Increase In Strain Rate Sensitivity Research Articles

Prediction Of Superplasticity Of Austenitic Stainless Steel -304 At Hot Working Temperatures

Abstract Austenitic stainless steels are widely used in many industrial, medical, marine and nuclear applications. The use of stainless steel is growing due to the ease of maintenance, excellent corrosion resistance, attractive appearance, ductility, fire resistance and toughness. Anisotropy is usually described with two parameters: as planar anisotropy and as a normal anisotropy. The strain rate sensitivity index plays an important role in characterizing superplastic deformation. In the present work experiments are conducted from room temperature 700°C to 900°C in intervals of 50°C in rolling [R0], perpendicular [R90] and transverse directions [R45] at strain rate in the range of [0.01S to 0.0001s-1]. Using tensile test experimental data anisotropy and strain rate sensitivity index is calculated. Strain rate sensitivity calculated is positive. The positive rate sensitivity shows improvement in forming and has a similar effect on strain hardening. It is shown that with the increase in strain rate sensitivity the amount of elongation in the tensile test piece increases after maximum load, necking and before failure. The normal anisotropy ratio, r - value, is a very important parameter of cold rolled thin sheet metals. It is a constant numeric value which has been used as an indicator of formability of sheet metals by deep drawing and used as property of materials in software for numeric simulation of a plastic forming process. This paper investigates the behavior of austenite stainless steel sheet, based on a sophisticated experiment show significant variations of the r value during the forming process.

Anneal hardening and elevated temperature strain rate sensitivity of nanostructured metals: Their relation to intergranular dislocation accommodation

Nanocrystalline materials exhibit various properties or phenomena not common in the conventional grain size regime, including enhanced strain rate sensitivity for FCC metals or strength increase during recovery annealing. These peculiarities are associated with the enhanced confinement of plasticity. Accordingly, the interaction of dislocations with the numerous grain boundaries, the boundary state as well as its local chemistry are of importance for a complete understanding. Due to the various influencing factors, determination of the dominant and rate controlling processes remains challenging. Here, we present a study on selected nanostructured FCC materials where dislocation-grain boundary interactions have been studied earlier for their coarse grained counterparts. High temperature nanoindentation revealed for all materials a pronounced increase of strain rate sensitivity when exceeding a certain temperature, peaking prior to the occurrence of significant grain growth. Static annealing of samples close to these peak temperatures leads, for sufficiently small grain sizes, to a maximum hardness increase. Interestingly, despite the nanocrystalline grain size, these temperatures perfectly agree with those obtained earlier for annihilation of lattice dislocations at grain boundaries in coarse-grained samples. This suggests that at elevated temperatures the dominant mechanism controlling the enhanced rate sensitivities in nanocrystalline metals is the thermally activated annihilation of lattice dislocations at grain boundaries. Measurements of activation energies being close to reported values for grain boundary diffusion further support this concept.

Fabrication of nanocomposites through diffusion bonding under high-pressure torsion

Abstract

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.

Influence of temperature on the strain rate sensitivity and deformation mechanisms of nanotwinned Cu

The mechanical behavior of nanotwinned Cu was studied through indentation creep and constant strain rate indentation tests from 25 °C to 200 °C. The results showed an enhanced strain rate sensitivity of nanotwinned Cu with temperature, which was higher than that found in coarse-grained Cu. Transmission electron microscopy revealed the same deformation mechanism in the whole temperature range: confined dislocation slip between coherent twin boundaries and formation of dislocation pile-ups at the coherent twin boundaries. The mechanisms responsible for large increase in strain rate sensitivity of nanotwinned Cu with temperature were discussed to the light of experimental observations.

Effect of Strain Rate Sensitivity and Strain Hardening Exponent of Materials on Plastic Strain Distribution and Damage Accumulation During Equal Channel Angular Pressing

Flow pattern and plastic strain distribution inside deformed sample are two important factors governing the microstructural uniformity of equal channel angular pressing (ECAP) processed materials. In the present study, the effects of strain hardening exponent (n) and strain rate sensitivity (m) on the characteristics of the ECAP process were investigated by the aid of finite element simulation. Investigated parameters are plastic strain distribution and accumulated damage. In addition, contour maps were generated to demonstrate mutual effects of n and m coefficients on the uniformity of plastic strain and maximum damage developed on ECAPed samples. Results showed that the effect of m and n values on plastic strain is more pronounced at downside regions of sample compared with upper regions. Also, at a constant strain rate sensitivity, the plastic strain is increased with decreasing strain hardening exponent due to the small corner gap formed in outer region of the intersecting area of two channels. Increasing strain rate sensitivity at constant strain hardening rate is accompanied by the formation of severely deformed area near downside of sample which leads to non-uniform strain distribution.

On the ductility of alpha titanium: The effect of temperature and deformation mode

Single phase α-titanium shows anomalous warm deformation behaviour. As the temperature increases, ductility increases in uniaxial tension and decreases in biaxial stretching. Previously, this behaviour was attributed to an increase in strain rate sensitivity and a decrease in twinning activity with temperature. In this study, we show that it can instead be explained by an increase in slip anisotropy with temperature. Grade 2 CP-Ti sheet was tested in uniaxial tension at 20 °C and 300 °C to determine ductility, work hardening behaviour and the coefficient of plastic anisotropy (R-value). The increase in uniaxial ductility with temperature was found to be a consequence of an increasing rate of saturation work hardening with temperature. In the absence of significant twinning, this unexpected work hardening behaviour was attributed to an increase in slip anisotropy with temperature. This hypothesis was supported by crystal plasticity finite element modelling, which can also predict the observed increase in surface roughness with temperature. The increase in anisotropy leads to higher strain localization which, coupled with the increasing work hardening rate helps explain why biaxial ductility decreases with increasing temperature. In addition to explaining the limitations in warm forming of Ti, understanding the origins of these effects contributes to our general understanding of the deformation of other hexagonal metals like Zr and Mg.

What governs ductility of ultrafine-grained metals? A microstructure based approach to necking instability

We demonstrate that the conditions for plastic instability, which are traditionally obtained from solid mechanics considerations, also follow from linear stability analysis of the intrinsic evolution laws for the dislocation density. However, the strain rate sensitivity effect enters these conditions in a way different to the known Hart criterion. The necking strain predicted from the dislocation-based models shows good agreement with experimental data, highlighting the primary role played by dynamic dislocation recovery in the stability of uniform plastic flow. In particular, a sharp drop of ductility after plastic deformation to modest strains has been accounted for. A result of special interest with regard to ultrafine-grained materials is the predicted increase of their tensile ductility with increased strain imparted to them by prior severe plastic deformation. Importantly, ductility of such materials was found not to be governed primarily by increased strain rate sensitivity of the flow stress, as is commonly assumed in literature. Rather, it emerges as a result of a decrease in the rate of dynamic recovery of dislocations and the history of the pre-processing by severe plastic deformation.

Comparative study of mechanical properties of pure nanocrystalline Ni and Ni-Tf nanocomposite

Mechanical properties of nanocrystalline (nc) Nickel (Ni) and Ni-Teflon (Ni-Tf) nanocomposite has been studied in detail using nanoindentation and conventional tensile tests. Nanoindentation tests carried out at room temperature over a range of loading rate exhibited higher hardness for the nanocomposite as compared to the nc-Ni due to Tf dispersion. An increase in strain rate sensitivity (SRS) and decrease in activation volume of the nanocomposite as compared to nc-Ni was also observed. Tensile tests were carried out over a temperature regime of room temperature (RT) to 500°C. Tensile data also showed enhancement of yield stress and ultimate tensile strength up to 150°C. But from 200°C and above, Tf dispersion is unable to provide superior strength over nc-Ni as both the materials show almost similar strength. However, the ductility of the Ni-Tf nanocomposite was observed to be higher at all temperature compared to the pure nc-Ni. SRS was found to increase with temperature while SRS in nanocomposite had higher value below 200°C. The present study reveals that Ni-Tf nanocomposite possess better strength and ductility compared to nc-Ni up to 200°C.

Evidence for exceptional low temperature ductility in polycrystalline magnesium processed by severe plastic deformation

An investigation was conducted to examine the mechanical behavior and microstructure evolution during deformation of ultrafine-grained pure magnesium at low temperatures within the temperature range of 296–373 K. Discs were processed by high-pressure torsion until saturation in grain refinement. Dynamic hardness testing revealed a gradual increase in strain rate sensitivity up to m ≈ 0.2. High ductility was observed in the ultrafine-grained magnesium including an exceptional elongation of ∼360% in tension at room temperature and stable deformation in micropillar compression. Grain coarsening and an increase in frequency of grain boundaries with misorientations in the range 15°–45° occurred during deformation in tension. The experimental evidence, when combined with an analysis of the deformation behavior, suggests that grain boundary sliding plays a key role in low strain rate deformation of pure magnesium when the grain sizes are at and below ∼5 μm.

Nano-structure evolution of secondary Al3(Sc1−xZrx) particles during superplastic deformation and their effects on deformation mechanism in Al-Zn-Mg alloys

The nano-structure evolution of secondary Al3(Sc1−xZrx) particles during high-strain-rate (0.01 s−1) superplastic deformation at 500 °C was investigated by high resolution transmission electron microscopy. The results show that by increasing the true stains from 0.69 to 2.40, the mean radii of spheroidal secondary Al3(Sc1−xZrx) particles increase from 9.8 ± 3.4 nm to 16.4 ± 6.8 nm, and the lattice misfit value increases from 1.04% to 1.30%. Before deformation, Al3(Sc1−xZrx) nano-particles are completely coherent with Al(α) matrix. As the deformation proceeds, the accumulated sever plastic deformation introduces misfit dislocations at the interface between particles and matrix. Superplastic flow deformation data indicate that with the increase of true strains, the strain rate sensitivity of Al-Zn-Mg alloy decreases from 0.29 to 0.19, and the deformation activation energy increases from 112 to 121 kJ/mol. However, for Al-Zn-Mg alloy with secondary Al3(Sc1−xZrx) nano-particles, the strain rate sensitivity increase from 0.33 to 0.45, and the deformation activation energy decreases from 107 to 84 kJ/mol. Based on the microstructural results and the established constitutive equations at different strains, it can be concluded that at the working hardening stage, two kinds of alloys are both controlled by dislocation viscous glide creep mechanism. At the dynamic softening stage, dislocation creep mechanism operates in Al-Zn-Mg alloy, whereas grain boundary sliding mechanism is dominant in Al-Zn-Mg-Sc-Zr alloy. Secondary Al3(Sc1−xZrx) nano-particles play an important role in accelerating the cooperative grain boundary deformation and only affect dynamic softening deformation mechanism of Al-Zn-Mg alloys.

Atomistic simulations of the effect of embedded hydrogen and helium on the tensile properties of monocrystalline and nanocrystalline tungsten

Uniaxial tensile properties of monocrystalline tungsten (MC-W) and nanocrystalline tungsten (NC-W) with embedded hydrogen and helium atoms have been investigated using molecular dynamics (MD) simulations in the context of radiation damage evolution. Different strain rates have been imposed to investigate the strain rate sensitivity (SRS) of the samples. Results show that the plastic deformation processes of MC-W and NC-W are dominated by different mechanisms, namely dislocation-based for MC-W and grain boundary-based activities for NC-W, respectively. For MC-W, the SRS increases and a transition appears in the deformation mechanism with increasing embedded atom concentration. However, no obvious embedded atom concentration dependence of the SRS has been observed for NC-W. Instead, in the latter case, the embedded atoms facilitate GB sliding and intergranular fracture. Additionally, a strong strain enhanced He cluster growth has been observed. The corresponding underlying mechanisms are discussed.

Hot deformation behaviour and microstructural evolution of biomedical Ti-13Nb-13Zr alloy in high strain rate range

The deformation behaviour and microstructural evolution of biomedical Ti-13Nb-13Zr alloy are investigated at strain rates of 1×103s−1, 2×103s−1 and 3×103s−1 and temperatures of 25°C, 650°C and 800°C using a compressive split-Hopkinson pressure bar (SHPB) system. The results show that the flow stress, strain rate sensitivity and temperature sensitivity increase with increasing strain rate or decreasing temperature. In addition, adiabatic shearing occurs in the specimens deformed at 25°C and 650°C under the maximum strain rate of 3×103s−1. For the specimen deformed at 25°C, the adiabatic shear band results in catastrophic failure. The microstructural observations reveal that single β phase is formed under the highest temperature of 800°C. The volume fraction of α' martensite and the dislocation density increase, but the grain size decreases, as the temperature is reduced or the strain rate increased. The average thickness of the α' laths reduces as the strain rate and temperature increase. The interaction between the grain size, dislocation density and α'martensite volume fraction leads to a significant strengthening effect. The higher flow stress caused by this strengthening effect can be estimated using an empirical model of the form σ=σo+KD−1/2+K'ρ1/2+Ωξ, where K, K'and Ω are equal to 69.82MPaμm1/2, 6.62 MPaμm and 105.4MPa, respectively. The average error in the flow stress estimates is found to be just 2.04% for the temperature and strain rate conditions considered in the present study. Overall, the results presented in this study provide a useful insight into the combined effects of strain rate and temperature on the hot working behaviour and deformability of Ti-13Nb-13Zr alloy. In general, the results confirm that Ti-13Nb-13Zr alloy is well suited to the fabrication of orthopaedic implant devices and structural components in the biomedical and engineering fields, respectively.

Heat Assisted Dieless Drawing Process of Superplastic Metal Microtubes - From Zn22Al to β Titanium Alloys

A heat assisted superplastic dieless drawing process that requires no dies or tools is applied to the drawing of a Zn-22Al and β titanium superplastic alloy for not only circular but also noncircular microtubes such as square, rectangular and noncircular multi core tubes having square inner and rectangular outer cross sections. As a result, the tendency has been to increase the limiting reduction in area with increasing strain rate sensitivity index m value. We successfully fabricate Zn-22Al alloy, AZ31 magnesium, β titanium circular microtubes with outer diameter of 191μm, 890μm and 180μm, respectively. Furthermore, a noncircular micro tube, which has inner square tubes with a 335μm side, and an outer rectangular tube of 533×923μm were fabricated successfully. During the dieless drawing process, the geometrical similarity law in cross section which the tube is drawn while maintaining its initial shape can be satisfied. The smooth surface can be obtained in case of superplastic dieless drawing process without contact situation with dies and tools. Consequently, it is found that the superplastic dieless drawing is effective for the fabrication of circular and noncircular multicore microtubes.

Mechanical response and microstructural evolution of Ti-13Zr-13Nb biomedical alloy under high strain rate load.

This study uses a split-Hopkinson bar to investigate the mechanical response and microstructural evolution of Ti-13Zr-13Nb biomedical alloy under dynamic loading. Dynamic compression tests are performed at room temperature (25°C) and 500°C under strain rates of 900 s - 1, 1900 s - 1 and 2800 s - 1, respectively. It indicates that the mechanical response of Ti-13Zr-13Nb biomedical alloy is very influenced by strain rate and temperature. The plastic flow stress and strain rate sensitivity increase with the increasing strain rate but decrease with increasing temperature. It reveals that the Ti-13Zr-13Nb alloy fail due to the intensive localized shearing by fracture observation. Furthermore, TEM observations reveal that the dislocation density increases with an increasing strain rate, but decrease with increasing temperature.

The influence of grain size and strain rate on the mechanical behavior of pure magnesium

Commercially pure magnesium was processed by severe plastic deformation techniques and thermo-mechanical routes to produce samples with grain sizes in the range from 1.0 to 300 µm. Tensile testing of a fine-grained material revealed a decrease in hardening and an increase in strain rate sensitivity with decreasing testing strain rate from 10?2 to 10?7 s?1. Compression testing of fine-grained samples not only revealed similar trend, but also there was a transition from twinning to slip-dominated flow. Dynamic ultra-microhardness testing showed an increase in strain rate sensitivity with decreasing grain size. The trend of decreasing hardening rate and increasing strain rate sensitivity with decreasing strain rate was not observed in samples with a coarser grain size. Compression testing at different temperatures suggests that a creep mechanism with stress exponent of ~7 and activation energy close to grain boundary diffusion operates at low strain rates and moderate temperatures. A model for a new deformation mechanism is proposed based on the present experimental results and published data.

Thermally Activated Deformation Behavior of ufg-Au: Environmental Issues During Long-Term and High-Temperature Nanoindentation Testing.

For testing time-dependent material properties by nanoindentation, in particular for long-term creep or relaxation experiments, thermal drift influences on the displacement signal are of prime concern. To address this at room and elevated temperatures, we tested fused quartz at various contact depths at room temperature and ultra-fine grained (ufg) Au at various temperatures. We found that the raw data for fused quartz are strongly affected by thermal drift, but corrected by use of dynamic stiffness measurements all the datasets collapse. The situation for the ufg Au shows again that the data are only useful with drift correction, but with this applied it turns out that there is a significant change of elastic and plastic properties when exceeding 200°C, which is also reflected by an increasing strain rate sensitivity.

Tensile Behavior of Polyetheretherketone over a Wide Range of Strain Rates

Polyetheretherketone (PEEK) is used in several engineering applications where it has to bear impact loads. Nevertheless, the tensile behavior has only been studied in the quasi-static range of loading rates. To address the lack of data in the impact strain rate range, the tensile mechanical behavior of PEEK is investigated at room temperature over a large range of strain rates (from 0.001 to 1000/s). The macroscopic volume change is studied under uniaxial tension using digital image correlation (DIC) method, showing a significant dilatation that reaches 16% at a logarithmic axial strain of 40%. The true stress-strain behavior is therefore established based on the measured volume change. Elsewhere, the yield stress shows a significant sensitivity to strain rate. Besides, a new constitutive equation is proposed to take into account the increase in strain rate sensitivity at high strain rates. It assumes an apparent activation volume which decreases as the strain rate increases. The new constitutive equation gives similar results when compared to the Ree-Eyring equation. However, only three material constants are to be identified and are physically interpreted.

Characterization and modeling of the strain rate sensitivity of polyetheretherketone’s compressive yield stress

Semi-crystalline polymers are increasingly used in structural applications where they can withstand dynamic loads. It is then, of highly importance, to measure and model their mechanical behavior over a wide range of strain rates. In this paper, the polyetheretherketone’s yield stress is investigated under quasi-static (0.0001–0.1/s), intermediate (5–500/s) and high (500–10,000/s) strain rates. Four experimental set-ups were used to achieve this task. It was shown that the mechanical behavior is highly sensitive to strain rate. The yield stress at 10,000/s is 115% higher than at 0.0001/s. Moreover, the strain rate sensitivity increases with increasing strain rate. A new three-material-constant constitutive equation is proposed to take into account the increase of strain rate sensitivity at very high strain rates. An identification approach is also developed to consider the influence of the strain rate range. The material constants, of the new constitutive equation and of three constitutive equations available in the literature, are identified. For each equation, we have reported the strain rate range where each model best fits the experimental data. The new model gives the best trade-off of fitting the experimental data with a good accuracy while minimizing the number of material constants.

Strain rate sensitivity of Al-based composites reinforced with MnO2 additions

Fine-grained Al-based composites reinforced with MnO2 particles were manufactured by means of powder metallurgy (PM) and mechanical alloying (MA) methods. It was found that the applied powder consolidation methods, including KOBO extrusion, did not induce any chemical reaction between thermodynamically unstable components. However, it was shown that addition of magnesium to the Al-matrix initiated a reaction in the vicinity of MnO2 particles that resulted in the nucleation and growth of nano-sized aluminum–magnesium oxides. This led to a local refining of structural components. The most intense refining of structural components was observed for the MA Al–MnO2 composite. Strain rate sensitivity (SRS) of as-extruded materials was tested in compression in the range 293–773K. SRS was determined by making a rapid change in the basic true strain rate from ε̇=1.2·10-3 to ε̇=1.2·10-2. It is found that SRS did not practically depend on strain. The highest value of SRS was observed for the PM Al–MnO2–Mg composite. SRS of PM materials evidently increases with deformation temperature; however, it becomes smaller above a temperature of ∼600K. For the MA Al–MnO2 composite, tested at high temperatures, primary mechanical alloying resulted in relatively low increase of SRS with temperature that also becomes smaller above ∼600K. Suppression of the increase in SRS at high temperatures can be attributed to the specific features of grain boundaries created by the adhesive bonding between powder particles. This could hinder grain boundary sliding mechanisms and micro-cracks development at the compound interfaces.