field splitting and spin-orbit coupling (SOC) effect effectively suppresses intervalley scattering, leading to high hole mobility. Moreover, the strong anharmonicity in Bi2TeSe2 monolayer results in the temperature-dependent softening of its optical phonons, along with a corresponding variation in interatomic force constants (IFCs). As a result, the lattice thermal conductivity is remarkably low and exhibits weak temperature dependence. Finally, the predicted dimensionless thermoelectric figure of merit exceeds unity in the range of 200–800 K, indicating the potential of Bi2TeSe2 monolayer for thermoelectric applications. This work provides new insights into the elimination of intervalley scattering by crystal field splitting and SOC effects, and paves the way for the evaluation of thermoelectric properties of materials with complex scattering mechanisms and strong anharmonicity.

Time and temperature dependent softening of a novel maraging steel fabricated by laser metal deposition

Durability is a critical factor for hot stamping dies from an economic point of view. Refurbishing the dies by depositing new material instead of replacement is a promising method to reduce the cost. For this reason, a newly developed maraging steel (NMS) was cladded on a hot work tool steel by means of directed energy deposition. After an optimized tempering, exposures at high temperatures were carried out on the cladded NMS in order to examine the softening resistance. The microstructural evolution of the material was systematically characterized using a combination of optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). The precipitate in the cladded steel was identified as Laves phase. The coarsening of this phase is considered as the main reason for the thermal softening of the steel at high temperatures. The coarsening behavior was also simulated by using the revised Langer-Schwartz-Wagner (LSW) model, which was in good agreement with experimental observations. Moreover, a mathematical model of precipitate strengthening was successfully applied to evaluate the softening behavior of the steel. This model can be used to predict the hardness/strength evolution of the investigated tool steel during its high-temperature service.

Dynamic phase diagram of plastically deformed amorphous solids at finite temperature.

The yielding transition that occurs in amorphous solids under athermal quasistatic deformation has been the subject of many theoretical and computational studies. Here, we extend this analysis to include thermal effects at finite shear rate, focusing on how temperature alters avalanches. We derive a nonequilibrium phase diagram capturing how temperature and strain rate effects compete, when avalanches overlap, and whether finite-size effects dominate over temperature effects. The predictions are tested through simulations of an elastoplastic model in two dimensions and in a mean-field approximation. We find a scaling for temperature-dependent softening in the low-strain rate regime when avalanches do not overlap, and a temperature-dependent Herschel-Bulkley exponent in the high-strain rate regime when avalanches do overlap.

Thermoplastic cohesive fracturing model of thermally-treated granite

Thermoplastic fracturing is a prominent characteristic of thermally-treated granite for temperature-dependent underground geotechnical engineerings , such as geothermal energy exploration and high-level nuclear waste disposal. More specifically, the tension-open of the cohesive crack in the fracture process zone (FPZ) presents a plastic softening feature dependent on temperature. However, understanding granite thermoplastic fracturing remains challenging. This work proposes a thermoplastic cohesive fracturing model of thermally-treated granite, incorporating temperature-dependent softening rule and completely coupled stress-strain-temperature constitutive relation. Several new thermoplastic fracturing properties in the proposed model are determined by further analyses of published fracturing test data (Miao et al., 2020) done on Beishan granite suffering high-temperature treatments (200–500 °C). The initial yield parameter (cohesive tensile strength) decreases linearly with the rising high-temperature treatments. The critical yield parameter (the critical crack opening displacement (COD)) follows a two-stage linear increasing rule with 300 °C as a boundary, similar to the temperature-dependent FPZ length. The plastic modulus, delineating the cohesive crack's softening rule, increases quadratically with the enhanced high-temperature treatment. The temperature sensitivity modulus ( T ), characterizing the contraction/expansion of yield surface with increasing temperatures, decreases from positive to negative with the increasing COD. Both the fracture energy and the accumulated dissipated energy present remarkable nonlinearities with several inflection points , implying the complex temperature-dependent fracture resistance . This proposed model was validated with three-point-bending fracturing tests of thermally-treated Beishan granite, fitting well (coefficient: 88.8%–99.7%) with the experimental cohesive tensile strength , critical COD, and the accumulated dissipated energy. Besides, by analyzing the softening curves of cohesive cracks in thermally-treated granite, the positive-negative change of T was consistent with the contraction-expansion transition of the yield surface. This work provides a modeling approach for temperature-dependent fracturing of rock-like materials. • The thermoplastic softening of FPZ is characterized via a thermoplastic framework. • This work proposes a thermoplastic cohesive fracturing model for rocks. • New model parameters were proposed to characterize temperature-dependent fracturing. • The correlation coefficient between the model and test data reaches 88.8%–99.7%. • This work provides a modeling approach for temperature-dependent fracturing of rock.

Time–temperature superposition for cavitation resistance of metals with nonequilibrium vacancy concentrations

Here, we provide a systematic molecular dynamics (MD) study of the pulse duration and temperature dependence of spall strength in Al and Mg with a nonequilibrium concentration of vacancies. A superconcentration of single vacancies are randomly introduced into an otherwise perfect lattice, and the systems are allowed to evolve for up to 500 nanoseconds as a proxy for the pulse duration of a shock compression wave. Since these systems are nonequilibrium in nature, a time-dependent and temperature-dependent evolution of the microstructure ensues, which leads to a time- and temperature-dependent softening of the spall strength. Here, we report a large parametric study consisting of 196 MD calculations to quantify this softening in Al and Mg. We invoke the notion of time–temperature superposition from the field of viscoelasticity, and show that the results of our 196 MD calculations remarkably collapse onto a single master curve when normalized by an appropriate relaxation timescale. Lastly, a favorable agreement between the MD calculations and experimental measurements of thermal softening of spall strength is reported.

Time and temperature dependent softening of H11 hot-work tool steel and definition of an anisothermal tempering kinetic model

The isothermal softening behaviour of the hot work tool steel Böhler W400 VMR (AISI H11) was investigated as a function of both temperature and time. Two mechanism-based tempering kinetic models were used to describe thermal softening and the suitability of each model was evaluated. Scanning electron microscopy analysis was carried out to represent the evolution of the microstructure during the isothermal softening resistance tests. A new model for hardness change prediction during anisothermal loading of steels was proposed and validated. The analysed material showed thermal softening behaviour, which appeared as time and temperature dependent loss of hardness. Differences in suitability of each tempering kinetic model used for the description of thermal softening were observed. Microstructure evolution during the isothermal softening resistance tests of the analysed material confirms that secondary carbides coarsening is the main cause for thermal softening. A good agreement between experimental and prediction results was obtained with use of the new proposed model for hardness change predictions during anisothermal loading.

Extended Gurson–Tvergaard–Needleman model for damage modeling and control in hot forming

During hot working, internal damage of the workpiece is not only controlled by the stress state, but also by time- and temperature-dependent softening processes such as recovery and recrystallization. These processes may be used to delay or prevent damage initiation and hence to improve part performance. This work proposes an extended Gurson–Tvergaard–Needleman damage model for hot forming which couples softening processes to void formation at inclusions. Based on this model, optimal control problems are solved to find optimal ram speed profiles in hot working reducing damage in the workpiece or reducing process time while controlling damage.

Temperature dependent cyclic mechanical properties of a hot work steel after time and temperature dependent softening

In this paper, the temperature dependent cyclic mechanical properties of the martensitic hot work tool steel 1.2367 after tempering are investigated. To this end, hardness measurements as well as monotonic and cyclic tests at temperatures in the range from room temperature to 650 °C are performed on material tempered for different tempering times and temperatures. To describe the observed time and temperature dependent softening during tempering a kinetic model for the evolution of the mean size of secondary carbides based on Ostwald ripening is developed. Furthermore, mechanism-based as well as phenomenological relations for the cyclic mechanical properties of the Ramberg-Osgood model depending on carbide size and temperature are introduced. A good overall agreement of the measured and the calculated stress-strain hysteresis loops for different temperatures and heat treatments is obtained using the determined material properties of the kinetic and mechanical model.

Large area chemical vapor deposition of monolayer transition metal dichalcogenides and their temperature dependent Raman spectroscopy studies.

We investigate the growth mechanism and temperature dependent Raman spectroscopy of chemical vapor deposited large area monolayer of MoS2, MoSe2, WS2 and WSe2 nanosheets up to 70 μm in lateral size. Further, our temperature dependent Raman spectroscopy investigation shows that softening of Raman modes as temperature increases from 80 K to 593 K is due to the negative temperature coefficient and anharmonicity. The temperature dependent softening modes of chemical vapor deposited monolayers of all TMDCs were explained on the basis of a double resonance phonon process which is more active in an atomically thin sample. This process can also be fundamentally pertinent in other emerging two-dimensional layered and heterostructured materials.

Temperature dependent phonon shifts in few-layer black phosphorus.

Atomically thin two-dimensional (2D) sheets of black phosphorus have attracted much attention due to their potential for future nanoelectronic and photonics device applications. Present investigations deal with the temperature dependent phonon shifts in a few-layer black phosphorus nanosheet sample prepared using micromechanical exfoliation on a 300 nm SiO2/Si substrate. The temperature dependent Raman spectroscopy experiments were carried out on a few-layer black phosphorus sample, which depicts softening of Ag(1), B2g, and Ag(2) modes as temperature increases from 77 to 673 K. The calculated temperature coefficients for Ag(1), B2g, and Ag(2) modes of the few-layer black phosphorus nanosheet sample were observed to be -0.01, -0.013, and -0.014 cm(-1) K(-1), respectively. The temperature dependent softening modes of black phosphorus results were explained on the basis of a double resonance process which is more active in an atomically thin sample. This process can also be fundamentally pertinent in other promising and emerging 2D ultrathin layer and heterostructured materials.

Reactive Blending of Nitrile Butadiene Rubber and In situ Synthesized Thermoplastic Polyurethane‐Urea: Novel Preparation Method and Characterization

AbstractBlends of nitrile butadiene rubber (NBR) and in‐situ synthesized thermoplastic polyurethane‐urea (PUU) are prepared by a reactive blending method. The PUU is synthesized in‐situ from the polyaddition of 4,4′‐diphenylmethane diisocyanate‐terminated prepolymer with Bisaniline‐M during blending with NBR in an internal mixer. The 1H NMR spectroscopic analysis shows up to 93% conversion of Bisaniline‐M to PUU in NBR/PUU blends. The incorporation of PUU improves the tensile characteristic, tear and abrasion resistance however a temperature dependent softening of amorphous PUU phase diminishes the dynamic‐mechanical behavior of blend vulcanizates. Improvement in physical properties of blends is attributed to good interfacial adhesion between dispersed PUU domains and continuous NBR matrix as seen by TEM.

DSC/TG Analysis and Temperature-dependent Softening Characteristics of Thermoplastic Gutta-Percha Obturation Materials

DSC/TG Analysis and Temperature-dependent Softening Characteristics of Thermoplastic Gutta-Percha Obturation Materials

Dramatic softening of the negative thermal expansion material HfW2O8 upon heating through its WO4 orientational order-disorder phase transition

HfW2O8 undergoes a dramatic softening where the average bulk modulus (P = 52–414 MPa) drops from 69 GPa at 298 K to 48 GPa at 430 K as the temperature of the WO4 orientation order-disorder transition is approached. This is accompanied by increasingly negative thermal expansivity (−10 ppm·K−1 to −15 ppm·K−1) and reversible WO4 orientational disordering upon compression in α-HfW2O8. Additionally, α-HfW2O8 becomes elastically softer upon compression at constant temperature. The α→β phase transition temperature decreases by ∼30 K between 52 and 414 MPa. Above this phase transition, no further temperature-dependent softening or pressure-dependent changes in the coefficient of thermal expansion occurred.

Anharmonic suppression of charge density waves in 2H-NbS2

The temperature dependence of the phonon spectrum in the superconducting transition metal dichalcogenide 2H-NbS$_2$ is measured by diffuse and inelastic x-ray scattering. A deep, wide and strongly temperature dependent softening, of the two lowest energy longitudinal phonons bands, appears along the $\mathrm{\Gamma M}$ symmetry line in reciprocal space. In sharp contrast to the iso-electronic compounds 2H-NbSe$_2$, the soft phonons energies are finite, even at very low temperature, and no charge density wave instability occurs, in disagreement with harmonic ab-initio calculations. We show that 2H-NbS$_2$ is at the verge of the charge density wave transition and its occurrence is only suppressed by the large anharmonic effects. Moreover, the anharmonicity and the electron phonon coupling both show a strong in-plane anisotropy.

Vibrational entropy near glass transition in a chalcogenide glass and supercooled liquid

Abstract Raman spectroscopic measurements have been performed on Ge20Se80 glass and supercooled liquid at temperatures ranging between 298 and 500 K. Temperature dependent softening of vibrational mode frequencies has been used in conjunction with the available vibrational density of states data at ambient temperature to estimate the relative contributions of vibrational and configurational entropies across glass transition. Nearly 20% of the additional entropy above glass transition is estimated to be vibrational. Thermal expansion effect on vibrational mode softening is found to be insufficient to account for the anharmonic component of vibrational entropy implying possible coupling between the vibrational and configurational entropies at temperatures above Tg. These results may have important consequences in shaping our understanding of various aspects of glass transition.

Coupling between molecular vibrations and liquid crystalline order parameters.

Specific Raman active modes in two prototype cyanobiphenyl liquid crystals are shown to display a temperature dependent softening proportional to either the nematic or smectic order parameters, while other vibrations (like the C identical withN stretch mode) remain unaltered. This selective coupling between intramolecular vibrations and the liquid crystalline order is related to the intrinsic symmetry of the modes. The method provides a simple, microscopic, noninvasive optical technique with which the liquid crystalline order parameters can be qualitatively mapped out.

Determination of Softening Kinetics in a Material by Measuring the Evolution of Hot Flow Stress

Abstract A new technique for estimating the time and temperature-dependent softening response of a deformed material is proposed. The hot flow stress is measured at various intervals in a quasistatic softening experiment to determinse the kinetics of softening. A major advantage of the proposed technique is that a single specimen can be used to estimate the evolution of softening without having to cool and reheat the specimen for each measurement. High resolution strain measurement apparatus is used in the experiments, as minimizing strain is a critical requirement in property performing the measurement. Test control requires optimal performance from the servohydraulic system which must operate near its threshold for reliable information to be obtained. Experimental results indicate that age hardening response as well as recovery and recrystallization kinetics can be measured using this method.

Ultrasonic velocity and attenuation in a TaH0.51 hydride single crystal

Ultrasonic propagation of longitudinal waves (CL mode) has been investigated as a function of temperature in a single crystal of a TaH0.51 hydride. Stepwise changes of the elastic constant CL and of the attenuation A have been observed in the vicinity of the β ↔ ∊ and ∊ ↔ α phase transitions. These changes occur over narrow temperature ranges corresponding to regions of coexistence of two phases. A relatively small temperature dependent softening is displayed by CL on approaching Tβ→∊ from the low temperature side. At the transition temperatures no divergency has been observed in the attenuation, which appears to originate from domain boundary motions, rather than from the transitions themselves. A determination of the H diffusion coefficient by a permeation method at high temperature supports a view that adiabatic tunneling is an effective mechanism even at temperatures as high as 1173 K.

The anomalous bulk modulus of CeBe 13

Ultrasonic measurements of the elastic constants of CeBe 13 between 0.5 and 350 K show strongly temperature dependent softening of the bulk modulus. This is discussed in terms of a continuous and incomplete γ-α transition of Ce.