Instantaneous Strain Rate Sensitivity Research Articles

Assessment of activation volume in 304L SS at high temperatures

The variation in apparent and true activation volumes of 304L SS with true plastic strain has been evaluated at three temperatures (973 K, 1073 K and 1123 K) over the strain rates of 3 × 10−3 s−1, 3 × 10−4 s−1 and 3 × 10−5 s−1. The instantaneous strain rate sensitivity parameter was derived from strain-rate jump tests across different strain levels to estimate apparent activation volume. The values of apparent activation volume were higher at 973 K than at other temperatures. In contrast to the apparent activation volume, the estimated values of true activation volume using internal-stress-based kinetic rate law systematically increase with the increasing temperature from 973 K to 1123 K. The higher flow stress with a larger strain hardening rate at 973 K leads to a lower true activation volume in the steel compared to other temperatures. The model outcomes revealed that the instantaneous strain rate jump-up or jump-down leads to the change in the internal and effective stresses. The observed variations in geometrically necessary dislocation density (GND) and intra-grain reference orientation deviation (GROD) in the substructure substantiated the predicted variation in the internal stress level upon strain rate jump up/down. The true activation volume was estimated for a fixed value of mobile dislocation density (ρm = 5 × 1011 m−2) and for varying mobile dislocation density. Irrespective of ρm values, the estimated true activation volume values for 304L SS were in the order of 102 b3 to 103 b3, thereby implying that the intersection of dislocations coupled with the climb of jogged dislocations is the dominant mechanism during tensile deformation at high temperatures. Based on the analysis using the kinetic rate law with a fixed value of mobile dislocation density, it has been realized that the multiplication of effective stress and true activation volume remains constant across the plastic strain levels for the given combination of strain rate and temperature.

Abnormal strain rate strengthening and strain hardening with constitutive modeling in body-centered cubic {332} TWIP titanium alloy

Metastable body-centered cubic titanium alloys exhibiting {332}<113> twinning-induced plasticity (TWIP) have attracted extensive attention owing to their remarkable strain hardenability and unprecedented impact performance. However, the intrinsic correlation between the strain rate, {332}<113> twinning, and flow stress remains to be fully elucidated. In this study, the instantaneous strain rate sensitivity (ISRS) and strain rate sensitivity of strain hardening (SRSS) corresponding to strain rate strengthening and strain hardening, respectively, were systematically clarified in {332}<113> Ti-15Mo alloy exhibiting TWIP effect by conducting strain-rate-jump tests and constitutive modeling analyses. The positive ISRS with an increasing trend correlated well with the change in the dislocation activation volume, which was mainly controlled by local obstacles of interstitial O atoms, and was affected by geometrically necessary dislocations due to twin formation. The negative SRSS with a decreasing trend correlated well with the evolution of deformation microstructures, which was mainly due to the decrease in the twinning rate and an increase in the dislocation annihilation rate. The established constitutive model reasonably described not only the abnormal strain rate strengthening from the thermal activation of dislocation but also the abnormal strain hardening from concurrent back stress strengthening due to twin formation and dislocation strengthening.

Influence of Composition on Nanoindentation Response of Ni-Zr Alloy Thin Films

Variation in composition of an alloy thin film can alter its microstructure, which provides control over its nanomechanical behavior. To explore this idea, we fabricate thin films of Ni-Zr binary alloys with three different compositions and degrees of crystallinity. At low Zr-content, the microstructure is nanocrystalline, which becomes a mixture of amorphous and nanocrystalline phases at intermediate Zr-content. Further, the increase in Zr-content yields a predominantly amorphous film. Nanoindentations of the films reveal negative strain rate sensitivities over the investigated range of composition, although the effect becomes more pronounced with an increase in the Zr-content. Furthermore, the experiments render a closer view of the nanoindentation creep deformation of these Ni-Zr thin films. In particular, we have examined the influence of loading strain rate and composition on the creep compliance and retardation spectra, which provide valuable insight into the timescales associated with the time-dependent relaxation mechanisms. While the decrease in crystallinity mitigates the creep resistance, an increase in the loading strain rate is found to give rise to fast relaxation mechanisms corresponding to relatively smaller timescales. This study also introduces and highlights the prospects of analyzing the instantaneous strain rate sensitivity measured during the nanoindentation creep, which shows temporal features qualitatively analogous to that of the retardation spectra.

Correlation between strain rate sensitivity and characteristics of Portevin-LeChátelier bands in a twinning-induced plasticity steel

The effects of strain rates on the deformation behaviors of a twinning-induced plasticity (TWIP) steel (Fe23Mn0.6C) have been investigated using uniaxial continuous tensile tests at different strain rates and strain-rate jump tests, coupled with digital image correlation (DIC) method. A positive instantaneous strain rate sensitivity (ISRS), a negative strain rate sensitivity of work hardening (SRSW) and a negative overall strain rate sensitivity (SRS) have been revealed in the present TWIP steel. The critical strain for appearance of Portevin-LeChátelier (PLC) bands increases while the height of serrated flow behaviors decreases with increasing strain rate, which indicates that the dynamic strain aging (DSA) effect is suppressed at higher strain rates. The strain concentration in PLC bands is found to be more severe at higher strain rates. Moreover, the appearance/disappearance of PLC band in strain-rate jump tests is found to be highly dependent on the deformation and microstructure evolution histories. Thus, the negative SRSW can be attributed to the higher strain concentration in the PLC bands and the restriction of DSA at higher strain rates, which are related to the suppression of deformation twins at higher strain rates.

A model for the strain rate dependent plasticity of a metastable austenitic stainless steel

A continuum material model is developed for the dynamic plastic deformation behavior of metastable austenitic stainless steel EN 1.4318-2B. An incremental approach in both experimental testing and in the model is used to distinguish between the direct effects of strain rate and the macroscopic adiabatic heating effects. In the model a set of evolution equations is integrated over the deformation path, which makes the model flexible in terms of changes in the strain rate and material temperature. The strain-induced phase transformation from austenite to α′-martensite is accounted for with evolution equations based on the Olson-Cohen transformation model. In order to describe the phase transformation accurately during dynamic loading, the original model is modified by adding instantaneous strain rate sensitivity to the α′-transformation rate. Comparison with experimental results shows that the model can be used to describe the strain rate and temperature dependent behavior of a metastable austenitic alloy with a reasonable number of material parameters. Finally, the model gives realistic results in a set of validation experiments.

Experimental Investigations on the Densification and Deformation Behaviour of Al-TiB2 Composite Preforms

The present investigation dealt with the study on the workability, densification and strain hardening behaviour of sintered aluminium-titanium diboride composite (Al-5%TiB2, Al-10%TiB2) preforms subjected to upsetting process. The workability data and densification mechanisms were essential for the industrial practitioners to design the process and to predict the failure strains. In the present study, Al-TiB2 composite preforms with different aspect ratios and initial preform densities were compressed under triaxial stress state condition. The densification and strain hardening behaviour were analyzed by incrementally loading them till the onset of failure. The effect of the addition of TiB2 content to aluminium and the effect of aspect ratio on formability were discussed in detail. The work hardening exponent, stress ratio parameters, the instantaneous density coefficient, instantaneous strain rate sensitivity and densification attained were studied.

Strain rate sensitivity and evolution of dislocations and twins in a twinning-induced plasticity steel

The present work investigated the effect of strain rates (10−3 to 103s−1) on the deformation behaviour of a twinning-induced plasticity (TWIP) steel. The strain rate sensitivity was studied in terms of instantaneous strain rate sensitivity (ISRS) and strain rate sensitivity of work-hardening (SRSW). While ISRS concerns the instantaneous flow stress change upon strain rate jump, SRSW deals with the subsequent modification in microstructure evolution, i.e. change of work-hardening rate. The present TWIP steel demonstrates a positive ISRS which remains stable during deformation and a negative SRSW, i.e. lower work-hardening rate at higher strain rate. Synchrotron X-ray diffraction experiments indicate that the negative SRSW should be attributed to the suppression of dislocations and deformation twins at high strain rate. This unexpected finding is different to conventional face-centred cubic (fcc) metals which generally show enhanced work-hardening rate at higher strain rate. A constitutive model which is strain rate- and temperature-dependent is developed to explain the stable ISRS and the negative SRSW. The modelling results reveal that the stable ISRS should be attributed to the thermally-activated dislocation motion dominated by interstitial carbon atoms and the negative SRSW should be due to the suppression of the dislocations and deformation twins caused by the adiabatic heating associated with high strain rate deformation.

Instantaneous strain rate sensitivity of wrought AZ31 magnesium alloy

The strain rate sensitivity evolution of a wrought AZ31 magnesium alloy was studied using strain rate jump (SRJ) testing technique. The strain rate jump tests were conducted in a wide temperature range of 200–550°C under the base strain rates of 0.01 and 0.001s−1. The material exhibited the negligible strain-rate sensitivity at relatively low temperatures. However, the rate sensitivity of the alloy was found to be increased with increasing deformation temperature due to thermally activated nature of dislocation motion. In addition, the results indicated a rate sensitivity drop above 500°C. This ascending/descending trend in positive rate sensitivity domain is systematically discussed relying on the main mechanisms operating in the related deformation conditions. Besides the effect of deformation temperature on the strain rate sensitivity, the influences of strain and strain rate are also properly addressed through this manuscript.

Load Relaxation Behavior of Metastable Austenitic Stainless Steels at Quasi-Static Strain Rates

Instantaneous strain rate sensitivity of metastable austenitic stainless steel EN 1.4318 was studied at quasi-static strain rates (2 × 10−4 ... 10−1 s−1) using load relaxation tests carried out at room temperature. The results indicate that at the studied deformation conditions the instantaneous strain rate sensitivity of the test material decreases with decreasing strain rate but increases with increasing plastic strain. Rapid strain aging was also observed to occur at the studied deformation conditions.

Deformation Behavior of Semisolid A356 Alloy Prepared by Low Temperature Pouring

The deformation behaviors of A356 alloy with various initial grain structures, from a very coarse dendrite to a fine equiaxed structure, prepared at different pouring temperatures, from 625°C to 725°C, were investigated. Cylindrical specimens with 12 mm in diameter and 10 mm in height were compressed in the semisolid state to a height reduction of 6 mm at an initial strain rate of 1.39 × 10−3/s and then jumped to 1.39 × 10−2/s. The instantaneous strain rate sensitivity was calculated under iso-structure conditions. Three very different deformation behaviors have been observed, which are believed to have resulted from their different starting microstructures. The materials produced at low pouring temperatures evolve into a fine globular structure after isothermal holding at semisolid region and shows a very low compression stress, while the materials cast at high pouring temperatures remain as coarse and dendritic in its microstructure and have much higher compression stress. The macrostructural breakage and microstructural segregation of hot compressed samples were analyzed, and the deformation mechanisms were discussed. The low temperature poured material is not sensitive to an instantaneous change in strain rate, while the high temperature poured material is very sensitive.

Some aspects on plastic deformation of copper and copper–titanium carbide powder metallurgy composite preforms during cold upsetting

The densification, workability and strain hardening behaviour of sintered copper and Cu–7.5%TiC powder metallurgy (P/M) composite preforms during cold upsetting were investigated by the constitutive model using the experimental data. Cold upsetting of copper and Cu–7.5% TiC composite preforms having different aspect ratios were carried out and the formability behaviour of the preforms under triaxial stress state was determined. The mechanisms most likely involved in the constitutive model, namely, densification and strain hardening were studied. The effects of aspect ratio and addition of titanium carbide to copper on the formability behaviour and various constants involved in the constitutive model, namely, instantaneous density coefficient, instantaneous strain hardening index, instantaneous strain rate sensitivity and instantaneous strength coefficient were discussed in detail.

Effect of geometric work-hardening and matrix work-hardening on new constitutive relationship for aluminium–alumina P/M composite during cold upsetting

The densification and strain hardening behaviour during cold deformation of sintered aluminum–3.5% alumina powder metallurgy preforms were investigated by constitutive model and experimental data obtained with no and with subsequent annealing. The mechanisms most likely involved in the constitutive model, namely, densification and strain hardening were studied. The effect of geometric and matrix work hardening on the various constants involved in the constitutive model, namely, instantaneous density coefficient, instantaneous strain hardening index, instantaneous strain rate sensitivity and instantaneous strength coefficient were discussed in detail.

Study of the superplasticity of inconel 718 using tests with cyclic perturbation of strain rate

The imposition of a sinusoidal perturbation of variable frequency to the strain rate of a material subjected to a monotonic tensile test is an easy technique to study the kinetics of plastic deformation that has been scarcely used. The application of this technique to an Inconel 718 in its superplastic regime allows for obtaining has permitted to obtain the value of the instantaneous strain rate sensitivity, m, in a continuous way as deformation. It is then possible a more complete characterization than that offered by conventional techniques, where strain rate sensitivity is obtained from the response to several instantaneous strain rate changes. The proposed method has revealed the influence that the time derivative of strain rate, e, has on the value of m. For a comparison of the rexults of both types of measurement, tests with sinusoidal perturbation and with instantaneous strain rate changes have been performed. The temperature and strain rate ranges studied span from 925 C to 1000 C and from 10 -4 s -1 to 10 -2 s -1 . The dynamic response of the system has been analyzed too in terms of amplitude and phase shift of the resulting stress signal response to the imposed sinusoidal perturbation of strain rate.

高ひずみ速度域におけるアルミニウム合金6061-0材の変形応力のひずみ速度依存性(S11-1 材料・構造の衝撃工学(1),S11 材料・構造の衝撃工学)

Strain rate sensitivity in FCC metals is known to increase dramatically when the strain rate exceeds about 5×10^3/s. In order to evaluate the strain rate sensitivity of a dynamic flow stress of aluminium alloy at very high strain rates, constant strain rate tests and strain rate reduction tests are conducted for AA6061-O in a strain rate range from about 1×10^3/s to 3×10^4/s. The flow stress increases linearly with increasing strain rate itself and the instantaneous strain rate plays a dominant role in the flow stress at very high strain rates. An effect of alloying is to increase an internal stress and to decrease somewhat a instantaneous strain rate sensitivity.

Superplastic behaviour of commercial duplex stainless steel SAF 2304

The superplasticity of duplex stainless steel SAF 2304 was investigated at temperatures from 900 to 1050°C and strain rates from 10-4 to 10-2 s-1 using uniaxial tensile tests. Within the range of temperatures and strain rates at which superplasticity occurred, the flow stress was under 20 MPa, and the instantaneous strain rate sensitivity was between 0·45 and 0·75. The microstructural evolution was investigated using optical, scanning electron, and transmission electron microscopes. During superplastic deformation, the initially banded two phase structure progressively broke up and evolved into a homogeneously distributed structure. Also, the strain rate sensitivity index initially rose with strain. Two possible models were considered for this behaviour, one based on slip, and the other a more conventional grain boundary sliding/grain switching model.

Mechanical behavior of alloys with equiaxed microstructure in the semisolid state at high solid content

The mechanical response in compression of A2014 and Al–4 wt% Cu alloys with equiaxed microstructure in the semisolid state at high volume fraction of solid (>0.6) is studied. When the amount of liquid is low, deformation in compression is highly non-uniform due to strain localization. In the range of conditions examined no phase segregation was detected. The stress–strain curve exhibits a peak at low strain, around 5–10%, and then decreases to a plateau value. Secondary phase particles in A2014 that remain solid in the semisolid range of the alloy are responsible for the lower peak and higher plateau stress compared to Al–4 wt% Cu alloy. The effect of the grain size for the alloys tested was small. Strain rate jump experiments show that the instantaneous strain rate sensitivity in the semisolid state is similar to that of the solid phase. As the volume fraction of solid decreases, the plateau stress becomes rate insensitive to the strain rate jump. The results allow the relative importance to be assessed of various mechanisms responsible for resistance to flow of semisolid alloys with equiaxed microstructure in compression.

Temperature dependence of the strain-rate sensitivity of persistent slip bands in copper single crystals

Abstract Copper single crystals were fatigued until saturation at different temperatures T between 77 and 430 K using an amplitude of the applied resolved plastic shear strain appropriate to establish persistent slip bands (PSBs) in about 40% of the crystal volume. Strain-rate changes were performed in order to measure the instantaneous strain-rate sensitivity S of the temperature-specific PSB structures at constant microstructure. For 77K ≤ T ≤ 190 K, S was found to be independent of the temperature, while S increases in proportion to T− for 190 K < T ≤ 430 K. The temperature dependence of the corresponding activation volumes indicates that cross-slip of screw dislocations is not the rate-controlling mechanism. It is proposed that the cutting of forest dislocations determines the strain-rate sensitivity.

The ductility of a HSLA steel at temperatures below 300 K

High Strength Low Alloy (HSLA) steel exhibits good ductility at low temperatures down to 77 K in both ferrite-pearlite and dual-phase structures. The instantaneous strain-rate sensitivity (m) increases with decreasing temperature and reaches a maximum at 100 K. This peak temperature is much lower than that for plain carbon steels. The post-uniform elongation, which is more sensitively dependent onm, shows only slight increase around 100 K. The effect ofm on the post-uniform elongation is probably mitigated by the increasing dependence ofm on strain rate above 180 K. The uniform elongation, normally associated with the strain hardening characteristics of the material, also shows some increase around 100 K. Therefore, both strain-hardening and strain-rate hardening are responsible for the enhanced ductility of HSLA steel around 100 K.