High Strain Rate Region Research Articles

Effect of Sn Addition on Superplastic Properties of Zn-22mass%Al Eutectoid Alloy

Superplastic behavior of a Zn 22 mass % Al eutectoid alloy (SPZ) with small addition of Sn (SPZSn) was investigated. Granular grain size of about 0.3 μm was obtained by water quench after annealing SPZ and SPZ05Sn (addition of 0.05 mass % Sn into the SPZ) at 653 K for 2 h. The fundamental microstructure of the SPZ05Sn was similar to that of the SPZ, but, microstructure observation by STEM showed additive Sn was present at the α’ grain boundary in the SPZ05Sn. Excellent high strain rate superplasticity was achieved in the SPZ05Sn, with elongation of more than 1300 % at 523 K at strain rate of 10-1 S-1. Furthermore, large elongation of about 1100 % was recorded at 473 K at strain rate of 10-1 S-1. The large elongation and high strain rate sensitivity value of the SPZ05Sn tend to shift to higher strain rate region as compared to those of the SPZ. It was considered that the small addition of Sn into the SPZ effectively suppressed the grain growth of α and β phase during the superplastic deformation, because granular grains less than 2 μm is maintained after superplastic deformation at 523 K.

Data characterizing compressive properties of Al/Al2O3 syntactic foam core metal matrix sandwich.

Microstructural observations and compressive property datasets of metal matrix syntactic foam core sandwich composite at quasi-static and high strain rate (HSR) conditions (525–845 s−1) are provided. The data supplied in this article includes sample preparation procedure prior to scanning electron and optical microscopy as well as the micrographs. The data used to construct the stress–strain curves and the derived compressive properties of all specimens in both quasi-static and HSR regions are included. Videos of quasi-static compressive failure and that obtained by a high speed image acquisition system during deformation and failure of HSR specimen are also included.

Strain rate change tests with the Split Hopkinson Bar method

In this paper, methods to produce rapid strain rate changes for strain rate sensitivity measurements in Split Hopkinson Bar arrangements are presented and discussed. Two different cases are considered: a strain rate change test within the high strain rate region in compression, and a tension test incorporating a large strain rate jump directly from the low strain rate region to high strain rates. The former method is based on the loading wave amplitude manipulation, while the latter method is based on the incorporation of a low strain rate loading device into a Tensile Split Hopkinson Bar apparatus.

Two-dimensional homogeneous isotropic fluid turbulence with polymer additives.

We carry out an extensive and high-resolution direct numerical simulation of homogeneous, isotropic turbulence in two-dimensional fluid films with air-drag-induced friction and with polymer additives. Our study reveals that the polymers (a) reduce the total fluid energy, enstrophy, and palinstrophy; (b) modify the fluid energy spectrum in both inverse- and forward-cascade régimes; (c) reduce small-scale intermittency; (d) suppress regions of high vorticity and strain rate; and (e) stretch in strain-dominated regions. We compare our results with earlier experimental studies and propose new experiments.

Plastic Instability in ISO 5832-9 High-nitrogen Austenitic Stainless Steel

ISO 5832-9 high-nitrogen austenitic stainless steel has shown promising results in the fabrication of temporary and permanent orthopedic prostheses, exhibiting better mechanical strength and corrosion resistance than the traditional ISO 5832-1 (ASTM F-138) steel. Recent studies have revealed that this alloy possesses unique properties, such as high mechanical strength and corrosion resistance and the presence of second phase particles (Z phase) in the matrix. However, it is not known how the microstructural and mechanical properties and the corrosion rate are correlated in regions of industrial processability of this alloy during hot forming. In this study, continuous and interrupted isothermal hot torsion tests were performed after solubilization heat treatment at 1 473 K for 300 s, with temperature intervals varying from 1 273 to 1 473 K and strain rates ranging from 0.05 to 5 s –1 . The purpose of these tests was to investigate the alloy’s workability and stress corrosion rate by means of electrochemical impedance spectroscopy (EIS) and to characterize its microstructure by optical microscopy. The results indicate that the yield stress is sensitive to the strain parameters. The peak stress decreases with increasing temperature and decreasing strain rate, with a high rate of dynamic recovery, with elongated grains generating areas of stress build-up. These regions of plastic instability present a higher degree of corrosion than as-received samples. The regions of high temperature and low strain rate exhibit good workability with a refined final microstructure of dynamically recrystallized grains.

Topo-Iberia project: CGPS crustal velocity field in the Iberian Peninsula and Morocco

A new continuous GPS network was installed under the umbrella of a research project called “Geociencias en Iberia: Estudios integrados de topografía y evolución 4D (Topo-Iberia)”, to improve understanding of kinematic behavior of the Iberian Peninsula region. Here we present a velocity field based on the analysis of the 4 years of data from 25 stations constituting the network, which were analyzed by three different analysis groups contributing to the project. Different geodetic software packages (GIPSY–OASIS, Bernese and GAMIT) as well as different approaches were used to estimate rates of present day crustal deformation in the Iberian Peninsula and Morocco. In order to ensure the consistency of the velocity fields determined by the three groups, the velocities obtained by each analysis center were transformed into a common Eurasia Reference Frame. After that, the strain rate field was calculated. The results put in evidence more prominent residual motions in Morocco and southernmost part of the Iberian Peninsula. In particular, the dilatation and shear strain rates reach their maximum values in the Central Betics and northern Alboran Sea. A small region of high shear strain rate is observed in the east-central part of the peninsula and another deformation focus is located around the Strait of Gibraltar and the Gulf of Cadiz.

Dynamic Deformation Behaviors of High Performance Steels

The responses of three high strength steels under impact loading were examined, specifically on their strain rate dependence. Split Hopkinson pressure bar test was used in this study. Over a wide strain rate range, the Johnson-Cook model and modified Johnson-Cook mode were adopted to determine the strain rate hardening behavior of the materials. The group determined the material parameters for each metallic material tested. Obtained material parameters were used to predict the behavior of each steel at high strain rate region. The modified Johnson-Cook model was not able to represent well enough the plastic deformation behavior of steels, specifically the steel that exhibited strain softening behavior at high strain rate region.

Hot Deformation Behavior of As-cast AISI M2 High-speed Steel Containing Mischmetal

The hot deformation behavior of as-cast AISI M2 high-speed steel containing mischmetal (RE) has been investigated on a Gleeble-3500 simulator in the temperature range of 1000–1150 °C and strain rate range of 0. 01 – 10 s−1 at true strain of 1.0. The mechanical behavior has been characterized using stress-strain curve analysis, kinetic analysis, processing maps, etc. Metallographic investigation was performed to evaluate the mechanism of flow instability. The results show that the deformation activation energy decreases with increasing deformation temperature; the efficiency of power dissipation increases with decreasing strain rate and increasing temperature; flow instability is observed at low-to-medium temperature and higher strain rate region when the strain is smaller, but extends to lower strain rate and high temperature regions with the increment of strain, in which it is manifested as flow localization near the grain boundary. Hot deformation equations and processing maps are obtained. The optimal processing window is suggested and the deformation mechanism is dynamic recrystallization (DRX).

Structures and dynamics of small scales in decaying magnetohydrodynamic turbulence

The topological and dynamical features of small scales are studied in the context of decaying magnetohydrodynamic turbulent flows using direct numerical simulations. Joint probability density functions (PDFs) of the invariants of gradient quantities related to the velocity and the magnetic fields demonstrate that structures and dynamics at the time of maximum dissipation depend on the large scale initial conditions at the examined Reynolds numbers. This is evident in particular from the fact that each flow has a different shape for the joint PDF of the invariants of the velocity gradient in contrast to the universal teardrop shape of hydrodynamic turbulence. The general picture that emerges from the analysis of the invariants is that regions of high vorticity are correlated with regions of high strain rate S also in contrast to hydrodynamic turbulent flows. Magnetic strain dominated regions are also well correlated with region of high current density j. Viscous dissipation (\documentclass[12pt]{minimal}\begin{document}${\propto } \bm S^2$\end{document}∝S2) as well as Ohmic dissipation (\documentclass[12pt]{minimal}\begin{document}${\propto } \bm j^2$\end{document}∝j2) resides in regions where strain and rotation are locally almost in balance. The structures related to the velocity gradient possess different characteristics than those associated with the magnetic field gradient with the latter being locally more quasi-two dimensional.

Creep behaviour of eutectic SnBi alloy and its constituent phases using nanoindentation technique

Creep behaviour of eutectic tin–bismuth (SnBi) and its constituent phase materials was studied using constant strain rate (CSR) nanoindentation. Eight strain rates from 5×10−4 to 0.1s−1 were used to assess their strain rate–stress relationship. The stress exponents of 12.25 and 10.09 were found for Sn–3%Bi and pure Bi, respectively, suggesting power-law breakdown (PLB) as the rate controlling mechanism for the two constituent single-phase materials. Strain bursts that appear at the initial stage of loading of the single-phase materials were found to be a prerequisite to cause dislocation creep at the later stage of deformation. Grain boundary sliding (GBS) was found as the complementary mechanism to accommodate grain shape change in Sn–3%Bi and pure Bi as proven by post-indent microstructural examination. Eutectic SnBi showed bi-linear strain rate dependent creep behaviour with the transition at around 2×10−3s−1. Stress exponent of 2.35 was found at low strain rate region suggesting that GBS dominates the creep deformation. At the high strain rate region, stress exponent of 5.20 suggests that dislocation climb in the crystal lattice is the creep mechanism.

Motion of inertial particles in Gaussian fields driven by an infinite-dimensional fractional Brownian motion

We study the motion of an inertial particle in a fractional Gaussian random field. The motion of the particle is described by Newton's second law, where the force is proportional to the difference between the background fluid velocity and the particle velocity. The fluid velocity satisfies a linear stochastic partial differential equation driven by an infinite-dimensional fractional Brownian motion with an arbitrary Hurst parameter H ∈ (0, 1). The usefulness of such random velocity fields in simulations is that we can create random velocity fields with desired statistical properties, thus generating artificial images of realistic turbulent flows. This model also captures the clustering phenomenon of preferential concentration, observed in real world and numerical experiments, i.e. particles cluster in regions of low-vorticity and high-strain rate. We prove almost sure existence and uniqueness of particle paths and give sufficient conditions to rewrite this system as a random dynamical system with a global random pullback attractor. Finally, we visualize the random attractor through a numerical experiment.

Quantitative evaluation in hot workability of SUS303 free-cutting steel

The hot forging behavior of S-containing SUS303 free-cutting steel was investigated in detail by carrying out compression tests; the tests were carried out in the temperature range of 1073–1323K and strain rates ranging from 0.01 to 30s−1. Quantitative relationships among the Zener–Hollomon parameter Z, dynamic recrystallization (DRX) fraction and power dissipation efficiency were discussed for the first time. The processing map exhibited high reliability in predicting the stable condition of low strain rate and high temperature, while at high strain rate region the reliability is relatively low. At the optimum condition predicted by the processing map, refined and uniaxial microstructure could be obtained, and at the same time the cutting ability was also enhanced greatly due to the re-distribution of MnS precipitates into a more suitable condition for the cutting process.

Sub-slab mantle anisotropy beneath south-central Chile

Knowledge of mantle flow in convergent margins is crucial to unravelling both the contemporary geodynamics and the past evolution of subduction zones. By analysing shear-wave splitting in both teleseismic and local arrivals, we can determine the relative contribution from different parts of the subduction zone to the total observed SKS splitting, providing us with a depth constraint on anisotropy. We use this methodology to determine the location, orientation and strength of seismic anisotropy in the south-central Chile subduction zone. Data come from the TIPTEQ network, deployed on the forearc during 2004–2005. We obtain 110 teleseismic SKS and 116 local good-quality shear-wave splitting measurements. SKS average delay times are 1.3s and local S delay times are only 0.2s. Weak shear-wave splitting from local phases is consistent with a shape preferred orientation (SPO) source in the upper crust. We infer that the bulk of shear-wave splitting is sourced either within or below the subducting Nazca slab. SKS splitting measurements exhibit an average north-easterly fast direction, with a strong degree of variation. Further investigation suggests a relationship between the measurement's fast direction and the incoming ray's back-azimuth. Finite-element geodynamic modelling is used to investigate the strain rate field and predicted LPO characteristics in the subduction zone. These models highlight a thick region of high strain rate and strong S-wave anisotropy, with plunging olivine a-axes, in the sub-slab asthenosphere. We forward model the sub-slab sourced splitting with a strongly anisotropic layer of thick asthenosphere, comprising an olivine a-axis oriented parallel to the direction of subduction. The subducting lithosphere is not thick enough to cause 1.2s of splitting, therefore our results and subsequent models show that the Nazca slab is entraining the underlying asthenosphere; its flow causes it to be strongly anisotropic. Our observation has important implications for the controlling factors on sub-slab mantle flow and the movement of asthenospheric material within the Earth.

Unusual grain-size and strain-rate effects on the serrated flow in FeMnC twin-induced plasticity steels

The purpose of the paper is to clarify the serrated flow behavior and to explore the grain-size and strain-rate effects on serrations in coarse- and fine-grained FeMnC twin-induced plasticity (TWIP) steels at room temperature. Tensile tests were performed under extensometer-measured strain control, rather than under conventional cross-head displacement control, at strain rates ranging from 6 × 10−6 to 6 × 10−3 s−1. The results indicate that both the coarse- and fine-grained steels show different types of serrations, which depend on strain rate, and demonstrate a nonmonotonic strain-rate sensitivity of the critical strain for the onset of serrations, i.e., a positive strain-rate sensitivity of the critical strain in the high strain-rate region typified by type A serrations, and a negative strain-rate sensitivity in the low strain-rate region typified by type C ones. As compared to the coarse-grained steel, the fine-grained steel increases the critical strains, showing an inverse grain-size effect. The findings suggest that the fine-grained FeMnC TWIP steel suppresses the serrated flow, possibly due to the enhanced dynamic recovery associated with the decreased planar slip length in the fine-grained low stacking-fault-energy steel.

Extension Flow Induced Crystallization of Poly(ethylene oxide)

Crystallization of poly(ethylene oxide) (PEO) induced by extensional flow has been studied by in-situ small-angle X-ray scattering (SAXS). A constant Hencky strain was applied to the melt with strain rates varying in 3 orders of magnitude. The evolution of the long period with crystallization time is qualitatively different for high and low strain rates. In the region of high strain rates the long period first increases, reaches a plateau, and decreases in the later stage of crystallization. In contrast, for low strain rates only a monotonic decrease is observed. Given the high orientation in the high strain rate region, we propose a localized nucleation mechanism (like row-nuclei). The initial increase of the long period for high strain rates indicates a decrease of nucleation density, which is the inverse of the long period. On the basis of the microrheological model proposed by Coppola et al., in which the flow induced free energy change under steady state flow is calculated with the Doi–Edwards model, we develop a framework describing the nucleation after a step strain with high strain rates. By introducing the memory function, the dynamic process of relaxation is taken into account, leading to a continuous decrease of the nucleation density. Numerical fitting the model to the experimental data gives a good agreement with the terminal relaxation time τd, the volume filling rate B, and a constant C3 determined by surface free energy of the nuclei as fitting parameters. The results confirm the localized nucleation mechanism in highly oriented melt. Furthermore, the fitted value of C3 indicates a drop in surface free energy of nuclei under strong flow.

Development of Processing Maps for DC Cast Modified and Unmodified Hypereutectic Al-Si Alloys

The hot deformation behavior of DC cast phosphorous modified and unmodified hypereutectic Al-Si alloys was studied in the temperature range of 400-500 °C and strain rate range of 0.001-1 s-1. Processing maps were developed to evaluate the efficiency of the hot deformation and to identify the instability region. The results show that the peak stresses of the unmodified alloy are higher than that of the modified alloy at the strain rate of 1 s-1 and temperatures of 400 and 440 °C. The maximum power dissipation efficiencies for both the alloys are in the region of T=480-500 °C and =0.01-0.1 s-1. The flow instabilities for both the alloys occur in regions of high strain rate about 1 s-1 and temperature about 400 and 500 °C. The instability region area of the unmodified alloy is larger than that of the modified alloy. In addition, the primary Si cracking frequencies of the unmodified alloy are higher than that of the modified alloy when compared at the same deformation rate and temperature. The coarser primary Si particles of the unmodified alloy cause higher stress concentration around them when deformed at low temperature and high strain rate.

Experimental study on rate dependence of macroscopic domain and stress hysteresis in NiTi shape memory alloy strips

NiTi polycrystalline shape memory alloys, when stretched, can deform through the formation and growth of localized macroscopic martensite domains. In this paper, we study the effects of stretching rate on the stress-induced domains and stress hysteresis in NiTi strips. Synchronized measurements of the nominal stress–strain curve, macroscopic domain pattern and the associated temperature field were conducted in the strain rate range of 10 −4–10 −1/s. It was found that the nominal stress–strain curve changed from the near-isothermal plateau-type with distinct stress drops at the very low strain rate to the near-adiabatic smooth hardening-type in the high strain-rate region. The corresponding deformation mode changed from the nucleation propagation mode with a few parallelepiped martensite domains to the near-homogeneous multiple-nucleation mode with many fine alternating austenite–martensite stripes. The number of the domains (domain spacing) increased (decreased) monotonically with the strain rate and followed a power law scaling, while the stress hysteresis (or material damping capacity) changed non-monotonically with the strain rate, reaching a peak at strain rate of 2.0×10 −3/s. We show that, though the rate dependence of both pattern and hysteresis originates from the transfer of the released/absorbed heat and the thermo-mechanical coupling, the domain spacing in the test of static air is mainly controlled by heat conduction and the hysteresis change is mainly controlled by the heat convection with the ambient.

Reconciling surface plate motions with rapid three-dimensional mantle flow around a slab edge

The direction of tectonic plate motion at the Earth's surface and the flow field of the mantle inferred from seismic anisotropy are well correlated globally, suggesting large-scale coupling between the mantle and the surface plates. The fit is typically poor at subduction zones, however, where regional observations of seismic anisotropy suggest that the direction of mantle flow is not parallel to and may be several times faster than plate motions. Here we present three-dimensional numerical models of buoyancy-driven deformation with realistic slab geometry for the Alaska subduction-transform system and use them to determine the origin of this regional decoupling of flow. We find that near a subduction zone edge, mantle flow velocities can have magnitudes of more than ten times the surface plate motions, whereas surface plate velocities are consistent with plate motions and the complex mantle flow field is consistent with observations from seismic anisotropy. The seismic anisotropy observations constrain the shape of the eastern slab edge and require non-Newtonian mantle rheology. The incorporation of the non-Newtonian viscosity results in mantle viscosities of 10(17) to 10(18) Pa s in regions of high strain rate (10(-12) s(-1)), and this low viscosity enables the mantle flow field to decouple partially from the motion of the surface plates. These results imply local rapid transport of geochemical signatures through subduction zones and that the internal deformation of slabs decreases the slab-pull force available to drive subducting plates.

Energy Absorption Capability of Zn-22Al Superplastic Alloy Foam Manufactured through Melt Foaming Process

Superplastic Zn-22Al alloy foams were manufactured through the melt foaming process using titanium hydride powder as a foaming agent. The compressive properties of the Zn-22Al foams were investigated under quasi-static and dynamic loading conditions. Experimental results show that the flow stress and the energy absorption of the Zn-22Al foam significantly increased with increasing the strain rate. At high strain rate region, the energy absorption of the Zn-22Al foams is also larger than that of conventional aluminum foams. These behaviors are due to superplastic deformation of cell walls.

Role of time scale in the deformation patterns and hysteresis of NiTi SMA

In this paper we present our recent modelling and experiment on the roles of loading time scale and thermo-mechanical two-field coupling in the deformation domain patterns and stress hysteresis in NiTi SMA. We first report the effects of loading time or stretching rate on the domain patterns and stress hysteresis observed in NiTi strips in the strain rate range of 10-5/s ~ 10-1/s. It was found that the nominal stress-strain curve of the specimen changed from near-isothermal plateau-type with distinct stress-drops in the low strain rate region to the near-adiabatic smooth hardening-type in the high strain rate region. The pseudoelastic hysteresis varies non-monotonically with the external loading rate and the maximum hysteresis occurs at certain intermediate loading rate. The corresponding deformation mode changed from the localized-propagation mode with a few parallelepiped martensite domain patterns to the near-homogeneous multiple nucleation mode with fine alternating austenite-martensite stripes. The analysis shows that the number of the macroscopic domains (domain spacing) of the specimen increased (decreased) with the applied stretching rate in a power-law form, i.e., the higher the loading rate, the finer the deformation pattern. Such observed power-law dependency is originated from the coupling between the material’s nonlinear property (formation and growth of domains) and the transfer of heat in the phase transition process. The important roles of heat transfer dynamics and loading time in imposing the emerging length scale in the deformation pattern of the material is demonstrated. For the hysteresis, a simple two-scale thermo-mechanical coupling model is developed to explain the observed rate-dependent hysteresis phenomenon. We incorporate the effects of the released/absorbed heat of the phase transition and the heat transfer on the pseudoelastic hysteresis. An analytical expression for the rate-dependent hysteresis is derived and the theoretical predictions agree quantitatively well with the experiments.