Ultrahigh Strain Research Articles

The Microstructure and Properties of LY12 Aluminum Alloy by Ultra-High Plastic Strain

The ultra-high plastic deformation behavior by laser shock processing on the LY12 aluminum alloy had been investigated. The morphology of the materials had been analyzed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). It was found that the grain refinement layer was formed in the thickness of about 100μm .The dislocation density of LY12 aluminum alloy should be large increased after laser shocking because the accumulation of dislocation was appeared on the grain boundary. With the laser energy density increased there formed subgrain structure and eventually generate ultra-fine grain. Hardness test results also show that the surface hardness obtains a big growth about 60% after laser shock processing. The results showed that the formation of ultra-high plastic strain can improve the surface hardness of LY12 aluminum alloy, and thus effectively improve the comprehensive mechanical properties.

Evaluation of Shear Failure of Strain Hardening Cementitious Composite Beams

The mechanical behavior and fracture mechanisms of SHCC member failed in diagonal shear were evaluated both experimentally and analytically. Ultra High Performance Strain Hardening Cementitious Composite (UHP-SHCC) was applied as structural material. In the analysis, shear transfer model for SHCC was proposed based on its characteristics of cracking, that is, multiple fine cracking during strain hardening region and localization in one crack during strain softening region. As results, it was confirmed that proposed model was suitable for the mechanisms of shear transfer of SHCC and reasonable to predict the behavior of SHCC beam failed in diagonal shear.

Influence of Laser Shock Processing on Surface Microstructures and Properties of Carbon Steel

Abstract In order to understand the correlations between the laser shock processing and materials’ response, an as annealed plain carbon steel 45 (equivalent to AISI 1045) was investigated by means of a Nd:glass laser setup with 1054 nm wavelength and 20 ns short pulse. The microstructure evolution was analyzed with optical microscope, scanning electron microscope, etc. The experimental results indicate that the strengthening effects can be obtained under the used confinement conditions and the ultra-high strain rate of laser shock. After laser shocking, the cementite in pearlite crashes the subject to their orientations, and the pro-eutectoid ferrite produces a great number of dislocation etched pits and lens-like twins. The hardness of pearlite and pro-eutectoid ferrite increases by 70 and 55 Hv, respectively, and the residual compressive stress of the steel reaches 425 MPa on the shocked spot.

Effects of microstructure on high strain rate deformation and flow behaviour of Al–Mg–Si alloy (AA 6061) under uniaxial compression and combined compression and shear loading

In the present study, high strain rate deformation and flow characteristics of AA 6061 were investigated under dynamic uniaxial compression and combined compression and shear loading conditions. The uniaxial compressive behaviour was obtained at strain rates in the range of 102–104 s−1 using a split Hopkinson pressure bar (SHPB); ultrahigh strain rate plate impact compression and shear experiments were conducted to investigate the material behaviour at plastic strain rates in excess of 104 s−1. In order to understand the effect of material microstructure on the mechanical behaviour of the AA 6061 alloy, both the T6 and overaged (OA) heat treatments were investigated. The results of the SHPB experiments indicate that the two heat treatments show positive strain rate sensitivity at strain rates up to 4660 s−1. However, at strain rates in excess of 4660 s−1, Al6061-T6 exhibits a somewhat negative strain rate sensitivity. Estimates for temperature rise, assuming that all the plastic work is converted to heat, indicate that the specimen temperature may be elevated into a regime where both thermal effects on the flow stress and the resulting strain rate sensitivity change the flow stress response. This suggests that one contributing reason for the observed decrease in flow stress at the highest strain rates is due to thermal softening. Moreover, at strain rates in excess of 104 s−1, the flow stress levels for both the Al6061-T6 and Al6061-OA heat treatments are observed to approach their saturation stress levels.

Microstructure Evolution of Pearlitic Lamella Impacts at Ultra-High Strain Rates

The microstructure evolution of eutectoid steel with lamellar pearlite was investigated by SEM and TEM during ultra-high strain rate loading. The results indicate that ultrafine microduplex structure (ferrite + cementite) with the grain size to sub-micrometer level was observed at the surface of eutectoid steel after single pass ultra-high strain rate loading. Equiaxed ferrite grain was about 0.8 μm and the cementite lamella was spheroidized fully, and the diameter of the cementite particle was about 50 nm. The bent or fractures can occur at the edge of shock wave. Ultra-high strain rate shocking induced severe plastic deformation at the surface of materials and the cementite lamella has better plastic deformation capacity.

Forming Limit and Fracture Mode of Microscale Laser Dynamic Forming

The microscale laser dynamic forming (LDF) process is a high strain rate microfabrication technique, which uses a pulse laser to generate high pressure by vaporizing and ionizing an ablative coating, and thus produces complex 3D microstructures in thin foils. One of the most important features of this technique is ultrahigh strain rate (typically 106–7 s−1), which is theoretically favorable for increasing formability. However, due to the lack of measurement techniques in microscale and submicroscale, the formability of workpieces in LDF is hardly studied. In this article, experiments were carried out on aluminum foils to study the forming limits and fracture of thin films in LDF. The deformation depth was measured by an optical profilometer and the formed feature was observed using a focused ion beam and a scanning electron microscope. Meanwhile, a finite element model based on a modified Johnson–Cook constitutive model and a Johnson–Cook failure model was developed to simulate the mechanical and fracture behaviors of materials in LDF. Experimental results were used to verify the model. The verified model was used to predict the forming limit diagram of aluminum foil in LDF. The forming limit diagrams show a significant increase in formability compared with other metal forming processes.

State-of-the-Art in Micro-Forming of Ultra-High Strain Rate

Most lithography-based micro-forming technology has the limitations in materials selection, forming efficiency and the complexity of micro-parts etc. It is urgent to develop the efficient low-cost micro-forming method in batch. This paper introduced the state-of-the-art of sheet metal micro-forming technology with laser-induced ultra-high strain rate involving the loading modes, processes and forming fundamental. Then, the application of the micro-forming of ultra-high strain rate induced by laser shock is extended into the bulk forming domain, micro bulk forming. It is pointed out that the further investigation on micro bulk forming method focuses on scale effects, forming mechanism, loading and control modes.

Microstructural evolution in electroformed nickel shaped-charge liners with nano-sized grains undergone deformation at ultrahigh strain rate

Nickel shaped-charge liners with nano-sized grains were prepared by the electroforming technique, and the deformation at ultrahigh strain rate was performed by explosive detonation. The as-formed and post-deformed microstructures of electroformed nickel shaped-charge liners with nano-sized grains were observed by means of transmission electron microscopy, and the orientation distribution of the grains was analyzed by the electron backscattering pattern (EBSP) technique. The melting phenomenon in the jet fragment and the recovery and recrystallization in the slug after plastic deformation at ultrahigh-strain rate were observed in the ultrafine-grained nickel shaped-charge liners. The research evidence shows that dynamic recovery and recrystallization play an important role in plastic deformation at ultrahigh strain rate.

Anomalous Piezoresistance Effect in Ultrastrained Silicon Nanowires

In this paper we demonstrate that under ultrahigh strain conditions p-type single crystal silicon nanowires possess an anomalous piezoresistance effect. The measurements were performed on vapor-liquid-solid (VLS) grown Si nanowires, monolithically integrated in a microelectro-mechanical loading module. The special setup enables the application of pure uniaxial tensile strain along the <111> growth direction of individual, 100 nm thick Si nanowires while simultaneously measuring the resistance of the nanowires. For low strain levels (nanowire elongation less than 0.8%), our measurements revealed the expected positive piezoresistance effect, whereas for ultrahigh strain levels a transition to anomalous negative piezoresistance was observed. For the maximum tensile strain of 3.5%, the resistance of the Si nanowires decreased by a factor of 10. Even at these high strain amplitudes, no fatigue failures are observed for several hundred loading cycles. The ability to fabricate single-crystal nanowires that are widely free of structural defects will it make possible to apply high strain without fracturing to other materials as well, therefore in any application where crystallinity and strain are important, the idea of making nanowires should be of a high value.

Grain refinement mechanism of multiple laser shock processing impacts on ANSI 304 stainless steel

Micro-structural evolution and grain refinement in ANSI 304 stainless steel subjected to multiple laser shock processing (LSP) impacts were investigated by means of cross-sectional optical microscopy and transmission electron microscopy observations. The plastic strain-induced grain refinement mechanism of the face-centered cubic (fcc) materials with very low stacking fault energy was identified. The micro-structure was obviously refined due to the ultra-high plastic strain induced by multiple LSP impacts. The minimum grain size in the top surface was about 50–200 nm. Multidirectional mechanical twin matrix (MT)–MT intersections led to grain subdivision at the top surface during multiple LSP impacts. Furthermore, a novel structure with submicron triangular blocks was found at the top surface subjected to three LSP impacts. The grain refinement process along the depth direction after multiple LSP impacts can be described as follows: (i) formation of planar dislocation arrays (PDAs) and stacking faults along multiple directions due to the pile up of dislocation lines; (ii) formation of submicron triangular blocks (or irregularly shaped blocks) by the intersection of MT–MT (or MT–PDA or PDA–PDA) along multiple directions; (iii) transformation of MTs into subgrain boundaries; (iv) evolution by continuous dynamic recrystallization of subgrain boundaries to refined grain boundaries. The experimental results and analyses indicate that a high strain with an ultra-high strain rate play a crucial role in the grain refinement process of fcc materials subjected to multiple LSP impacts.

Grain refinement of LY2 aluminum alloy induced by ultra-high plastic strain during multiple laser shock processing impacts

The plastic deformation behavior and the effects of the impact time on the LY2 aluminum (Al) alloy during multiple laser shock processing (LSP) impacts were investigated. The residual stress in the near-surface region was determined by X-ray diffraction. In addition, the micro-structural features of the hardening layer were characterized by scanning electron microscopy, optical microscopy and transmission electron microscopy. It was found that the micro-structure was obviously refined due to the ultra-high plastic strain induced by multiple LSP impacts. The minimum grain size in the top surface after multiple LSP impacts was about 100–200 nm. The grain refinement process after multiple LSP impacts can be described as follows: (i) the formation and development of dislocation lines in original grains; (ii) dislocation tangles (DTs) and the formation of dense dislocation walls (DDWs); (iii) transformation of DTs and DDWs into subgrain boundaries; and (iv) evolution of the continuous dynamic recrystallization in subgrain boundaries to refined grain boundaries.

Laser-Induced High-Strain-Rate Superplastic 3-D Microforming of Metallic Thin Films

Microforming of metals has always been a challenge because of the limited formability of metals at the microscale. This paper investigates an innovative microforming technique: microscale laser dynamic forming (¿LDF), which induces 3-D superplastic forming in metallic thin films. This forming process proceeds in a sequence of laser irradiation and ionization of ablative coating, shockwave generation and propagation in metallic thin films, and conformation of metallic thin films to the shape of microscale molds. Because the deformation proceeds at ultrahigh strain rates, it is found that materials experience superplastic deformation at the microscale. In this paper, experiments are systematically carried out to understand the deformation characteristics of ¿LDF. The topologies and dimensions of the deformed samples are characterized by scanning electron microscopy and optical profilometry. The thickness variations are characterized by slicing the cross section of the deformed material using the focused ion beam. The magnitude of deformation depth in ¿LDF is determined primarily by three critical factors: film thickness, mold geometry, and laser intensity. The relationships between these factors are explored in process maps to find suitable processing conditions for ¿LDF. Nanoindentation tests are conducted to show that the strength of the thin films is increased significantly after ¿LDF.

Characterizations of Stress and Strain Variation in Three-Dimensional Forming of Laser Micro-Manufacturing

A micro-manufacturing technology is presented to form three-dimensional metallic micro-structures directly. Micro grid array structures are replicated on a metallic foil surface, with high spatial resolution in micron levels. The numerical simulation results indicate that the material deformation process is characterized by an ultrahigh strain rate. With increasing pulse duration, the sample absorption strain energy increases, and the sample deformation degree enlarges. The stress state of the central point fluctuates between tensile stress and compression stress. The stress state of the angular point is altered from compressive stress to tensile stress due to geometry and loading conditions. The duration length of pulse stress has an effect on the stress state, as with the increase of pulse duration, fluctuation in the stress state decreases. Therefore, laser micro-manufacturing technology will be a potential laser micro forming method which is characterized by low cost and high efficiency.

Thermal behavior of Ni (99.967% and 99.5% purity) deformed to an ultra-high strain by high pressure torsion

Polycrystalline Ni of two purities (99.967% (4N) and 99.5% (2N)) was deformed to an ultra-high strain of ε vM = 100 ( ε vM , von Mises strain) by high pressure torsion at room temperature. The 4N and 2N samples at this strain are nanostructured with an average boundary spacing of ∼100 nm, a high density of dislocations and a large fraction of high angle boundaries (>15°) of 0.68–0.74, as determined by transmission electron microscopy, and 0.8–0.83, as determined by electron backscattering diffraction. The deformed samples were annealed isochronally for 1 h at temperatures from 100 to 600 °C, and the evolution of the structural parameters (boundary spacing, average boundary misorientation angle and the fraction of high angle boundaries), crystallographic texture and hardness were determined. Based on microstructural parameters the energy stored in the deformed state was estimated to be 14 MPa and 24 MPa for 4N Ni and 2N Ni, respectively. The isochronal annealing leads to a drop in hardness in three stages: a relatively small decrease at low temperatures (recovery), followed by a rapid decrease at intermediate temperatures (recrystallization) and a slow decrease at high temperature (grain growth). Both recovery and recrystallization of the 2N Ni are strongly retarded by the presence of impurities reducing the mobility of boundaries. In the recrystallization stage, changes in hardness, microstructure and texture show that the 4N Ni recrystallizes discontinuously, in spite of a large fraction of high angle boundaries in the deformed state. This finding contradicts previous experiments and theory, which suggest that recrystallization is continuous when the fraction of high angle boundaries is high. In the 2N Ni, the observations suggest that some structural coarsening (continuous recrystallization) may take place simultaneously with discontinuous recrystallization. The findings emphasize the importance of alloying, which can delay the process of recovery and recrystallization and thereby enable tailoring of the microstructure and properties through an optimized annealing treatment.

Effect of electrical fatigue on the electromechanical behavior and microstructure of strontium modified lead zirconate titanate ceramics

The fatigue behavior of strontium doped lead zirconate titanate (PSZT) ceramics is investigated. Different compositions near the antiferroelectric–ferroelectric (AF–F) phase boundary are synthesized by tape casting and sintering route. The influence of electric field-induced AF to F phase transition on piezoelectric and strain behaviors is studied. Very high maximum polarization (∼41 μC/cm 2) and ultrahigh strain (0.8%) are seen for some of the PSZT compositions near the morphotropic phase boundary. The samples are subjected to low frequency (30 Hz) electric field up to 10 7 cycles. Although the maximum polarizations of most of the PSZT ceramics showed fatigue-free behavior (less than 10% degradation), the strains in most of them showed degradation as high as over 50%. Electron microscopy of the fractured surface of the electrically cycled samples showed some intergranular fracture below the electroded surface. Results indicated that diffusion of silver electrode into the PSZT ceramics is responsible for the electro-coloration and degradation in strain response with fatigue cycles. Thermal annealing and removal of the damaged layer under the electrode showed the complete recovery of the strain to its original as-sintered value. X-ray diffraction technique is used before and after the fatigue cycles to investigate the influence of electrical cycles on the changes at the crystal structure level, which are also related to the fatigue-induced changes to electromechanical properties of PSZT ceramics.

Deformation Modes in Stainless Steel During Laser Shock Peening

In order to study the deformation mechanisms during ultrahigh strain rate deformation of face centered cubic metals, laser shock peening of a 304L stainless steel is systematically investigated. Two deformation modes—microtwins and microbands—and their interrelationship during high strain rate deformation are discussed in detail. Transmission electron microscopy and selected area electron diffraction are employed to study the deformation modes. It is found that twinning takes place even when the shock pressure is much less than the critical twinning stress in stainless steels. Theoretical critical twinning stress is not the only criteria to decide the deformation modes of twinning or slip. The formation of twinning and slip can be affected by the factors such as loading profile, loading stress/strain rate, stacking fault energy, grain sizes, and cell substructures. Factors that influence twin-slip transition in shock loading are discussed. The formation of dislocation structure is compared with those predicted using 3D dislocation dynamic simulation.

Deformation Behaviors and Critical Parameters in Microscale Laser Dynamic Forming

Microscale laser dynamic forming (μLDF) is a novel microfabrication technique to introduce complex 3D profiles in thin films. This process utilizes pulse laser to generate plasma to induce shockwave pressure into the thin film, which is placed above a microsized mold. The strain rate in μLDF reaches 106–107 S−1. Under these ultrahigh strain rates in microscale, deformation behaviors of materials are very complicated and almost impossible to be measured in situ experimentally. In this paper, a finite element method model is built to simulate the μLDF process. An improved Johnson–Cook model was used to calculate the flow stress, and the Johnson–Cook failure criterion was employed to simulate failure during μLDF. The simulation results are validated by experiments, in which the deformation of Cu thin foils after μLDF experiments are characterized by scanning electron microscopy and compared with simulation results. With the verified model, the ultrafast μLDF process is generally discussed first. A series of numerical simulations are conducted to investigate the effects of critical parameters on deformation behaviors. These critical parameters include the ratio of the fillet radius to film thickness, the aspect ratio of mold, as well as laser intensities. The relationship of laser pulse energy and the deformation depth is also verified by a series of μLDF experiments.

On measuring the strength of metals at ultrahigh strain rates

The strain rate sensitivity of materials is normally measured through a combination of quasistatic, Hopkinson bar, and pressure-shear experiments. Recent advances in uniaxial strain ramp loading provide a new means to reach strain rates significantly higher than achievable in pressure-shear experiments. One way to determine strength in ramp loading is by comparing the uniaxial stress-strain response to an appropriate pressure-density response obtained from an equation of state for the material. Using this approach, strengths for aluminum are obtained for strain rates of 105–108 s−1. Two issues arise in this calculation: heating due to plastic work and the effect of the superimposed hydrostatic stress on the strength. Heating due to plastic work is calculated and accounted for within the context of the equation of state for the material in a straightforward manner, but neglecting this heating can lead to significant errors in the calculated strength at higher compression levels. A simple scaling of strength with the pressure-dependent shear modulus is utilized to estimate the strength at zero pressure for ramp loading and pressure-shear experiments. When examined in this manner, the strain rate dependence of aluminum is found to be less than previously reported, with little increase in strength below strain rates of about 107s−1. The effects on ramp loading strength measurements of heating due to plastic work and of hydrostatic pressure are also examined for copper and tantalum using simple equation of state and strength models. The effect of plastic heating is similar for the three materials for a given strain level but quite different for a constant stress, with aluminum showing greater effects than the other materials. The effect of hydrostatic pressure in ramp loading experiments is similar for all three materials, but the effect is likely to be much greater in pressure-shear experiments for aluminum than the other materials.

Tensile and flexural properties of ultra high toughness cemontious composite

The tensile and flexural properties of polyvinyl alcohol (PVA) fiber reinforced ultra high toughness cementitious composite (UHTCC) were investigated. The composite, tested at the age of 14 d, 28 d and 56 d, shows extremely remarkable pseudo strain hardening behavior, saturated multiple cracking and ultra high ultimate strain capacity above 4% under uniaxial loading. Also, the corresponding crack widths are controlled under 50 μm even at 56 days age. In the third point bending tests on thin plate specimens, the composite shows ultra high flexural ductility and multiple cracking on the tension surface. The high ultimate flexural strength/first tensile strength ratio of about 5 verifies the pseudo strain hardening behavior of UHTCC. SEM observation on fracture surfaces provides indirect evidence of optimal design for the composite.

Reverberation technique for yield strength experiments at ultrahigh pressure and strain rate

Ultrahigh pressure and strain-rate (pressure ⪢10 GPa and strain rate ⪢105 s−1) are now attainable using high-power laser systems. A laser pulse drives a shock through a reservoir material which then unloads onto the target specimen. Laser velocimetry measurements at the back surface of the specimen are used to infer the material response. Material strength experiments have been proposed in which the pressure wave reverberates within the specimen and the cumulative resistance to compression by material strength is measured from the recorded velocimetry signature. In this work, ultrahigh pressure and strain-rate reverberation experiments are performed in tantalum, aluminum, and copper to investigate the yield strength behavior in this extreme regime. The experimental results indicate that the measurement sensitivity to yield strength is dominated by the lateral wave effects or impeded by the occurrence of spall. However, computer hydrodynamic code simulations are used to develop an experimental design based on shaping of the laser drive pulse that circumvents these difficulties. Simulations using the Steinberg–Guinan yield strength model and applying the design indicate the capability of measuring a factor-of-2 strength deviation from that predicted by the model with accuracy equal to or greater than the velocimetry measurement error.