Dependence Of Strain Rate Sensitivity Research Articles

On the substructure evolution and its distinct role on dynamic strain aging in a multi-component cobalt base alloy at intermediate temperatures (300 to 700 °C)

Manifestations of dynamic strain aging (DSA) in a solid-solution multi-component cobalt-base alloy was evaluated over wide temperature [25–1000 °C] and strain rate range [10−4–10−1 s−1]. Trends in serration onset strain with temperature together with kinetic analysis revealed the features of Portevin-Le Chatelier effect (PLC) and inverse-PLC. Besides, distinct dependence of strain rate sensitivity on the strain rate jump direction is observed between 10−4 and 10−3 s−1 and >550 °C. Detailed post-deformation characterization was done using transmission electron microscopy. In the initial stages (∼0.05 strain), multipoles were found to be the predominant local obstacles responsible for DSA, and strain accumulation occurs through their unzipping. The substantial presence of widely separated (∼23 nm) extended dislocations indicates the strong dominance of Suzuki segregation. Besides, a continuous evolution of heterogeneous to homogeneous slip vis-à-vis multipole density and their arrangement with temperature is observed. This study provides new insights into substructural evolution and its role on DSA mechanisms in multi-component alloys.

Unravelling the Size‐Dependent Mechanical Properties of Nanocrystalline Face‐Centered‐Cubic Metals: From the Dislocation Point of View

The grain size dependence of strain rate sensitivity, apparent activation volume, and yield stress of nanocrystalline (NC) face‐centered‐cubic metals (FCC) are modeled based on the bow‐out model of single dislocation. It is found that the grain size limitation on the dislocation length gives rise to the grain size effects on the mechanical properties. The model gives predictions that when temperature is at or below room temperature and the strain rate is in the quasistatic range, dislocation‐mediated mechanisms play a dominant role in the plastic deformations of FCC NC metals even for the grain size of several nanometers, which is consistent with known experimental and simulation results. Based on the model, abnormal mechanical phenomena of nanostructured metals, such as the appearance of the inverse Hall–Petch relationship, the existence of the critical grain size, and its strain rate dependence, are well predicted and interpreted.

Deformation and tribological behavior of ductile refractory high-entropy alloys

Refractory high-entropy alloys have attracted widespread attention as potential structural materials for high temperature applications. However, there is limited understanding of their high temperature deformation behavior and tribological response. Here, the temperature dependence of strain rate sensitivity and wear behavior was evaluated for two refractory high-entropy alloys, namely HfNbTiZr and HfNbTaTiZr. HfNbTaTiZr showed higher hardness, strength, and lower strain rate sensitivity compared to HfNbTiZr. The extrinsic strain rate sensitivity of both alloys obtained from micro-pillar compression was higher compared to the intrinsic values from nanoindentation due to the larger surface to volume ratio in micro-pillars leading to annihilation of dislocations and their easier movement. The decrease in strain rate sensitivity with increase in temperature was attributed to activation of both edge and screw dislocations as well as phonon drag controlled screw dislocation motion at high temperatures. Wear rate for both alloys increased with increase in sliding cycle frequency due to frictional heating. Wear rate also increased with increase in temperature from 298 K to 423 K and then decreased with further increase in temperature to 573 K. The underlying mechanism changed from abrasive wear at room temperature to extensive ploughing with fine wear debris at intermediate temperatures and oxidative wear at the higher temperatures.

Opposite grain size dependence of strain rate sensitivity of copper at low vs high strain rates

The grain size dependence of the strain rate sensitivity (SRS) of copper were systematically investigated via tensile deformation at strain rates of ~10−4 s−1 and ~103 s−1. In contrast to the general perception that SRS increases with decreasing grain size at low strain rates in FCC metals, the SRS increases monotonously with grain size under deformation at high strain-rates of ~103 s−1. Analytical formulation based on the Nemat-Nasser-Li (NNL) and Modified-Rusinek-Klepaczko (MRK) models was established to reveal the essential dependence of SRS on grain size, at the change of strain rate: that the opposite dependence of SRS on grain size at low vs high strain rates can be attributed to the transformation of the dominant rate-controlling deformation mechanism from thermal activation at low strain rates to viscous drag at high strain rates. It is demonstrated that the thermal activation component of SRS, m*, which is nearly strain rate independent, increases with reducing grain size; while the viscous drag component of SRS, mvs, which is enhanced significantly at high strain rates, decreases with reducing grain size. Microstructural observation based on local misorientation characterization and statistics confirms that the viscous-drag dominates at high strain-rate deformation, and becomes progressively influential as grain size increases. This unveils the essence of viscous drag in the opposite grain size dependence of SRS at low vs high strain rates.

Crystallographic orientation sensitive measurement of strain rate sensitivity in Zircaloy-2 via synchrotron X-ray diffraction

A series of stress relaxation experiments were performed on samples of rolled Zircaloy-2 plate. In situ synchrotron x-ray diffraction was used to observe changes in interatomic lattice spacing during both deformation and relaxation. The concepts of strain rate sensitivity and activation volume are extended from the macroscopic to the microscopic scale using the results of the diffraction analysis. The technique outlined in this paper allows for the precise measurement of thermally activated properties at the level of subsets of grains sharing similar crystal orientations relative to the loading direction. Self-consistent modelling results are used to connect the observed strain rate sensitivity of these grain families with the rate sensitivities of the various active slip systems in zirconium alloys. The results show that the dependence of strain rate sensitivity on crystal orientation occurs primarily due to the differing strain rate sensitivities of the individual slip systems, which ultimately determines the macroscopic value.

Grain size dependence of strain rate sensitivity in a single phase FCC high entropy alloy Al0.3CoCrFeNi

High-entropy alloys (HEAs) can offer exceptional strain-rate sensitivity (SRS) due to high lattice-friction and strong solid-solution strengthening, and in some cases, low stacking fault energy. In this study, SRS of a single phase FCC HEA- Al0.3CoCrFeNi was investigated at two grain sizes. Due to its outstanding Hall-Petch coefficient, grain size becomes a crucial microstructural feature in determination of strength and SRS. Change in SRS due to grain refinement was derived to be inversely proportional to associated strength gain, in coarse microstructures where grain size >> dislocation forest cell size. This correlation was proven using two microstructures with grain sizes of 12 µm and 150 µm, with yield strengths of 313 MPa and 145 MPa, and demonstrated SRS of m = 0.029 and 0.064, respectively. SRS was also derived to increase linearly with strength contribution from thermal short-range obstacles. The slope reflects the maximum upper limit on SRS possible upon elimination of all other obstacles of long-range nature. This limiting value of SRS was derived to be 0.118 for this HEA.

Temperature dependence of strain rate sensitivity, indentation size effects and pile-up in polycrystalline tungsten from 25 to 950 °C

Elevated temperature nanoindentation measurements were performed on polycrystalline tungsten to 950 °C under high vacuum conditions with very low thermal drift. The temperature dependence of the hardness, elastic modulus, strain rate sensitivity, activation volume and the indentation size effect in hardness were studied. More significant time-dependent deformation was observed from 850 °C. Strain rate sensitivity determined by analysis of indentation creep data increased with temperature. Activation volume reached a peak of ~50 b3 at 750–800 °C. Decreasing activation volume >800 °C was a consequence of the increased strain rate sensitivity. For a bcc metal lattice resistance depends on T/Tc (where Tc, the critical temperature, at which flow stress becomes insensitive to temperature, is 527 °C for W); size effects would be expected scale with this relative temperature. Stronger indentation size effects in hardness were found at elevated temperatures. The influence of the time-dependent deformation on the unloading data was accounted for by a viscoelastic compliance correction. After correction the elastic moduli were to within ~1% of literature values at 750–800 °C and to within 6% at 950 °C. These small remaining differences are consistent with AFM measurements which show that pile-up is significant in these high temperature indentations.

Microstructural dependence of strain rate sensitivity in thermomechanically processed Al0.1CoCrFeNi high entropy alloy

Al0.1CoCrFeNi is a single-phase FCC high entropy alloy (HEA) that promises remarkable work-hardening due to its low stacking fault energy (SFE) resulting in suppression of cross-slip and dynamic recovery. The cast material of low yield strength was cold-worked to enhance strength; and then subjected to recovery and recrystallization treatments to improve ductility. Mechanical responses from standard tensile testing at quasi-static strain rate of 10−3 s−1 were coupled with dynamic deformation from split-Hopkinson pressure bar (SHPB) testing at 103 s−1 to study strain rate sensitivity (SRS) and its microstructural dependence in various conditions generated by thermomechanical processing. While dynamic work-hardening remained high in all microstructural conditions, SRS was highly sensitive to the nature of obstacles in each condition. The cast condition showed a moderate SRS of 0.017, but introduction of dislocation tangles and large deformation twins with cold work rendered a sharp drop in SRS to ~ 0. As the density of these defects is reduced during low-temperature annealing treatments, the recovered microstructures showed SRS recuperating back to original SRS level of 0.017. Higher temperature treatments resulted in partial recrystallization and lower SRS < 0.008 due to additional athermal strength contribution from grain refinement in the recrystallized portions and remnant cold work in unrecrystallized portions. Dynamic work-hardening remained very high at ~ 1600 MPa in all conditions from a combination of dynamic recovery suppression and intense twinning that is inherent to the HEA from its low SFE.

Dependence of Strain Rate Sensitivity on the Slip System: A Molecular Dynamics Simulation

The strain rate of deformation might affect mechanical properties of metallic materials, especially at elevated temperatures. Due to the nature of dislocation slip, it is anticipated that strain rate sensitivity (SRS) depends on the slip system. While the dependency of SRS on the temperature and strain rate of the deformation is well recognized, its dependence on the slip system is not well understood. Accordingly, the molecular dynamics simulation was used to investigate the dependence of strain rate sensitivity of pure Al single crystals on the slip system. In this study, the embedded atom method (EAM) potential for Al was employed. SRS and shear strength of the material were studied in four different slip systems and at two temperatures of 300 and 500 K. It was found that SRS of the material depends on the slip system in addition to the temperature, and SRS was higher in less compact systems with more difficult slip. The dislocation theories were used to rationalize the simulation results.

Size dependent strain rate sensitivity transition from positive to negative in Ti/Ni multilayers

This work presents a length scale dependent strain rate sensitivity (SRS) transition from positive to negative in Ti/Ni multilayers with unequal thickness of individual layer. The multilayers exhibit distinct interfacial structure and no Ti-Ni amorphous intermixed layer between Ni and Ti layers. It can be found that the hardness first increases with strain rate at large modulation period (λ), showing a positive SRS. Subsequently, a tunable (positive to negative) size dependent SRS phenomenon occurs with further decrease of λ. The present negative SRS is attributed to deterioration of the plasticity with decreasing λ and at larger strain rate, which induces microcracks formation.

Texture dependent strain rate sensitivity of ultrafine-grained aluminum films

We investigated the strain rate sensitivity (SRS) of freestanding ultrafine-grained aluminum films with similar thickness (~240nm) and mean grain size (~275nm) but highly dissimilar texture. For (110) textured bicrystalline films, flow stress increased by 14% (m=0.017) as the strain rate was varied from ~7×10−6/s to 5×10−3/s. In contrast, for non-textured films the flow stress increased by more than 90% (m=0.103) over a similar strain rate range. The drastic difference in SRS can be explained by texture-induced changes in deformation mechanisms of the films, as revealed by in situ TEM straining experiments.

Dynamic softening behavior of TC18 titanium alloy during hot deformation

Abstract Dynamic softening behavior of TC18 titanium alloy have been investigated in this paper based on isothermal compression experiment. Strain-rate sensitivity coefficient m and deformation activation energy Qdef under different deformation conditions have been calculated through Arrhenius constitutive model. The dependence of strain-rate sensitivity and activation energy on deformation parameters has been analyzed from the perspective of internal variables. A function of deformation temperature has been proposed to estimate the self-diffusion activation energy Qself for dual-phase titanium alloys. The dependence of DRX/DRV on deformation parameters has been demonstrated by novel methods. From comparisons between self-diffusion and deformation activation energy, deformation undergoing DRX has been identified with low strain rates and elevated temperatures. Finally, microstructure observation has been taken carried out to verify the applicability of the new methods. Meanwhile, Poliak–Jonas criterion and power dissipation efficiency distribution has been employed to discuss the dependence of softening behavior on deformation condition.

Correlation between strain-rate sensitivity and dynamic softening behavior during hot processing

This paper has quantitatively investigated strain-rate sensitivity (SRS) coefficients m for hot processing with different dynamic softening (DS) behavior. Isothermal compression experiment of TC18 titanium alloy has been conducted for verification and relevant sensitivity coefficients have been obtained from Arrhenius constitutive function. Kocks–Mecking model has been employed to estimate SRS coefficients for dynamic recovery (DRV) process and SRS values maintain stable at 0.2. SRS coefficients for dynamic recrystallization (DRX) process have been calculated from JMAK kinetic model and the values have been obtained over 0.3 with obvious sensitivity to parameter variation. Dependence of strain-rate sensitivity on dynamic softening behavior has been demonstrated in comparison with dislocation evolution map. Microstructure evolution incorporating different SRS values has been analyzed under deformation parameter variation. Power dissipation efficiency distribution map for hot deformation of TC18 alloy has been constructed from SRS values in quantized characterization on DS behavior.

Strain-rate sensitivity of glasses

We report on the loading-rate dependence of localized plastic deformation in inorganic covalent, metallic, ionic and superionic glasses. For this, the strain-rate sensitivity is determined through instrumented nanoindentation in a load-controlled strain-rate jump test. Through relating the strain-rate sensitivity to the reduced temperature, the packing density, the network dimensionality and the average single bond strength of the system, a qualitative mechanistic description of the strain-mediating process is possible. A strong variability of strain-rate sensitivity is obtained only at intermediate values of packing density, network connectivity or bond strength, when other parameters such as chemical composition and specific structural arrangement are dominating the deformation process. On the other side, for high bond strength and connectivity or for high packing density, the strain-rate sensitivity of the considered glasses is always low, which is also confirmed through the dependence of strain-rate sensitivity on Poisson ratio. Here, only for glasses with a Poisson ratio of ~0.3–0.4 we observe a wide variability of the loading-rate dependence of local deformation. For higher or lower Poisson ratio, the observed dependence is always low: when the limiting factor in deformation is primarily network connectivity and bond strength or packing density, respectively, once an activation barrier is overcome, deformation is only weakly loading-rate-dependent. This is regardless of the height of the activation barrier. When approaching the glass transition temperature, high strain-rate sensitivity is observed only in glasses where non-Newtonian flow is expected also in the corresponding liquid.

Depth dependent strain rate sensitivity and inverse indentation size effect of hardness in body-centered cubic nanocrystalline metals

Size effects on hardness (H) and strain rate sensitivity (m) of nanocrystalline (NC) body-centered cubic Mo thin film were examined under nanoindentation testing. Contrast to existing reports that there was no indentation size effect on hardness in NC metals, inverse indentation size effect (ISE) in NC Mo was observed for the first time at penetration depths ranging from 15 to 200nm, at all the loading strain rates applied. In addition, the strain rate sensitivity of NC Mo exhibited strong dependence on penetration depth, increasing dramatically with decreasing penetration depth. Surface effects related to two deformation mechanisms were proposed to be responsible for the observed inverse ISE on H and depth dependent m. Specifically, the mobility of screw dislocation/component and the diffusion length of interfacial diffusion were altered as the deformed region underneath the indenter was approaching the free surface, resulting in the unusual size effects in NC Mo.

Dependence of strain rate sensitivity upon deformed microstructures in nanocrystalline Cu

The creep behavior of nanocrystalline copper was experimentally characterized with nanoindentation using two sequential regimes, i.e., loading and holding. Significantly enhanced strain rate sensitivity was found within an unusually narrow range of creep rate in the holding regime, which is attributed to the deformed microstructure generated during the loading regime. By quantitatively analyzing the creep rate and rate sensitivity exponent of NC Cu in the holding regime, both the grain boundary sliding (GBS) and dislocation activities are found to be responsible for the observed abnormal behavior, with the contribution of GBS decreasing with increasing grain size and increasing with decreasing loading strain rate. These findings provide a potential way of adjusting the mechanical properties of nanocrystalline metals by pre-straining.

차체용 강판의 온도에 따른 동적 구성방정식에 관한 연구 (I) - 온도에 따른 동적 물성 특성 -

This paper is concerned with the thermo-mechanical behavior of steel sheet for an auto-body including temperature dependent strain rate sensitivity. In order to identify the temperature-dependent strain rate sensitivity of SPRC35R, SPRC45E and TRIP60, uniaxial tensile tests are performed with the variation of the strain rates from 0.001/sec to 200/sec and the variation of environmental temperatures from ? 40℃ to 200℃. The thermo-mechanical response at the quasi-static state is obtained from the static tensile test and that at the intermediate strain rate is obtained from the high speed tensile test. Experimental results show that the variation of the flow stress and fracture elongation becomes sensitive to the temperature as the strain rate increases. It is observed that the dynamic strain aging occurs with TRIP60 at the temperature above 150℃. Results also indicate that the flow stress and fracture elongation of SPRC35R are more dependent on the changes of strain rates and temperature than those of SPRC45E and TRIP60.

Deformation characteristics of isothermally forged UDIMET 720 nickel-base superalloy

The hot deformation behavior of nickel-base superalloy UDIMET 720 in solution-treated conditions, simulating the forging process of the alloy, was studied using hot compression experiments. Specimens were deformed in the temperature range of 1000 °C to 1175 °C with strain rates of 10−3 to 1 s−1 and total strain of 0.8. Below 1100 °C, all specimens showed flow localization as shear band through the diagonal direction, with more severity at higher strain rates. A uniform deformation was observed when testing between 1100 °C and 1150 °C with dynamic recrystallization as the major flow softening mechanism above 1125 °C. Deformation above γ′ solvus temperature was accompanied with grain boundary separation. The hot working window was determined to be in the interval 1100 °C to 1150 °C. Thermomechanical behavior of the material was modeled using the power-law, the Sellars-Tegart, and an empirical equation. The flow stress values showed a nonlinear dependence of strain rate sensitivity to strain rate. The analysis indicated that the empirical method provides a better constitutive equation for process modeling of this alloy. The apparent activation energy for deformation was calculated and its variations with strain rate and temperature are discussed.

Development and validation of a processing map for AFNOR 7020 aluminium alloy

A simple instability condition is applied to delineate the regions of unstable metal flow in the processing map utilising flow stress data for AFNOR 7020 aluminium alloy, which was fitted in a constitutive equation. Unstable regions identified from the simple instability condition at different strain levels were superimposed in the processing map. This phenomenon takes into account the dependence of strain rate sensitivity and strain hardening effect on plastic instability during hot deformation of the material. The processing map has been developed and validated by comparing microstructural observations of deformed compression specimens.

Interaction between a dislocation and an impurity in KCl: Mg2+ single crystals

The interaction between a dislocation and the impurity in KCl: Mg2+ (0.035 mol% in the melt) was investigated at 77–178 K with respect to the two models: one is the Fleischer's model and the other the Fleischer's model taking account of the Friedel relation. The latter is termed the F-F. The dependence of strain-rate sensitivity due to the impurities on temperature for the specimen was appropriate to the Fleischer's model than the F-F. Furthermore, the activation enthalpy, ΔH, for the Fleischer's model appeared to be nearly proportional to the temperature in comparison with the F-F. The Friedel relation between effective stress and average length of the dislocation segments is exact for most weak obstacles to dislocation motion. However, above-mentioned results mean that the Friedel relation is not suitable for the interaction between a dislocation and the impurity in the specimen. Then, the value of ΔH(Tc) at the Fleischer's model was found to be 0.61 eV. ΔH(Tc) corresponds to the activation enthalpy for overcoming of the strain field around the impurity by a dislocation at 0 K. In addition, the Gibbs free energy, ΔG0, concerning the dislocation motion was determined to be between 0.42 and 0.48 eV on the basis of the following equation