Enhanced Strain Rate Sensitivity Research Articles

Dynamic deformation mechanism in submicro-laminated copper with interlamellar graphene multilayers

The plastic deformation of metals is usually sensitive to loading rates, and such rate-dependent behaviors are influenced by various factors, among which inclusions especially nanoinclusions such as graphene and carbon nanotubes were found to have dramatic influence on the deformation mechanisms of metallic materials. However, relative studies are basically focused in quasi-static regimes. Here, the full spectrum of deformation behaviors of graphene/copper (Gr/Cu) submicro-laminated composites at wide-range strain rates (from 10−4 /s to ≥108 /s) and various temperatures (room temperature and -70 °C) are provided, and relevant mechanisms, especially when deformed at high strain rates and low temperatures, are studied elaborately. Unlike conventional copper materials whose strain rate sensitivity drops dramatically (∼50%) with decreasing temperature, Gr/Cu submicro-laminates has an enhanced strain rate sensitivity that decreases limitedly (∼10%) with temperature. Microstructural analysis revealed that a graphene-assisted deformation mechanism dominated the dynamic compression of Gr/Cu composites, where graphene inclusions act as lubricants that enhanced the deformation ability of the submicro-laminates. Further analysis on copper matrix revealed that dislocation dominated deformation switch to twinning dominated deformation from high to extreme strain rates loading, and extreme loading conditions as well as graphene inclusions are prerequisites for high-density deformation twins in Gr/Cu submicro-laminates.

Solid solution hardening effects on structure evolution and mechanical properties of nanostructured binary and high entropy alloys after high pressure torsion

Two different alloy series (Cu-X, Ni-X) have been selected to investigate the effects of solutes on the saturation grain size, the thermal stability and mechanical properties after high pressure torsion. The results of the Cu-X series indicate that the saturation grain size does not correlate with the stacking fault energy but shows good agreement with solid solution hardening according to the Labusch model. This correlation does not only hold for binaries, but also for chemically complex high entropy alloys (Ni-X) in the form of (CrMnFeCo)xNi1-x, where the Varvenne model is used to describe solid solution hardening. The alloy series exhibit a grain size in the range of 50 – 425 nm after high pressure torsion and the solutes increase the strength as well as the thermal stability of the alloys after annealing. The nanostructured alloys exhibit an enhanced strain rate sensitivity exponent, as determined from nanoindentation strain rate jump and constant contact pressure creep testing, whereas an enhanced rate sensitivity is found at low strain rates. The relatively lower rate sensitivity of the alloys as well as their higher thermal stability indicate, that defect storage and annihilation is strongly influenced by a complex interaction of solutes, dislocations and grain boundaries.

Thermo-viscoplastic behavior of DP800 steel at quasi-static, intermediate, high and ultra-high strain rates

Using the shear-compression specimen (SCS), thermo-viscoplastic behavior of DP800 steel has been investigated systematically over a wide range of strain rates, including quasi-static (0.001–1 s−1), intermediate (10–100 s−1), high (2000–8000 s−1) and ultra-high (16,000–50,000 s−1) regions. Temperatures between 293 and 1073 K were considered. Experimental results show: (1) DP800 steel exhibits non-linear strain rate sensitivity. For tests at strain rates lower than 4000 s−1, the flow stress increases slightly. However, for tests at strain rates above 8000 s−1, a significant flow stress upturn exists; (2) dynamic strain aging (DSA) occurs within the temperature range of 473–873 K and strain rates below 100 s−1; (3) both strain rate sensitivity and temperature sensitivity of DP800 steel are influenced by DSA, and bell-shaped sensitivity curves form; (4) with increasing strain rates, peaks of the bell-shaped temperature sensitivity curves move to higher temperature regions, and this law is equally valid for the effect of temperature on the strain rate sensitivity curves; (5) strain hardening behavior of DP800 steel is dominated by a competition between strain rate hardening, DSA hardening and thermal softening. With increasing strain rate, single peak or double peaks are observed in average strain hardening rate-strain rate curves. According to experiments, a semi-physical constitutive relation has been established, taking into account both the viscous drag effect and the DSA phenomenon. The established constitutive model can predict flow stress curves of DP800 steel accurately with an average error of 7.12%. Both the enhanced strain rate sensitivity due to viscous drag and the bell-shaped flow stress curves caused by DSA are captured correctly.

Ductility and formability of ultrafine-grained 5754 aluminium alloy under various strain rates and temperatures

The paper discusses the formability and ductility, under various strain rates and temperatures, of ultrafine grained plates of commercial 5754 aluminium alloy processed by a hybrid process combining multi-turn Equal Channel Angular Pressing and upset forging. The proposed process offers a simple way to produce UFG sheets that are characterized by high formability at elevated temperatures. It was proven that a proper selection of temperature and strain rate makes it possible to obtain elongations in tensile tests exceeding 80%, as well as a two-fold increase in formability, expressed as an increase in the cup height value in a cupping test of a UFG sheet compared with room temperature. This increase in ductility was attributed to enhanced strain rate sensitivity, since the m value exceeds 0.2, indicating a change in the deformation mechanisms at work. Based on apparent activation volume, which was equal to 13 b3, the nucleation and propagation of dislocations through grain and diffusion processes are indicated as the operating deformation mechanisms.

Deformation mechanisms in ultrafine-grained metals with an emphasis on the Hall–Petch relationship and strain rate sensitivity

Ultrafine-grained materials display almost no strain hardening, an enhanced strain rate sensitivity and grain boundary offsets during plastic deformation. It is expected that dislocation climb is active in order to enable prompt recovery. The present analysis proposes a deformation mechanism that includes these effects and follows from the mechanism for high temperature grain boundary sliding. This mechanism predicts the relationship between strain rate, flow stress, grain size, temperature and basic material properties such as the Burgers vector modulus, the shear modulus and the grain boundary diffusion coefficient. The model may be used to estimate the final grain size achieved by severe plastic deformation and the strain rate sensitivity. An analysis shows that the predicted behavior agrees with the data from multiple experimental investigations and provides a good estimate of the Hall–Petch slope for different materials which includes breakdown and inverse Hall–Petch behavior under some conditions. The incorporation of a threshold stress provides an opportunity to predict the relationship between flow stress and grain size for a broad range of grain sizes, strain rates and temperatures. An excellent agreement is observed between the predictions of the model and experimental data for Al, Cu, Fe (α), Fe(γ), Mg, Ni, Ti and Zn.

Enhanced strain rate sensitivity in thermal-cycling-rejuvenated metallic glasses

Strain rate sensitivity (SRS) is an effective parameter to demonstrate the deformation mechanism of metallic materials. However, despite extensive research, the meanings of the SRS of metallic glasses (MGs) were still vague and controversial results largely remained. In the present study, with altering the microstructural features of the magnetron sputtering CuZr MG before and after thermal cycling treatment, the effects of size of both positive and negative flow units on SRS were evaluated by amplitude-modulation atomic force microscopy (AM-AFM) and nanoindentation testing. Positive SRS index m was derived in the CuZr MG, and m increased from 0.014 to 0.040 after thermal cycling. The mechanism of the enhanced SRS and underlying microstructural evolution were explored and proposed. It shows the competition between time scales of atom rearrangement within flow units and applied loading strain rate causes the change of SRS, and both the size of positive and negative flow units play crucial roles in determining the magnitude of SRS.

Micro-compression studies of face-centered cubic and body-centered cubic high-entropy alloys: Size-dependent strength, strain rate sensitivity, and activation volumes

High-entropy alloys (HEAs) are evolving multi-component metallic systems, wherein generally five or more principal elements tend to provide strong solid-solution strengthening. In contrast to typical metals and alloys, the deformation mechanisms in HEAs have not been well understood so far. Here, we employ strain rate jump micro-compression testing to study the deformation mechanisms of two face-centered cubic (fcc) (FeCoNiCuPd and CrMnFeCoNi) and two body-centered cubic (bcc) HEAs (VNbMoTaW and NbMoTaW). The size-dependent strength, strain rate sensitivity, and activation volumes of the HEAs are compared with those of pure fcc and bcc metals. Both the fcc and bcc HEA pillars exhibit relatively low size dependence of strength compared to their corresponding pure metals, which can be attributed to the increased lattice distortion. Both the fcc and bcc HEAs exhibit enhanced strain rate sensitivity (SRS) with reducing pillar sizes. The FeCoNiCuPd HEA exhbits slightly lower size depedence of flow stress and a larger activation volume than those of the CrMnFeCoNi HEA (Cantor alloy), which could be associated with the higher dislocation motion resistance by adding palladium atoms with a markedly larger atomic size. The VNbMoTaW HEA also shows slightly lower size depedence of flow stress than that of the NbMoTaW HEA, which could be due to higher Peierls stress by adding the fifth element vanadium. Although there is no significant difference regarding activation volumes between HEAs and their corresponding pure metals, further studies are still needed to give a fundamental understanding of deformation mechanisms in a broad range of HEAs.

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.

A Model of Enhanced Strain Rate Sensitivity in Nanocrystalline and Ultrafine-Grained Metals

Abstract A model is suggested that describes enhanced strain rate sensitivity of nanocrystalline and ultrafine-grained metals. Within the model, plastic deformation of such metals incorporates dislocation transmission across grain boundaries (GBs) in the stress fields of dislocation pileups, the emission of individual dislocations from GBs as well as GB sliding accommodated by GB dislocation climb and/or Coble creep. The model predicts a strong increase in the strain rate sensitivity and a decrease in the activation volume with decreasing grain size, in accord with experimental data.We also considered the effect of GB sliding and Coble creep on the anomalous dependence of the activation volume on temperature observed in nanocrystalline Ni. It is demonstrated that although an account for GB sliding and Coble creep leads to the appearance of cusps in the temperature dependence of the activation volume, these mechanisms alone cannot be responsible for the observed anomalous dependence of the activation volume on temperature.

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.

Enhanced strain rate sensitivity of Zr-based bulk metallic glasses subjected to high pressure torsion

The Zr62Cu22Al10Fe5Dy1 bulk metallic glass (BMG) was subjected to high pressure torsion (HPT) at room temperature. The state after HPT processing demonstrates presence of large number of heterogeneities in amorphous structure – shear bands. Nanoindentation was used to study the effect of HPT processing on deformation behavior and strain rate sensitivity (m). It was found that HPT significantly affects mechanical behavior of BMG during nanoindentation. Decrease in hardness and Young's modulus was observed after HPT. At the same time HPT processing led to an increase in m from 0.014 for the as-cast material to 0.036. The lack of serration flow and shear bands around imprints was observed in addition to above mentioned changes. A much less localized deformation mode under nanoindentation along with an increase in values of strain rate sensitivity of HPT-processed state in comparison with the initial state imply homogeneous deformation behavior after processing. The observed drastic changes in deformation behavior and structure can be explained as the formation of a highly-heterogeneous amorphous structure resembling nanoglasses in result of HPT processing.

Deformation of small-volume Al-4Cu alloy under electron beam irradiation

Energetic electron beams (e-beams) are used in the welding, 3D printing and imaging of various materials. It is commonly accepted that the e-beam irradiation inside transmission electron microscope has a negligible effect on the intrinsic mechanical properties of small-volume metallic materials during in situ investigation. However, as small-volume Al is usually covered by a thin layer of native oxide, we show here that high-energy (200 keV) e-beam irradiation can dramatically alter the deformation behavior. E-beam irradiation can drastically rejuvenate the amorphous structure of the oxide layer, producing a defective interface/surface, and promoting the transition of the deformation mode from internal dislocation multiplication to interface-dominated dislocation nucleation mediated plasticity, as indicated by the enhanced strain rate sensitivity and reduced activation volume. Moreover, the amorphous oxide layer undergoes viscous flow under the e-beam irradiation, such that nanoscale single-crystal Al confined by a 29% volume fraction of native oxide shell exhibits more than 60% uniform elongation.

Effect of incremental equal channel angular pressing (I-ECAP) on the microstructural characteristics and mechanical behaviour of commercially pure titanium

Incremental equal channel angular pressing (I-ECAP) is one of the continuous severe plastic deformation (SPD) processes. This paper presents the processing of commercial purity titanium (CP-Ti) using a double billet variant of I-ECAP process. Ultrafine-grain (UFG) structure was successfully achieved after six passes of I-ECAP at 300°C. Microstructural evolution and texture development were tracked using EBSD. Analysis revealed continuous dynamic recrystallization (CDRX) as one of the grain refinement mechanism during processing. Room temperature tensile tests carried out before and after six passes, shows significant increase in strength with acceptable levels of ductility. The yield strength was increased from 308 to 558MPa and ultimate tensile strength from 549 to 685MPa. Compression tests conducted at different strain rates shows considerable increase in strength and enhanced strain rate sensitivity after processing. A distinct three-stage strain hardening was observed during compression. However the processed material displayed a loss in strain hardening ability during tensile as well as in compression tests. Detailed microhardness measurements show the evolution of hardness after subsequent passes with a reasonable level of homogeneity after the sixth pass. It is demonstrated that I-ECAP is an effective method for grain refinement in CP-Ti and subsequently improving its mechanical properties.

Compressive mechanical compatibility of anisotropic porous Ti6Al4V alloys in the range of physiological strain rate for cortical bone implant applications.

Porous titanium and its alloys are believed to be promising materials for bone implant applications, since they can reduce the "stress shielding" effect by tailoring porosity and improve fixation of implant through bone ingrowth. In the present work, porous Ti6Al4V alloys for biomedical application were fabricated by diffusion bonding of alloy meshes. Compressive mechanical behavior and compatibility in the range of physiological strain rate were studied under quasi-static and dynamic conditions. The results show that porous Ti6Al4V alloys possess anisotropic structure with elongated pores in the out-of-plane direction. For porous Ti6Al4V alloys with 60-70% porosity, more than 40% pores are in the range of 200-500μm which is the optimum pore size suited for bone ingrowth. Quasi-static Young's modulus and yield stress of porous Ti6Al4V alloys with 30-70% relative density are in the range of 6-40 GPa and 100-500MPa, respectively. Quasi-static compressive properties can be quantitatively tailored by porosity to match those of cortical bone. Strain rate sensitivity of porous Ti6Al4V alloys is related to porosity. Porous Ti6Al4V alloys with porosity higher than 50% show enhanced strain rate sensitivity, which is originated from that of base materials and micro-inertia effect. Porous Ti6Al4V alloys with 60-70% porosity show superior compressive mechanical compatibility in the range of physiological strain rate for cortical bone implant applications.

Microstructure-dependent deformation behaviour of bcc-metals – indentation size effect and strain rate sensitivity

In this work, the indentation size effect and the influence of the microstructure on the time-dependent deformation behaviour of body-centred cubic (bcc) metals are studied by performing nanoindentation strain rate jump tests at room temperature. During these experiments, the strain rate is abruptly changed, and from the resulting hardness difference the local strain rate sensitivity has been derived. Single-crystalline materials exhibit a strong indentation size effect; ultrafine-grained metals have nearly a depth-independent hardness. Tungsten as a bcc metal shows the opposite behaviour as generally found for face-centered cubic metals. While for UFG-W only slightly enhanced strain rate sensitivity was observed, SX-W exhibits a pronounced influence of the strain rate on the resulting hardness at room temperature. This is due to the effects of the high lattice friction of bcc metals at low temperatures, where the thermally activated motion of screw dislocations is the dominating deformation mechanisms, which causes the enhanced strain rate sensitivity. For the SX-materials, it was found that the degree of the indentation size effect directly correlates with the homologous testing temperature and thus, the material specific parameter of the critical temperature Tc. However, for the resultant strain rate sensitivity no depth-dependent change was found.

Elevated temperature, strain rate jump microcompression of nanocrystalline nickel

Nanocrystalline and ultrafine-grained materials show enhanced strain rate sensitivity (SRS) in comparison to their coarse grained counterparts. Majority of SRS measurements on nanocrystalline thin films reported in literature have focused on nanoindentation-based approaches. In this paper, micropillar strain rate jump tests were demonstrated on an electrodeposited nanocrystalline nickel film from 25 to 100 °C. SRS exponent, m, and activation volume, V, values were determined as a function of temperature. The measured values were found to be in good agreement with previously reported literature on bulk and nanoindentation measurements. Apparent activation energy for deformation was found to be about 100 kJ/mol, which is close to that for grain boundary diffusion in nickel. Grain boundary sliding was observed in the deformed pillars from scanning electron microscopy images.

Ultra-fine grained bulk CP-Ti processed by multi-pass ECAP at warm deformation region

In order to refine the grain size of commercially pure titanium (CP-Ti) to a submicrometer scale, equal channel angular pressing (ECAP) was attempted at a temperature range of 200–300 °C. The experiments revealed that, 250 °C was the minimum temperature at which ten passes of ECAP could be performed in a 105° die without the cracking of billets. An ultrafine-grained (UFG) microstructure with a mean grain size of 183 nm was achieved after 10 passes. The processed CP-Ti displayed high tensile strength of 892 MPa and high elongation to failure of 20.5%. The enhancement in mechanical properties is explained in terms of grain refinement and dislocation density increasing. The high ductility of UFG pure Ti with the absence of strain hardening behavior is attributed to its enhanced strain rate sensitivity.

Strain rate sensitivity studies in an ultrafine-grained Al–30wt.% Zn alloy using micro- and nanoindentation

The characteristics of plastic deformation of an ultrafine-grained (UFG) Al–30 wt.% Zn alloy were investigated using depth-sensing micro- and nanoindentation. Emphasis was placed on the effects of grain boundaries and the unusually high strain rate sensitivity. It is shown that there is a close relationship between enhanced strain rate sensitivity and ductility in this UFG material and this is associated with grain boundary sliding and enhanced diffusion along the Al/Al grain boundaries which appear to be wetted by Zn-rich layers.

Experimental investigation on strain rate sensitivity of ultra-fine grained copper at elevated temperatures

Quasi-static and dynamic compression tests on ultra-fine grained (UFG) copper (Cu 99.9) were performed at temperatures ranging from 77 to 573 K and strain rates ranging from 10 −3 to 5 × 10 3 1/s using an electronic universal testing machine and the split Hopkinson pressure bar system, respectively. We focused on the strain rate sensitivity and its dependence on experimental temperatures. The results show that UFG-Cu has enhanced strain rate sensitivity that is apparently temperature dependent. The activation volume of UFG-Cu is estimated to be on the order of ∼10 b 3 at current experimental temperature and strain rate range. The plastic deformation mechanism is suggested to be dislocation–grain boundary interactions.

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.