Change In Strain Rate Sensitivity Research Articles

Study on the Flow Behavior of 5052 Aluminum Alloy over a Wide Strain-Rate Range with a Constitutive Model Based on the Arrhenius Model Extension

The formability at room temperature and low speed limits the application of aluminum alloy, while high strain rates positively improve the formability of materials. The constitutive behaviors of materials under high strain rates or impact loadings are significantly different from those under quasi-static conditions, while few constitutive models consider the effect of the mobile dislocation and forest dislocation evolution on the dynamic strain aging (DSA) over a wide strain-rate range. The 5052 aluminum alloy, of which the primary source of strain-hardening is dislocation–dislocation interaction, is widely used in manufacturing automotive covering parts and is considered one of the most promising alloys. Therefore, this study conducts uniaxial tensile tests on AA5052-O under conditions of temperatures ranging from 293 K to 473 K and strain rates ranging from 0.001 s−1 to 3000 s−1, and compares the stress–strain relationships of AA5052-O under different conditions to illustrate the constitutive relationship affected by the dislocation evolution over a wide strain-rate range. The Arrhenius model based on the thermal activation mechanism is modified and extended by considering the effects of dynamic strain aging (DSA), drag stress, and the evolution of mobile dislocation and forest dislocation. Thus, a new physics-based constitutive model for AA5052-O is proposed, which can well reflect the change in strain-rate sensitivity with the strain rate increasing. The mobile dislocation density and total dislocation density are predicted with a modified Kubin–Estrin (KE) model, and the influences of variable mobile dislocation on DSA and dislocation drag are discussed as well. In order to verify the reliability of the new constitutive model, the dislocation densities of the specimens before and after deformation are obtained with TEM and XRD, which are in good agreement with the predicted values. This study also compares the newly proposed model with classic constitutive models using multiple statistical evaluation methods, which shows that the new physics-based constitutive model has not only more clear physical meanings for its parameters but also has a higher prediction accuracy.

Plasticity of solid solution Al-Mn and Al-Mn-Si alloys

The influence of Mn and Si on the mechanical properties of solid solution Al-Mn and Al-Mn-Si alloys have been studied at 4 K, 78 K, and 298 K. The solute strengthening potency of the yield strength increases with decreasing temperature. The addition of Si stimulates the precipitation of Mn-containing dispersoids and decreases the solubility of Mn in the Al matrix, reducing the strengthening effect. The alloys exhibit Stage III of declining work hardening rate with the flow stress during the plastic flow at three temperatures. The mean slip length Λ in the work hardening model of Nabarro, Basinski, and Holt (NBH) evaluated from σΘ curves correlates with the scale of the microstructure. The strain rate sensitivity (SRS) measurements conducted at 78 K show positive SRS of Al and the alloys. At 298 K, one observes a change in SRS due to the change of the mechanism from solute-dislocation to dislocation-dislocation dominated interactions. Mn and Si decrease SRS at 298 K. Al-Mn-Si alloys exhibit negative SRS and jerk flow at a sufficiently low strain rate at 298 K. Analysis of the apparent activation volume, distance, and work has been conducted at 78 K and 298 K to understand the rate-controlling mechanisms. The results indicate that dislocation intersections are rate-limiting processes, but the atomistic mechanism of dislocation cutting varies vastly depending on the temperature and alloy composition. It is suggested that at 78 K, forest obstacles are passed by the formation of jogs and sequential activation of individual partials without making constriction to the perfect defect. At 298 K, the cutting involves constriction, the creation of jogs, and the creation of point defects at each intersection site. The calculated activation work and distance provide signatures of these processes.

Micro-mechanisms of deformation and strengthening during high pressure torsion of CoCuFeMnNi high entropy alloy

The equiatomic FCC CoCuFeMnNi HEA subjected to high pressure torsion (HPT) till 5 turns, showed a fourfold increase in hardness and three orders of magnitude decrease in grain size with an insignificant change in strain rate sensitivity and activation volume. There is a gradual transition from planar slip at low strain to twinning followed by shear banding at high strain accompanied by conversion of low angle to high angle grain boundaries, resulting in average grain size of 55 ± 33 nm at the periphery of 5 turn HPT sample. Atom probe tomography revealed that nano-clusters present in FCC matrix in the homogenized sample underwent partial dissolution during HPT to increase the copper concentration from 8.24 ± 3.29 to 17.02 ± 1.51 atomic%, contributing to significant solid solution strengthening in addition to dislocation and grain size strengthening, leading to tremendous increase in hardness (539 to 1941 MPa). Transient instrumented micro and nano-indentation tests yield activation volume between 12 and 17 b3 for the homogenized and HPT processed samples with three orders of magnitude difference in grain size and dislocation density. This indicates that the dislocation–solute environment interaction is the rate controlling mechanism during the deformation of the investigated HEA. The present work provides a unique pathway to design high entropy alloys that can explore and utilize non-equilibrium solid solution strengthening as a major strengthening mechanism to achieve high strength.

Exploring the high-temperature deformation behavior of monocrystalline silicon – An advanced nanoindentation study

Knowing the mechanical behavior of silicon at elevated temperatures is crucial for many high-temperature applications and processes. This work aims to expand the knowledge of these properties, especially on length scales relevant to miniaturized silicon structures. Therefore, nanoindentation experiments were performed on two differently doped (1 0 0) silicon wafers with varying oxygen content, from room temperature up to 950 °C. Residual impressions were microscopically characterized via confocal laser scanning microscopy. From this dataset covering an exceptionally large temperature range the elastic modulus and hardness as well as strain rate sensitivity, activation volume, and energy were evaluated. Generally, a change in deformation behavior can be identified between 300 °C and 400 °C. Above this transition temperature, the hardness drops exponentially. Also, the material becomes strain rate sensitive. These observations support the assumption that the deformation mechanism shifts from high-pressure phase transformation to dislocation-controlled plasticity with increasing temperature. Furthermore, a change in strain rate sensitivity, activation energy, and activation volume above 800 °C implies a further shift in the rate-controlling mechanism of dislocation motion. Lastly, the more strongly doped sample with the higher oxygen content showing higher mechanical strength is discussed regarding solid-solution strengthening and dislocation locking by oxygen atoms.

Strain rate sensitivity and activation volume of AISI 321 stainless steel under dynamic impact loading: Grain size effect

The rapid change in strain rate sensitivity (SRS) of metals at a critical true strain rate (ε̇Tc) is commonly attributed to change in rate-controlling deformation mechanism from thermally-activated process to viscous drag. In this study, metastable AISI 321 stainless steel with grain sizes of 0.24 μm (ultrafine [G1]), 3 μm (fine [G2]) and 37 μm (coarse [G3]) was deformed at high strain-rate. A change in SRS with increasing ε̇T is observed only in G2 and G3 and it occurs at ε̇Tc of ~5900 and 6800s−1, respectively; suggesting that grain size have an effect on the switch of rate-controlling deformation mechanisms. While the SRS of G1 is 0.101 and unchanged, those of G2 and G3 change from 0.094 to 0.326 and 0.091 to 0.634, respectively, once ε̇Tc is reached. Similarly, the estimated activation volume in G1 specimen is ~1.57b3, while those of G2 and G3 specimens change from ~2.95b3 to ~0.65b3 and from ~4.10b3 to ~0.45b3, respectively. Analysis of the deformed specimens using XRD, EBSD and TEM techniques revealed that the complexities in the activated deformation mechanisms in G2 and G3 specimens led to a change in ε̇Tc. Constant SRS in G1 specimen indicated no change in the operational deformation mechanisms.

Modelling of the mechanical response of Zr–Nb and Ti–Nb alloys in a wide temperature range

This article presents the results of modeling the mechanical behavior of Zr–Nb and Ti–Nb alloys in a range of strain rates from 0.001 to 1000 1/s and temperature range 297–1273 K. A modification of constitutive equations describing the mechanical response of fine-grained and coarse-grained Zr–1Nb and Ti–13Nb–13Zr alloys in a wide temperature range is proposed. It was shown that the phase transition between the hexagonal closed packed and body-centered cubic crystal structure at elevated temperatures leads to a sharp change in strain rate sensitivity of the yield strength of Zr–Nb and Ti–Nb alloys. The proposed modifications of constitutive equations make it possible to describe the strain hardening and the strain rate sensitivity of the plastic flow stress over a wide temperature range in the coarse-crystalline and ultrafine-grained Zr–Nb and Ti–Nb alloys. The results can be used for engineering analysis of structural elements of technical systems and design of manufacturing technologies for biomedical products.

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.

The Effect of Electric Current and Strain Rate on Serrated Flow of Sheet Aluminum Alloy 5754

Electrically assisted tensile tests are carried out on sheet aluminum alloy AA5754 at electric current densities ranging from 0 to 30.4 A/mm2 and strain rates ranging from 10−3 to 10−1 s−1. The strain rate sensitivity and the serrated flow behavior are investigated in accordance with dynamic strain aging mechanism. The strain rate sensitivity changes from negative to positive and keeps increasing with current density. The tendency toward serrated flow is characterized by the onset of Portevin-Le Chatelier (PLC) instabilities, which are influenced by strain rate, temperature, and electric current. The evolutions of three types of serrated flow are observed and analyzed with respect to strain rate and current density. The magnitude of serration varies with strain rate and current density. The serrated flow can be suppressed by a high strain rate, a high temperature, or a strong electric current. The threshold values of these parameters are determined and discussed. Conventional oven-heated tensile tests are conducted to distinguish the electroplasticity. The flow stress reduces more in electrically assisted tension compared to oven-heated tension at the same temperature level. The electric current helps suppress the serrated flow at the similar temperature level of oven-heating.

Rubber Structural Coupon Behaviour Study under Pressure Impulse

This study focuses on the rubber material behaviour assessment under dynamic loading using numerical methods. Consequently, dynamic simulations of the rubber structural coupon subjected to dynamic velocity loading were performed using the explicit integration procedure with central difference scheme with modified time integration of the equation of motion implementation. During investigations two impulse velocities were used and compared for two different constitutive materials: Mooney-Rivlin without rate-dependency and Mat 181 Simplified Rubber which includes strain rate effects. From the obtained results it was noticed that material behaviour in both cases is different and along with different values of velocity the strain rate sensitivity changes.

Nanoindentation strain rate sensitivity of thermo-oxidized PMR-15 polyimide

This article reports the study of rate-dependent mechanical properties of thermal-oxidized PMR-15 polyimide resins with a nanoindenter. A series of PMR-15 resin specimens have been isothermally aged at various temperatures, times, and pressures. The strain rate sensitivity of oxidized surface layer obtained at each aging condition has been determined from nanoindentation creep experiments using constant displacement-rate \( \left( {\dot{h}} \right) \) method. Results show that the average strain rate sensitivity in the oxidized surface layer is notably higher than that in the unoxidized interior, indicating that the oxidized surface layer has limited ductility and thus is susceptible to fracture. Effects of aging environments (time, temperature, and pressure) on mechanical properties are also examined. After passing the initial oxidation stage, the change in strain rate sensitivity become insignificant and less sensitive to aging parameters.

Compressive behaviour of ultrafine-grained AA6063T6 over a wide range of strains and strain rates

Abstract The flow stress behaviour of commercially available AA6063-AlMg0.5Si0.4 with ultrafine-grain size is investigated over a wide range of strain and strain rates under uniaxial compression. The ultrafine-grained microstructure is achieved by equal channel angular extrusion and characterised by grain sizes well below 1 μm. Results of quasi-static loading show that the ultrafine-grained states behave in an elastic – nearly perfect plastic manner with significantly reduced strain hardening capacity. When compared to the coarse grained counterpart, no change in strain rate sensitivity was measured for the material after two extrusions. In contrast to that, after eight extrusions the material shows significantly increased strain rate sensitivity especially in the range of highest rates above 102 s−1.

Molecular-dynamics study of ductile and brittle fracture in model noncrystalline solids

Molecular-dynamics simulations of fracture in systems akin to metallic glasses are observed to undergo embrittlement due to a small change in interatomic potential. This change in fracture toughness, however, is not accompanied by a corresponding change in flow stress. Theories of brittle fracture proposed by Freund and Hutchinson indicate that strain rate sensitivity is the controlling physical parameter in these cases. A recent theory of viscoplasticity in this class of solids by Falk and Langer further suggests that the change in strain rate sensitivity corresponds to a change in the susceptibility of local shear transformation zones to applied shear stresses. A simple model of these zones is developed in order to quantify the dependence of this sensitivity on the interparticle potential.

Theory of the effect of dynamic strain aging on mechanical properties

Dynamic strain aging (DSA) in solid solutions is described as diffusion of solute atoms to mobile dislocations, temporarily arrested at obstacles. As a consequence the solute concentration experienced locally by the dislocations depends on the time of arrest and the solute diffusion coefficient. Starting from this idea the dependence of the flow stress on strain rate, temperature, and strain in the presence of DSA is calculated. A criterion for the onset of serrated yielding is formulated. Finally the change in strain rate sensitivity due to DSA is calculated. The theory allows a qualitative and in some respects a quantitative comparison with experimental results of which some examples are given.