Rate Sensitivity Of Flow Stress Research Articles

Effect of solute distribution on the screw dislocation motion in bcc Fe-based systems

Screw dislocation motion in bcc Fe plays an important role in establishing the strong temperature and strain rate sensitivity of flow stress, which in turn promotes the ductile to brittle transition phenomenon in steel. Addition of solute atoms, possessing attractive interaction with screw dislocations, is believed to reduce the flow stress sensitivity of Fe. The present work intends to study the effect of Ni and Cu solute atom distribution on the screw dislocation motion in bcc Fe using molecular statics (MS) and molecular dynamics (MD) simulations. MS simulation results predict that the local distribution of Ni atoms around the screw dislocation greatly influences the Peierls stress of Fe at 0 K temperature, while Cu atom distribution has marginal effect. Moreover, the presence of Ni/Cu atoms ahead of the dislocation line helps to reduce the Peierls stress due to the dislocation-solute attractive interaction. MD simulation was performed on random Fe-Ni and Fe-Cu alloys to determine the effect of solute concentration on the temperature sensitivity of flow stress of Fe. The results showed that addition of Ni/Cu reduces the flow stress sensitivity of Fe by inducing solid solution softening effect at low temperature regime. Ni was found to be more effective than Cu in reducing the flow stress temperature sensitivity of Fe. The rationale behind the softening action of Ni and Cu is discussed in terms of changing the differential displacement plot and unstable stacking fault energy of Fe.

Fundamental insights on ductile to brittle transition phenomenon in ferritic steel

Ductile to brittle transition (DBT) is a well-known phenomenon responsible for deteriorating the low temperature fracture toughness of ferritic steels. It is unambiguous to state that the transition phenomenon is a direct consequence of the strong temperature and strain rate sensitivity of flow stress, which in turn is regulated by the thermally activated motion of screw dislocation at the atomic scale. Due to very high Peierls stress, screw dislocation motion is accompanied by the nucleation and propagation of kink-pairs along the dislocation line. Thus, the activation energy required for kink-pair nucleation is an essential ingredient for correctly predicting the flow stress sensitivity and hence the ductile to brittle transition temperature (DBTT) of ferritic steel. However, experimental determination of kink-pair activation energy is rather tedious and there exists a strong discrepancy between atomistic simulation results (using Density Functional Theory and empirical potentials). Therefore, the present review is aimed at understanding the mechanism of screw dislocation motion in BCC Fe lattice and comparing the results obtained for kink-pair activation energy through experiments and simulations. The effect of different alloying elements on the overall flow stress sensitivity of Fe is also discussed with reference to the interaction of screw dislocation with solute atoms or clusters. Finally, the importance of screw dislocation on predicting the DBTT is explored to identify the present knowledge gaps and the future research directions.

Negative strain rate sensitivity of yield strength of Al-Si alloy additive-manufactured using laser powder bed fusion

This article reports an unexpected finding of the mechanical behavior of Al-12%Si alloy manufactured using the laser powder bed fusion (L-PBF) process. The as-manufactured specimen has a negative strain rate sensitivity of flow stress at an early stage of tensile deformation (below 3% in strain). The post heat-treated specimens exhibited a positive strain-rate sensitivity and none of these specimens exhibited a serrated stress-strain curve, indicating a mechanism different from the Portevin–LeChatelier effect. Transmission electron microscopy revealed the dynamic precipitation of the nano-sized Si-phase inside the supersaturated solid solution of the α-Al matrix in the as-manufactured specimen deformed at ambient temperatures. The dynamically precipitated Si phase interacted with the introduced dislocations, enhancing the flow stress during the early stages of plastic deformation. The dynamic precipitation could contribute to the negative strain-rate sensitivity of the flow stress at an early stage of tensile deformation (including the yield strength).

Improving the workability of materials during the dieless drawing processes by multi-pass incremental deformation

The paper explores the new method of improving the workability of materials in the dieless drawing processes. The proposed method is based on the implementation of a multi-pass incremental deformation. Moreover, in each pass, strain and strain rate sensitivity of flow stress should be positive and significant. An approach based on the finite element calculation of instability coefficient of plastic deformation and simultaneous modeling of material ductility were applied for prediction of the workability. Two dieless drawing processes have been investigated. The difference was related to the heating system—induction heating and laser heating. FE simulations and experimental tests for three materials, two magnesium alloys (MgCa0.8 and MgNi19) and pure copper were performed. It was shown that the most effective increase in workability by multi-pass deformation can be achieved using laser dieless drawing. This is possible due to the shorter heating area and, as a consequence, the larger strain rate, which leads to better stability of the deformation process.

Strain Rate Sensitivity of Flow Stress Measured by Micropillar Compression Test for Single Crystals of 18Cr Ferritic Stainless Steel

Strain Rate Sensitivity of Flow Stress Measured by Micropillar Compression Test for Single Crystals of 18Cr Ferritic Stainless Steel

Effect of Strain Rate on the Tensile Behavior of CoCrFeNi and CoCrFeMnNi High-Entropy Alloys

High-entropy alloys (HEAs), a novel class of metal alloys, have been receiving increasing attention from the scientific community. HEAs have the potential to be used in critical load-bearing applications in replacement of conventional alloys such as stainless steel and nickel-base superalloys. Tensile experiments at quasi-static to dynamic strain rates (10−4-103 s−1) were performed on two single-phase face-centered cubic HEAs, CoCrFeNi and CoCrFeMnNi. Electron backscatter diffraction was used to study the microstructure of the samples before the experiments, and transmission electron microscopy was performed postmortem. The dominant deformation mechanisms were dislocation slip at quasi-static strain rates with the addition of deformation nano-twins at dynamic strain rates. Ultimate dynamic tensile strength and ductility improved with the increase in strain rate, which can be attributed to the activation of deformation nano-twins in HEAs. CoCrFeNi and CoCrFeMnNi both have low stacking fault energies, which could promote twinning at high strain rates to accommodate plastic deformation. The strain rate sensitivity of the flow stress increased with increasing strain rate, beginning with negligible strain rate sensitivity in the quasi-static range to high strain rate sensitivity in the dynamic range. CoCrFeMnNi showed greater strain rate sensitivity of flow stress. CoCrFeNi, with less configurational entropy, had higher mechanical properties and strain-hardening rates compared to CoCrFeMnNi, which was attributed to the weakening effect of the addition of Mn on the solid solution hardening.

Small-Scale Plastic Deformation of Nanocrystalline High Entropy Alloy.

High entropy alloys (HEAs) have attracted widespread interest due to their unique properties at many different length-scales. Here, we report the fabrication of nanocrystalline (NC) Al0.1CoCrFeNi high entropy alloy and subsequent small-scale plastic deformation behavior via nano-pillar compression tests. Exceptional strength was realized for the NC HEA compared to pure Ni of similar grain sizes. Grain boundary mediated deformation mechanisms led to high strain rate sensitivity of flow stress in the nanocrystalline HEA.

Strain-rate effect on work-hardening behavior in β-type Ti-10Mo-1Fe alloy with TWIP effect

The strain rate effect (2.8 × 10−5–2.8 × 10−1s−1) on the tensile properties and microstructure evolution of a β-type Ti-10Mo-1Fe (wt%) alloy has been investigated. With increasing strain rate, the yield strength increased, while the ultimate tensile strength, total elongation and uniform elongation decreased. It was found that deformation at a lower strain rate led to an enhanced work hardening rate (θ). This is reflected in the decreasing strain rate sensitivity of flow stress, m, with increasing strain. Strain rate sensitivity was positive at a smaller strain level (< 0.13), and decreased to a negative at a larger strain. The strain rate dependence of work-hardening behavior has been investigated and discussed in terms of the microstructure evolution, such as {332}<113> twins and dislocations. Electron Backscattered Diffraction (EBSD) and X-ray diffraction (XRD) analyses revealed lower increasing rates of {332}<113> twins and dislocation density at higher strain rates, which may be caused by adiabatic heating. This may lead to the reduced work-hardening rate on deformation at the higher strain rates.

Anomalous Strain Rate Sensitivity of Flow Stress in Ni3Ge Single Crystals. The role of Point Defects

The paper considers the strain rate sensitivity of flow stress in Ni3Ge single crystals having high-energy antiphase boundaries detected in experiments for the strain rate variation. The concentration of point defects is estimated by the model of plastic deformation of alloys having L12 structure. These point defects generated by plastic deformation, annihilate both mutually and on dislocations. It is shown that at higher temperatures of deformation, the anomalous strain rate sensitivity of the flow stress observed during the strain rate variation, can be caused by the migration of and interaction between the point defects and dislocations.

Analisys of hot deformation behavior of super duplex stainless steel UNS S32760 through processing maps

The need for materials with higher strength and corrosion resistance in corrosive environments, such as in the oil extraction in saline media, has led to the use of super duplex stainless steels in projects such as the Pre-sal. The manufacture of these materials involves the step of thermomechanical processing, whose performance depends on the workability of the material. Processing conditions in which the super duplex stainless steel UNS S32760 can be worked safely and in which the material can fail were investigated in this presentation. The physical simulation was performed by means of hot torsion testing. The tests were performed at temperatures ranging from 900°C to 1200°C and strain rates of 0.01s-1 to 10s-1. The evolution of strain rate sensitivity of flow stress (m) for deformation of 0.5 at all temperatures investigated here was determined. After attaining the values of m for each deformation condition, the values of the power dissipation efficiency (η) were calculated, an instability criterion (ξ) was applied, and processing maps were constructed. Using these maps, the effects of deformation conditions on the power dissipation efficiency and the material plastic instability were discussed. The domains of processing maps, the observed microstructures and the shape of plastic flow stress curves were associated.

Effect of manganese content on hot deformation behaviour of Fe–(20/27)Mn–4Al–0.3C non-magnetic steels

The influence of Mn content on flow behaviour and constitutive equations of Fe–(20/27)Mn–4Al–0.3C austenitic steels was investigated by uniaxial hot compression tests. The recrystallisation fraction was also measured using electron backscattered diffraction. Although dynamic recrystallisation is slightly delayed by the increased Mn content, the peak stress still has a pronounced decline especially during the deformation at relatively low temperature. The reason should be ascribed to the enhanced stacking fault energy, by which workhardening rate before the onset of dynamic recrystallisation is diminished significantly with the reinforced cross-slip process. The hot deformation activation energy is decreased under the condition of higher Mn addition, while the strain rate sensitivity of flow stress almost remains unchanged.

Thermodynamic formulation of a viscoplastic constitutive model capturing unusual loading rate sensitivity

Metallic materials and alloys may exhibit serrated flow, rate insensitivity or negative rate sensitivity in a certain range of strain, strain rate and temperature. This behavior is referred to as dynamic strain aging phenomenon. A rate-dependent nonlinear kinematic hardening model was proposed by Ho (2008) in order to model positive, zero, negative rate sensitivity of flow stress and other rate-dependent inelastic behavior in a consistent way. But it has not been demonstrated that the proposed model can be accommodated by the framework of thermodynamics of irreversible processes. The present contribution thus aims at thoroughly examining the thermodynamic consistency of the constitutive model. The evolution equations for the internal state variables are derived through the introduction of two potentials, the thermodynamic potential and the dissipation potential.

Effects of Cr Content on the Hot Compression Deformation Behavior of Ti5Mo5V3Al-xCr Alloys

The compression deformation behavior of a newly developed Ti5Mo5V3Al-xCr (x = 1, 3, 5, 7, and 9) alloy is investigated at temperatures ranging from 923 to 1173 K and under constant strain rate loadings ranging from 0.001 to 1.0 s−1. Each alloy’s dynamic stress-strain response, temperature sensitivity, strain rate sensitivity, and hot deformation activation energy are discussed. The results show that the temperature sensitivity of flow stress of Ti5Mo5V3Al-xCr alloys decreases with the increase of Cr content and strain rate. The strain rate sensitivity of flow stress of alloys increases with the increase of deformation temperature and which is insensitive to the Cr element content of alloys over the range investigated (1.0-9.0%). The hot deformation activation energy of alloys both in (α+β) phase region and β-phase region is discussed. When the alloys were deformed in (α+β)-phase region, the deformation activation energies of alloys are consistent with the proportion of α-phase in alloys, and which are all decreased with the increase of Cr content. When the alloys were deformed in β-phase region, the deformation activation energies of alloys are associated with the starting grain size prior to deformation, and the activation energy increases with increasing grain size.

Modelling of Strain Rate and Temperature Dependent Flow Stresses of Supercooled Austenite for AISI 4140

The numerical modelling of heat treatment has become an essential tool in understanding distortion potentials for case hardening. When looking at other surface hardening processes such as induction or laser hardening, high heating and cooling rates automatically lead to higher strain rates during the heat treatment cycle. So far, there have been almost no investigations on the strain rate as well as temperature dependency of the mechanical properties of supercooled austenite. In this paper, the typical induction and laser hardening steel AISI 4140 has been used in order to determine the influence of strain rate and temperature on the mechanical behaviour. The experiments are based on tensile tests, using a specifically designed thermo-mechanical simulator. The experimental results show that a positive strain rate sensitivity for strain rates up to 1 s-1 results. Especially in the temperature interval where austenite formation occurs during heating, the strain rate sensitive flow stress might lead to an alteration of the plastic strains in comparison to conventional heat treatments at low heating rates. The material model presented in this paper allows a good reproduction of the experimental data over a wide range of strain rates and temperatures.

Estimation model for magnetic properties of stamped electrical steel sheet

AbstractLess deterioration in the magnetic properties of electrical steel sheets in the process of stamping out the iron core is necessary in order to maintain performance. First, the influence of plastic strain and stress on the magnetic properties is studied using test pieces, in which plastic strain is added uniformly and residual stress is not induced. Because the influence of plastic strain is expressed using the equivalent plastic strain, at each equivalent plastic strain state the influence of the load stress is investigated. Second, the elastic limit is determined to be about 60% of the macroscopic yield point (MYP), and this agrees with the stress limit inducing irreversible deterioration in the magnetic properties. Therefore, simulation models, in which excess elastic limit plastic deformation begins and the magnetic properties deteriorate sharply, are proposed. Additional points taken into consideration in the deformation analysis are the strain rate sensitivity of flow stress, the anisotropy under deformation, and the influence of stress triaxiality on fracture. Finally, the proposed models are shown to be valid because the magnetic properties of rectangular sheets 5 mm wide stamped out from a nonoriented electrical steel sheet (35A250 JIS grade) can be estimated with good accuracy. It is concluded that the elastic limit must be taken into account in both the stamping process simulation and magnetic field calculation. © 2013 Wiley Periodicals, Inc. Electr Eng Jpn, 183(2): 1–11, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22269

Influence of strain rate and temperature on tensile properties and flow behaviour of a reduced activation ferritic–martensitic steel

Tensile strength and flow behaviour of a Reduced Activation Ferritic–Martensitic (RAFM) steel (9Cr–1W–0.06Ta–0.22V–0.08C) have been investigated over a temperature range of 300–873K at different strain rates. Tensile strength of the steel decreased with temperature and increased with strain rate except at intermediate temperatures. Negative strain rate sensitivity of flow stress of the steel at intermediate temperatures revealed the occurrence of dynamic strain ageing in the steel, even though no serrated flow was observed. The tensile flow behaviour of the material was well represented by the Voce strain hardening equation for all the test conditions. Temperature and strain rate dependence of the various parameters of Voce equation were interpreted with the possible deformation mechanisms. The equivalence between the saturation stress at a given strain rate in tensile test and steady state deformation rate at a given stress in creep test was found to be satisfied by the RAFM steel.

Deformation resistance in the transition from coarse-grained to ultrafine-grained Cu by severe plastic deformation up to 24 passes of ECAP

Abstract Pure Cu was subjected to severe plastic predeformation by p = 1, 2, 4, 8, 16 and 24 passes of equal channel angular pressing (ECAP) on route B C at ambient temperature and subsequently tested in uniaxial compression parallel to the extrusion direction at constant rate or constant stress and temperatures from ambient temperature up to 418 K. The maximum compressive strength of the ECAPed Cu varies in a systematic fashion with p , until a steady state is finally reached between p = 8 and 16 where the rate sensitivity of flow stress is maximal. The results are quantitatively interpreted in terms of the boundary structure, considering the superposition of hardening due to refinement of low-angle boundaries and softening due to enhanced thermal recovery at high-angle boundaries. Beyond the maximum the compressive strength declines with strain for relatively low rate and/or elevated temperature of compression. This is explained by dynamic grain coarsening towards the new steady state developing in compression.

電磁鋼板の打抜き加工による磁気特性劣化量の推定技術の開発

Less deterioration in the magnetic properties of electrical steel sheets in the process of stamping out the iron core is necessary in order to maintain performance. First, the influence of plastic strain and stress on the magnetic properties is studied using test pieces, in which plastic strain is added uniformly and residual stress is not induced. Because the influence of plastic strain is expressed using the equivalent plastic strain, at each equivalent plastic strain state the influence of the load stress is investigated. Second, the elastic limit is determined to be about 60% of the macroscopic yield point (MYP), and this agrees with the stress limit inducing irreversible deterioration in the magnetic properties. Therefore, simulation models, in which excess elastic limit plastic deformation begins and the magnetic properties deteriorate sharply, are proposed. Additional points taken into consideration in the deformation analysis are the strain rate sensitivity of flow stress, the anisotropy under deformation, and the influence of stress triaxiality on fracture. Finally, the proposed models are shown to be valid because the magnetic properties of rectangular sheets 5 mm wide stamped out from a nonoriented electrical steel sheet (35A250 JIS grade) can be estimated with good accuracy. It is concluded that the elastic limit must be taken into account in both the stamping process simulation and magnetic field calculation. © 2013 Wiley Periodicals, Inc. Electr Eng Jpn, 183(2): 1–11, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22269

High Strain Rate Properties of High Strength Steel Sheets

High strain rate properties of sheet steel largely depend on its strength level and strengthening mechanisms. The strain rate sensitivity of flow stress has a close relation with the hardness of a softer phase, which indicates superior strain rate sensitivity of multi-phase steels such as DP and TRIP steels. Simple-shear tests have revealed an increase of the flow stress with strain even for ultra high strength steel at large strains. Based on these experimental evidences and physical understandings, an appropriate constitutive model for crash simulation is proposed and discussed.

Deformation behaviour of electrodeposited nanocrystalline Ni with broad grain size distribution

In the present work, nanocrystalline Ni (nc-Ni) with a broad grain size distribution (BGSD) of 5–120 nm and an average grain size of 27·2 nm was prepared. The BGSD nc-Ni sample shows a similar strength and good ductility in comparison with electrodeposited nc-Ni with a narrow grain size distribution. The intracrystalline dislocation network was observed in the post-deformed microstructure confirming the conventional intracrystalline dislocation sliding mechanism in BGSD nc-Ni. The uniaxial tensile loading–unloading–loading deformation shows BGSD nc-Ni has the capability to store dislocations in the grain interior, which is very limited compared with that of coarse grained metals. For BGSD nc-Ni, the strain rate sensitivity of flow stress m enhances with decreasing strain rate. At the strain rate of 5 × 10−6 s−1, m was estimated to be 0·055. At the corresponding strain rate, the enhanced ductility along with the decreased strength was achievable, indicating activation of other deformation mechanisms, e.g. grain boundary sliding or diffusion.