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The high-strain-rate response of pure magnesium and AZ31B magnesium alloy is examined in compression and in a forced shear-loading top-hat configurations. Compression specimens loaded in the direction normal to the plane of the rolled plate (TT) display higher-strain-rate sensitivity than specimens that were loaded within the plane of the rolled plate (IP). This effect is more pronounced for pure magnesium as compared to the alloy, due to increased twinning in the IP direction as compared to the TT. Additionally, top-hat shear specimens loaded at high strain rates are observed to display stable deformation during loading, and the development of adiabatic shear bands is not observed. We hypothesize that this result is due to adiabatic heating during deformation, which enhanced the contribution of slip, lessened the role twinning, and possibly activated dynamic recrystallization processes, thus, preventing the formation of distinct shear bands. more...

Recent work in both 304L and 316L stainless steel produced by additive manufacturing (AM) has shown that in addition to the unique, characteristic microstructures formed during the process, a fine dispersion of sub-micron particles, with a chemistry different from either the powder feedstock or the expected final material, are evident in the final microstructure. Such fine-scale features can only be resolved using transmission electron microscopy (TEM) or similar techniques. The present work uses electron microscopy to study both the initial powder feedstock and microstructures in final AM parts. Special attention is paid to the chemistry and origin of these nanoscale particles in several different metal alloys, and their impact on the final build. Comparisons to traditional, wrought material will be made. more...

For additive manufacturing (AM) of metallic materials, the certification and qualification paradigm needs to evolve as there is currently no broadly accepted “ASTM- or DIN-type” additive manufacturing certification process or AM-produced material specifications. Accordingly, design, manufacture, and subsequent implementation and insertion of AM materials to meet engineering applications requires detailed quantification of the constitutive (strength and damage) properties of these evolving materials, across the spectrum of metallic AM methods, in comparison/contrast to conventionally-manufactured metals and alloys. For this study, cylindrical samples of 316L SS were produced using a LENS MR-7 laser additive manufacturing system from Optomec (Albuquerque, NM) equipped with a 1 kW Yb-fiber laser. The microstructure of the AM-316L SS was characterized in both the “as-built” AM state and following a heat-treatment designed to obtain full recrystallization to facilitate comparison with annealed wrought 316L SS. The constitutive behavior as a function of strain rate and temperature was characterized and is compared to that of annealed wrought 316L SS plate material. The dynamic shock-loading-induced damage evolution and failure response of all three 316L SS materials was quantified using flyer-plate impact driven spallation experiments at peak stresses of 4.7 and 6.5 GPa. The spall strength of AM-produced 316L SS and the recrystallized-AM-316L SS were found to decrease with increasing peak shock stress while the annealed wrought 316L SS spall strength remained essentially constant. The damage evolution, characterized using optical metallography and electron-backscatter diffraction (EBSD), was found to vary significantly across the three 316L SS microstructures while the three samples loaded to a peak shock stress of 6.5 GPa displayed only ∼12% differences in spall strength. more...

A new specimen geometry, the compact forced-simple-shear specimen (CFSS), has been developed as a means to achieve simple shear testing of materials over a range of temperatures and strain rates. The stress and strain state in the gage section is designed to produce essentially “pure” simple shear, mode II in-plane shear, in a compact-sample geometry. The 2-D plane of shear can be directly aligned along specified directional aspects of a material's microstructure of interest; i.e., systematic shear loading parallel, at 45°, and orthogonal to anisotropic microstructural features in a material such as the pancake-shaped grains typical in many rolled structural metals, or to specified directions in fiber-reinforced composites. The shear-stress shear-strain response and the damage evolution parallel and orthogonal to the pancake grain morphology in 7039-Al are shown to vary significantly as a function of orientation to the microstructure. more...

The microstructural changes and mechanical response of an HT-9 sample shock loaded to a peak pressure of 11 GPa have been investigated by TEM, XRD, microhardness and EBSD techniques. Dislocation densities obtained by both direct measurements (via TEM) and indirect calculations (by XRD and hardness) indicate that shock loading results in ~2–3 fold increase in dislocation density. TEM analyses show that the shape, and density of the dislocations change after shock loading. In addition, shock loading causes local plastic deformation of the continuous parallel lath structure in some regions, together with an overall decrease in the aspect ratio of laths due to local plastic deformation and lath fragmentation. As a result of XRD analyses, the fraction of edge dislocations is determined to increase by ~24% after shock loading. Furthermore, hardness increases by ~40 HV after shock loading due to the increased dislocation density. EBSD analyses show that the fraction of CSL boundaries decreases by ~5–10% as a result of shock loading. more...

The influence of temperature on the dynamic tensile behavior of Zr has been investigated. Bullet-shaped Zr samples with two different textures were dynamically extruded at room temperature and 523 K (250 °C). A higher ductility was measured for samples deformed at elevated temperature as compared to those extruded at room temperature. This difference in ductility is discussed in terms of zirconium’s ability to accommodate plastic deformation via thermally enhanced slip activity, as evidenced by examination of the deformed microstructures. more...

Metallic based multi-layered nano-composites are recognized for their increased plastic flow strength and indentation hardness, increased ductility, improved radiation damage resistance, improved electrical and magnetic properties, and enhanced fatigue failure resistance compared to conventional metallic materials. One of the ways in which these classes of materials are manufactured is through accumulated roll bonding where the material is produced by several rolling and heat treatment steps during which the layer thickness is reduced through severe plastic deformation. In this article, a single rolling pass of the accumulated roll bonding process in which a Cu/Nb layered composite with an initial average layer thickness of 24 μm subjected to a 50% height reduction is examined. Experimental morphological and crystallographic texture data is presented and used to initialize numerical models for these layered material systems. This study focuses on the Cu/Nb combination of fcc/bcc materials for incoherent material interfaces. A single crystal model based upon thermally activated dislocation motion is used and the evaluation of material parameters is presented. Since the initial state of the composite is not in a fully annealed condition, nano hardness tests for both the Cu and Nb layers are used to initialize the model for each of the two materials. EBSD data of the heat treated material is used to characterize the initial state of the composite and used to produce 40 combined morphological and crystallographic numerical model realizations of the material. Each of the models is then imposed to a plane strain compression deformation height reduction of 50%. Numerical data from the 40 simulations was then combined to arrive at the statistically comparable crystallographic texture for each of the Cu and Nb materials. This data was compared with the experimental data and the results suggest very good agreement between the predicted and experimental textures for both the materials. Selected stress profile predictions for two realizations are presented and demonstrate a strong difference in stress state between the Cu and Nb layers. more...

The bulk compressive and tensile response of samples machined from the different regions of dynamically damaged OFHC copper is examined in combination with post-mortem analysis of the recovered sample. This was done using optical microscopy and electron back-scatter diffraction to understand the strength and mechanical behavior of a material deformed under dynamic loading conditions. Stored plastic work in the shocked samples produced significant hardening in all the samples. It was also observed that the samples containing a dynamically induced damage field were slightly harder and showed enhanced yield strength over material simply subjected to shock and release that showed no dynamic damage. This was attributed to the excess work hardening around voids induced during incipient spall. By comparing the responses of two specimens with differing amounts of dynamic damage, it was concluded that there appears to be a limit to the amount of dynamic damage, after which the enhanced yield strength in the materi... more...

For additive manufacturing (AM) of metallic materials, the certification and qualification paradigm needs to evolve as there currently exists no broadly accepted “ASTM- or DIN-type” additive manufacturing certified process or AM-material produced specifications. Accordingly, design, manufacture, and thereafter implementation and insertion of AM materials to meet engineering applications requires detailed quantification of the constitutive (strength and damage) properties of these evolving materials, across the spectrum of metallic AM methods, in comparison/contrast to conventionally-manufactured metals and alloys. This report summarizes the 316L SS research results and presents initial results of the follow-on study of 304L SS. For the AM-316L SS investigation, cylindrical samples of 316L SS were produced using a LENS MR-7 laser additive manufacturing system from Optomec (Albuquerque, NM) equipped with a 1kW Yb-fiber laser. The microstructure of the AM-316L SS was characterized in both the “as-built” Additively Manufactured state and following a heat-treatment designed to obtain full recrystallization to facilitate comparison with annealed wrought 316L SS. The dynamic shock-loading-induced damage evolution and failure response of all three 316L SS materials was quantified using flyer-plate impact driven spallation experiments at peak stresses of 4.5 and 6.35 GPa. The results of these studies are reported in detail in the first sectionmore » of the report. Publication of the 316L SS results in an archival journal is planned. Following on from the 316L SS completed work, initial results on a study of AM 304L SS are in progress and presented herein. Preliminary results on the structure/dynamic spallation property behavior of AM-304L SS fabricated using both the directed-energy LENS and an EOS powder-bed AM techniques in comparison to wrought 304L SS is detailed in this Level 2 Milestone report.« less more...

Experiments were conducted to characterize the influence of texture and impact velocity on the dynamic, high-strain, tensile extrusion of zirconium. Bullet-shaped samples were machined from a clock-rolled, highly textured Zr plate. Specimens in two orthogonal directions were tested: the extrusion direction aligned with either the in-plane (IP) rolling or the through-thickness (TT) direction of the plate. The post-extrusion microstructure and texture evolution were examined using electron backscatter diffraction microscopy and modeled using the viscoplastic self-consistent model. It was found that extrusion deformation was accomplished through a combination of twinning and slip with their relative activity greatly depending on the initial texture. In this regard, higher elongations in the IP samples as compared to the TT samples were observed at similar test velocities. This difference in ductility is discussed in terms of the material’s ability to accommodate plastic deformation. Due to the availability of a larger number of slip systems with relatively high Schmid factors in the IP samples under this configuration, plastic deformation by prismatic slip can be easily achieved, resulting in larger elongations. On the contrary, for TT samples, twinning preceded deformation by slip. This sequential deformation process, driven by the need to reorient the microstructure favorably to slip, led to diminished elongations to failure. more...

While numerous investigations have examined microstructural, substructural and damage evolution due to shock loading, few of these studies have directly linked substructural evolution as a function of crystallographic orientation with nucleation of damage during shock loading. In this work, quantitative characterization of damage and substructural evolution in bicrystal copper reveals that the density of dislocation cells based on activation of available slip systems due to Schmid factor analysis influences damage nucleation in copper. more...

The response of metallic materials to shock loading, like all loading regimes, is controlled largely by factors operating at the microscopic or atomic levels. Over the past few years, face-centred cubic (fcc) metals have received a level of attention where the role of features such as stacking fault energy and precipitation hardening have been investigated. We now turn our attention to body-centred cubic (bcc) metals. In the past, only tantalum, tungsten, and their alloys have received significant attention at high strain-rate conditions due to their use by the ordnance community. In particular, this investigation examines the shear strength of these materials at shock loading conditions. Previous results on tantalum, tungsten, and a tungsten heavy alloy are reviewed, and more recent experiments on niobium, molybdenum, and Ta–2.5 wt% W presented. Results are discussed in terms of known deformation mechanisms and variations of Peierl’s stress. more...

The behaviour of the magnesium alloy AZ61 during one-dimensional shock loading has been investigated in terms of the Hugoniot (shock-induced equation of state) and the shear strength variation in terms of variation with impact stress and pulse duration. The Hugoniot has been shown to be near identical with the similar AZ31, as would be expected with related dilute alloy systems. The shear strength has been observed to increase with applied shock stress, in common with many other materials. The shear strength has also been observed to increase with time behind the shock front. It has been suggested that this be due to twinning early in the deformation process, followed by dislocation based at later stages. more...

The purpose of this study is to quantify the influence of textural and microstructural anisotropy on spallation. This includes the influence of anisotropically-oriented MnS inclusion stringers in the HY-100 and 1080 steels on spallation, within two crystallographically-isotropic steels, and the influence of strong, anisotropic crystallographic texture in high-purity polycrystalline Ta and Zr materials to assess the role of texture on damage evolution and spallation responses. The effect of anisotropic crystallographic texture on the spallation response of Ta and Zr is shown to play a minimal role in the spallation response of each material, as seen in wave profile pull-back signals, compared to the effect of texture on the shock arrival time and the Hugoniot elastic limit that reflects strength in these two high-purity materials. In the case of both the 1080 and HY-100 steels, the influence of elongated MnS stringers, resident within the essentially crystallographically isotropic steels, was found to be dominated by the heterogeneous nucleation of damage orthogonal to the MnS stringers. Delamination between the pearlitic matrix microstructure and the MnS stringers in the 1080 steel, or inclusions and the martensitic matrix in the HY-100 steel, was seen to correlate to a lower pull-back signal during transverse loading than to that parallel to the stringer axis in each steel. more...

Shear localization is often a failure mechanism in materials subjected to high strain rate loading. While the constitutive behavior of copper has been extensively studied, the influence of cold work, strain rate, and temperature on the microstructural development in Cu under shear loading conditions has received less systematic quantification. The purpose of this study is to quantify the mechanical response and the microstructural evolution of as-annealed and cryogenically rolled copper loaded dynamically in shear and to understand the mechanisms controlling shear deformation as well as the role of stored defects due to cryo-rolling on shear localization. It was found that localization is promoted in the cryo-rolled copper as compared to the as-annealed material and this instability is linked to stored defect structure specific to the cryo-rolled microstructure and its influence on the subsequent defect generation and storage in dynamically loaded Cu. more...

The microstructure and texture variations near adiabatic shear localization regions in as-received and pre-shocked 304 stainless steels were investigated using electron backscatter diffraction (EBSD). Shear bands were developed in both steels during a forced dynamic shear deformation produced on a split-Hopkinson pressure bar. EBSD examination revealed large intragranular misorientation gradients toward the shear bands. Taylor factor and misorientation analyses were applied to explain deformation characteristics. Shear-induced deformation structure was observed to be sensitive to both initial microstructure and local orientation. Comparison of as-received and pre-shocked steels indicated that the pre-shocked condition yielded sharper shear band boundaries but moderate misorientation gradients, while the as-received condition led to substantial misorientation adjacent to the band. Shear-induced texture evolution generally resulted in the reorientation of 〈1 1 0〉 toward the shear direction for both cases. Grains with slip systems favorably oriented with respect to the applied shear path displayed less intragranular misorientation development compared to unfavorably oriented grains. more...

The response of polycrystalline metals, which possess adequate mechanisms for plastic deformation under extreme loading conditions, is often accompanied by the formation of pores within the structure of the material. This large deformation process is broadly identified as progressive with nucleation, growth, coalescence, and failure the physical path taken over very short periods of time. These are well known to be complex processes strongly influenced by microstructure, loading path, and the loading profile, which remains a significant challenge to represent and predict numerically. In the current study, the influence of loading path on the damage evolution in high-purity tantalum is presented. Tantalum samples were shock loaded to three different peak shock stresses using both symmetric impact, and two different composite flyer plate configurations such that upon unloading the three samples displayed nearly identical "pull-back" signals as measured via rear-surface velocimetry. While the "pull-back" signals observed were found to be similar in magnitude, the sample loaded to the highest peak stress nucleated a connected field of ductile fracture which resulted in complete separation, while the two lower peak stresses resulted in incipient damage. The damage evolution in the "soft" recovered tantalum samples was quantified using optical metallography, electron-back-scatter diffraction, and tomography. These experiments are examined numerically through the use of a model for shock-induced porosity evolution during damage. The model is shown to describe the response of the tantalum reasonably well under strongly loaded conditions but less well in the nucleation dominated regime. Numerical results are also presented as a function of computational mesh density and discussed in the context of improved representation of the influence of material structure upon macro-scale models of ductile damage. more...

The response of pure nickel (Ni), a binary Ni-60 at. pct cobalt (Co) alloy exhibiting a low stacking fault energy (SFE), and the ordered face-centered-cubic (fcc) alloy Ni-24Al-0.01B to shock loading has been studied using the technique of plate impact. Changes in the variation of mechanical properties with shock amplitude and pulse duration have been explained in terms of a shift from dislocation dominated to twin dominated plasticity in the case of the Ni-Co alloy and the increasing effect of brittle failure in the case of Ni3Al. more...

Commercial purity aluminum AA1050 billets with similar submicrometer grain sizes but distinctively different boundary structures were obtained by the use of different equal channel angular extrusion (ECAE) routes. Route “A” is defined as no rotation between extrusion passes, while route “C” is rotated 180° between extrusion passes. The microstructure processed by route “A” consists mainly of high-angle boundaries (HABs) while route “C” has primarily low-angle boundary (LAB) structures. Tensile tests conducted on these two microstructures at intermediate and dynamic strain rates at both 77 and 298 K were compared with the results of material tested at quasi-static strain rates. The boundary structure was found to play an important role in the mechanical properties at low temperature. more...

The microstructural evolution in a cold-rolled 316L stainless steel during shear localization was comprehensively studied using transmission electron microscopy (TEM). The TEM results indicate that the main substructure inside a shear band consists of elongated lath, fine rectangular and equiaxed subgrains. The substructure at an early stage of shear banding was found to strongly depend upon existing defects, especially deformation twin patterns. These twin structures determine the initial dimensions of the elongated subgrains inside the shear bands. The coexistence of both rectangular and equiaxed subgrains suggests that no melting occurred though the predicted temperature was much higher than the bulk melting temperature. Dynamic/static recovery and continuous dynamic recrystallization are thought to be the main mechanisms by which these substructures form inside the shear bands. A new mechanism for nanostructure deformation of subgrains within shear bands is proposed to explain the temperature divergence between the experimental and calculated results. more...

A systematic experimental investigation into the effects of temperature on the stress–strain response of NiTi (55.6 wt.% Ni) shape memory alloy at high strain rate was conducted and compared with the behavior at quasi-static strain rate. Dynamic compression and tension tests, to total strains of 16–24%, were performed at high strain rate (∼1200/s) and under quasi-static conditions at temperatures from −196 to 400 °C. Since the superelastic range for the NiTi alloy used in this work is Af (2 °C) 0 °C and the (thermally induced) martensite phase at lower temperatures. The results indicate differences in the stress–strain response of thermally induced martensite and stress-induced martensite, in terms of plateau stress characteristics and critical stress (as determined by 0.2% strain offset). The observations illustrate a complex interplay of test temperature, stress state (compression and tension) and martensite type (thermally induced vs. stress induced) that lead to the asymmetry in compression vs. tension response of the shape memory alloy in both quasi-static and dynamic loading conditions. This asymmetry was captured in the variation of critical stress with temperature that exhibited a three-stage character, with the critical stress being higher in compression than in tension. These findings have significant implications for the understanding and exploitation of the underlining functional characteristics of shape memory alloys, especially at high strain rates. more...

The evolution of adiabatic shear localization in an annealed AISI 316L stainless steel has been investigated and was reported in Part I of this paper (Met. Trans. A, 2006, Vol. 37A, pp. 2435–446). In the present research (Part II), a comprehensive transmission electron microscopy (TEM) examination was conducted on the microstructural evolution of shear localization in this material at different loading stages. The TEM results indicate that elongated subgrain laths and an avalanche of dislocation cells are the major characteristics in an initiated band. Development of the substructures within shear bands is controlled by dynamic recovery and continuous dynamic recrystallization. The core of shear bands was found to consist of fine equiaxed subgrains. Well-developed shear bands are filled with a mixture of equiaxed, rectangular, and elongated subgrains. The equiaxed subgrains, with a typical size less than 100 nm, are postulated to result from either the breakdown and splitting of subgrain laths or the reconstruction of subcells. more...

Adiabatic shear localization in an annealed AISI 316L stainless steel was examined through a forced shear technique using a split Hopkinson pressure bar and hat-shaped specimens. A well-controlled forced shear technique provided the possibility of correlating the microstructural evolution of adiabatic shear localization to its transient mechanical behavior. The initiation of adiabatic shear bands occurred when the shear stress peaked after substantial work hardening. The work-hardening rate was found to play a dominant role in the formation of adiabatic shear localization. The stress drop presupposed the development of the localized deformation. A core structure of shear bands was generated within the shear band, which characterized a narrow-down process in the early stage of the shear band evolution. The continuous expansion of the shear band core to the entire width of the band was seen to correlate with the full development of shear localization. more...

A rigorous experimentation and validation program is being undertaken to create constitutive models that elucidate the fundamental mechanisms controlling plasticity in uranium-6 wt.% niobium alloys (U-6Nb). These models should accurately predict high-strain-rate large-strain plasticity, damage evolution and failure. The goal is a physically-based constitutive model that captures 1) an understanding of how strain rate, temperature, and aging affects the mechanical response of a material, and 2) an understanding of the operative deformation mechanisms. The stress-strain response of U-6Nb has been studied as a function of temperature, strain-rate, and thermal aging. U-6Nb specimens in a solution-treated and quenched condition (ST/Q) and after subsequent aging at 473K for 2 hours were studied. The constitutive behavior was evaluated over the range of strain rates from quasi-static (0.001 sec -1 ) to dynamic (∼2000 sec -1 ) and temperatures ranging from 77 to 773K. The yield stress of U-6Nb was exhibited pronounced temperature sensitivity. The strain hardening rate is seen to be less sensitive to strain rate and temperature beyond plastic strains of 0.10. The yield strength of the aged material is less significantly affected by temperature and the work hardening rate shows adiabatic heating at lower strain rates (1/s). more...

The work-hardening behavior of hexagonal-close-packed (hcp) metals, such as hafnium, is influenced by temperature, strain rate, chemistry, and texture. In the case of hafnium, while slip on the prism and pyramidal planes is dominant during deformation, the propensity of deformation twinning is known to increase with decreasing temperature and increasing strain rate. In this study, hafnium was prestrained quasi-statically in compression at liquid nitrogen temperature (77 K), creating a heavily twinned microstructure. The specimens were then reloaded in compression at room temperature (298 K). Yield stress, flow stress, and work-hardening behaviors of the prestrained specimens were higher than room-temperature compression test data typical of the as-annealed material. The microstructure of each specimen was characterized optically and using a transmission electron microscope (TEM). Texture was measured by neutron diffraction and the texture evolution due to twinning, and the interaction of slip with the twins was seen to lead to higher work-hardening rates and flow stresses in the cold prestrained specimens. more...

The mechanical behavior and substructural evolution of highly textured hafnium (Hf) has been examined in tension and compression and compared to the mechanical response of zirconium (Zr). The quasi-static work-hardening rate as a function of strain for both metals exhibits a compression-tension asymmetry. Both Zr and Hf exhibit a downward work-hardening response in tension, while each displays a parabolic and then concave upward work-hardening behavior in compression. Additionally, Hf displays higher flow stresses than Zr both in tension and compression. The stress-strain and strain-hardening curves for Zr and Hf have been characterized in terms of their propensity for deformation twinning and evolution of substructure with strain. Differences in the work-hardening rates and flow stresses as a function of the sense of the applied load and material are discussed in terms of slip-twin interactions during deformation. more...

We have investigated the formation mechanisms of nanostructures within adiabatic shear bands formed in stainless steel samples deformed by high-strain-rate forced shear. Twinning is shown to play a critical role in the initiation of nanostructures. Secondary twins directly led to the formation of elongated subgrains. Microtwins inside shear bands promoted division and break-down of the subgrains, which further refined the microstructures. more...

Commercial purity aluminum AA1050 was subjected to one of two equal channel angular extrusion (ECAE) routes. Route “A” is defined as no rotation between passes and route “C” is rotated 180° between extrusion passes. The resulting microstructures have similar grain sizes but relatively different grain boundary structures. The microstructure processed by route “A” has approximately 70% high angle boundaries (HABs), while the route “C” has approximately 40% HABs. The boundary structure plays an important role in the mechanical properties of the material because these boundaries act as a source and sink of dislocations. Mechanical tests conducted on these two microstructures at intermediate and dynamic strain rates at both 77 and 298 K are compared with the results of material tested at quasi-static strain rate. more...

The mechanical responses of high-explosive (HE)-driven “Taylor-Wave” shock prestrained, 20 pct quasistatically uniaxially compressed, and 32 pct cross-rolled 316 L austenitic stainless steel (SS) samples were investigated in compression and tension at room temperature. The results of the compression and tension tests were compared to the deformation results of the annealed microstructure at a strain rate of 10−3 s−1. The mechanical behavior of the 316 L SS following explosive shock prestraining exhibits a factor of 2 increase in yield stress over that observed for the annealed material. A significant reduction in ductility due to the Taylor-wave shock prestraining of the 316 L SS was also observed in tension. The microstructure and substructure evolution of the shock-prestrained 316 L SS samples were investigated using optical metallography and transmission electron microscopy (TEM). Microstructural analyses revealed a high density of deformation twins following Taylor-wave shock prestraining and cross-rolling to 32 pct strain. These microstructures were compared to those of the 20 pct quasi-statically uniaxially prestrained 316 L SS. The current experimental results were found to agree with previous literature results on SS samples shock-prestrained utilizing “square-topped” and explosive Taylor-wave shock-pulse loading. more...

Adiabatic shear localization was studied in both annealed (as-received) and shock-prestrained 304 stainless steels (304 SS). A forced shear technique was used to probe the evolution of shear localization under high-strain-rate loading using a compression split-Hopkinson pressure bar (SHPB) and hatshaped specimens. The shearing responses of the shock-prestrained steel at high strain rate indicate that they exhibit a higher initial yield strength but a lower work-hardening rates than those observed in the annealed steel. The shock-prestrained specimens, having achieved an unstable condition at a smaller plastic displacement and a lower flow stress, were seen to exhibit distinct sharp-edged bands. Corresponding comparisons between the mechanical responses and the deformed microstructures demonstrate that loading-path dependency exerts a strong influence on shear-band formation. The shock-prestrained 304 SS displayed a higher incidence of deformation twins, slip bands, and dislocation debris prior to the shear tests, this substructure suppressed further dislocation storage (work hardening) and thereby facilitated the propensity for shear localization. more...

The mechanical response of copper (Cu) and 316 L austenitic stainless steel (SS) shock loaded to 6.6 GPa in both square-topped and triangular pulse wave configurations has been investigated in compression at room temperature. The microstructure and substructure evolution due to shock pre-straining has been characterized using optical metallography and transmission electron microscopy (TEM). Both Cu and 316 L SS following shock pre-straining exhibit an increase in yield stress. There is, however, a difference in the yield stress measured in Cu samples shocked with a triangular-shaped “Taylor wave-like” pulse versus with a square-top wave; Cu samples shocked with the “Taylor wave-like” pulse shape exhibit a 10% higher yield stress than Cu samples shocked with square-topped wave. Yield stresses for both 316 L shocked with the “Taylor wave-like” and square-topped waves are essentially the same. Cu samples shocked with a triangular-shaped “Taylor wave-like” pulse show subtle microstructural differences to those shocked with a square-top shaped shock pulse loading. In addition, low temperature resistivity measurements also showed differences. Triangular wave shocked samples exhibited the highest resistivity at 4 K indicating the highest defect (point and dislocation) formation in copper samples. TEM examinations of copper samples revealed a change in the microstructure after quasi-statically straining (10−3 s−1) to 5% plastic strain as compared to that observed in shocked samples (shocked with square-topped and triangular shaped waves) even though the microstructure consists of only dislocation cells. TEM examinations of 316 L SS revealed similar microstructural evolution in all 316 L SS samples: 5% quasi-statically strained, square-top wave and triangular-shaped “Taylor wave-like” shock pre-strained samples, consisting of dislocations, planar slip and twins; however, the amount of twinning appeared higher in the shocked samples. more...

The pressure of the α–ω phase transition in zirconium is quantified as a function of interstitial content. The pressure increases with increasing interstitial oxygen content and for the high purity (Zr 0 ) material occurs at 7.1 GPa. Increasing the oxygen content increases the number of octahedral sites occupied; this is postulated to increase the pressure for phase transformation. Deformation behavior and substructural evolution of as-annealed Zr 0 zirconium under quasi-static conditions is compared to its response following shock prestraining at 5.8 and 8 GPa. The reload response of Zr 0 zirconium shock prestrained to 8 GPa exhibits enhanced hardening, while specimens shocked to 5.8 GPa do not when compared to quasi-static constitutive behavior. more...

The mechanical response of high-explosive (HE)-driven “Taylor-wave” (TW) shock-prestrained 304 stainless steel loaded to 35 to 45 GPa has been investigated in compression and tension at room temperature and compared to results from compressive testing of the annealed microstructure at a strain rate of 10−3/s. The microstructure and substructure evolution due to TW shock prestraining has been investigated using optical metallography and transmission electron microscopy. The mechanical behavior of 304 stainless steel following explosive shock prestraining is shown to exhibit a factor of 2 increase in yield stress and a significant reduction in ductility under tension. Microstructural analyses reveal a high density of twins after TW prestraining and very little production/retention of deformation-induced martensite. The current experimental results are shown to agree well with previous literature results on stainless steel samples shock-prestrained utilizing “square-topped” (ST) shock-pulse loading. This finding suggests that the peak shock pressure, which controls the total plastic strain that the sample must accommodate, is the dominant variable in shock-wave-loading defect generation and storage in 304 stainless steel. more...

The dynamic deformation, damage evolution, and cracking in four cast gamma titanium aluminide alloys have been investigated experimentally and theoretically. In this study, the dynamic compressive constitutive properties of several cast TiAl alloys were measured at quasi-static and dynamic rates. Using the measured high-strain-rate constitutive properties, the deformation, damage evolution, and postmortem specimen geometries observed in Taylor cylinder experiments were predicted using the mechanical threshold stress (MTS) constitutive model. The utility of the Taylor impact test for validation of high-rate constitutive models for intermetallics is discussed. more...

The compressive mechanical behavior of highly textured, polycrystalline hafnium has been examined as a function of texture, strain rate, temperature, and material chemistry. The microstructural and substructural evolution in Hf was also examined as a function of texture. Decreasing temperature, increasing strain rate, and increasing impurity concentrations were found to increase the yield-stress and work-hardening rates, as well as increase the amount of twinning in Hf. Crystallographic texture was found to exhibit the most marked effect on the mechanical behavior of Hf. Differences in the orientation of the c-axis with respect to the loading direction were found to affect the yield stress, work-hardening behavior, and anisotropy of the tested specimen, with the highest yield stresses and rates of work hardening and the lowest anisotropies in specimens compressed along the c-axis. The amount of deformation twinning and the slip systems activated during deformation were seen to vary based on texture and are shown to correlate well with the observed yield stresses and work-hardening behaviors. more...

Lateral stresses generated by shock loading in tantalum have been determined using manganin stress gauges. These have been used in combination with the measured longitudinal impact stresses to determine the shear strength behind the shock. Results show that with an increase in impact stress, the shear strength in tantalum also increases. Analysis shows that during shock loading the lateral stress in tantalum increases behind the shock front. Since the longitudinal stress is nominally constant until arrival of the release, this implies that the shear strength is reducing behind the shock front. The shock-wave response of tantalum is discussed in the context of a previous weak-shock wave-profile analysis of tantalum, and in terms of the defect generation and storage response of pure face-centered- versus body-centered-cubic metals. more...

The shock induced mechanical response of two γ-titanium alurninides has been investigated using the technique of plate impact. It has been found that the HEL, spall strength and shear strengths are all dependent upon the initial microstructure, with the material possessed of the duplex microstructure being the stronger. This is in agreement with quasi-static measurements. Shear strength has been observed to increase rapidly with increasing longitudinal stress, more so than in Ti-6Al-4V, indicating that the high degree of work hardening is an important factor in determining the shear strength of metallic and intermetallic materials. more...

The influence of temperature, strain rate history, and shock-wave prestraining on the mechanical behavior of Armco-iron (Fe) is presented. Shock-wave prestraining of iron above the polymorphic α-e transition is seen to result in significant changes in the stress-strain response of Fe. The increase in the constitutive response of Fe is seen to correspond to an increase in hardness in the preshocked Fe. A high degree of correlation in the experimental results independently obtained in Russia and USA is presented. The observed mechanical property effects in preshocked Fe are correlated to the microstructure evolution in iron following explosive prestraining. more...

The behaviour of two different alloys based on the intermetallic phase TiAl, with different microstructures and grain sizes have been investigated during shock loading. Significant differences in the equation of state (stress–shock velocity–particle velocity), one-dimensional yield strength and shear strength have been observed. These have been explained in terms of the microstructural differences between the two alloys. In addition, differences between the equation of state and the calculated hydrostat have been noted. An increase in the shear strength with increasing stress has been suggested, and confirmed with independent lateral stress measurements. The increase in shear strength has been explained as being due to the high work-hardening rates displayed by such materials, although an alternative possibility—the positive dependence of the shear modulus to pressure—has also been proposed. more...

The response of alloys based on the intermetallic compound NiTi to high-strain-rate and shock loading conditions has recently attracted attention. In particular, similarities between it, and other shape memory materials such as the alloy U–6%Nb in the propagation of the plastic wave in Taylor cylinders are of significant interest. In this article, the Hugoniot is measured using multiple manganin stress gauges, either embedded between plates of the NiTi alloy, or supported with blocks of polymethylmethacrylate. In this way, the shock stress, shock velocity, and details of the shock wave profile have been gathered. An inflection at lower stresses has been found in the Hugoniot curve (stress-particle velocity), and has been ascribed to the martensitic phase transformation that is characteristic of the shape memory effect in this alloy. In a similar way, the variation of shock velocity with particle velocity has been found to be nonlinear, contrary to other pure metal and alloy systems. Finally, a break in slope in the rising part of the shock profile has been identified as the Hugoniot elastic limit in NiTi. Conversion to the one-dimensional stress equivalent, and comparison to quasistatic data indicates that NiTi exhibits significant strain-rate sensitivity. more...

The response of polycrystalline α-zirconium to various deformation conditions was investigated through electron backscattered diffraction (EBSD) characterization. The range of deformation conditions included quasi-static compression and tension at room and cryogenic temperatures, along with a Taylor cylinder impact experiment. The resultant data provided spatial resolution of individual with system activity as a function of the progression of deformation. Over 300 deformation twins were analyzed to identify the type of twin system and active variant, along with the Schmid factor in the parent orientation. These data supplied information on the distribution of Schmid factor and variant rank as a function of twin system and deformation condition. Results showed significant deviation from a maximum Schmid factor activation criterion and suggest deformation twinning is greatly affected by local internal stress heterogeneities and the sense of the applied stress. more...

This paper deals with the mechanical behavior of Cu and solid–solution Cu–Al alloys that were shock-deformed to 10 and 35 GPa. All the shock-deformed materials showed shock-strengthening that was greater at higher shock pressure and decreased with decreasing stacking fault energy (SFE) at both shock pressures. In the literature, shock-strengthening has been qualitatively ascribed to a greater dislocation density and the formation of deformation twins without addressing the question as to why shock-strengthening is lower in low SFE materials. This question is addressed in the present work by quantifying the twin contribution to the total post-shock strength. The twin contribution was found to increase with decreasing SFE suggesting that the contribution of dislocations concurrently decreases. The stored energy of as-shock-deformed materials was measured and found to decrease with decreasing SFE implying a lower net stored dislocation density in the lower SFE alloys. It is suggested that a lower net stored dislocation density in low SFE alloys results in the observed lower shock strengthening. more...

The role of stacking fault energy (SFE) in deformation twinning and work hardening was systematically studied in Cu (SFE ∼78 ergs/cm2) and a series of Cu-Al solid-solution alloys (0.2, 2, 4, and 6 wt pct Al with SFE ∼75, 25, 13, and 6 ergs/cm2, respectively). The materials were deformed under quasi-static compression and at strain rates of ∼1000/s in a Split-Hopkinson pressure bar (SHPB). The quasi-static flow curves of annealed 0.2 and 2 wt pct Al alloys were found to be representative of solid-solution strengthening and well described by the Hall-Petch relation. The quasi-static flow curves of annealed 4 and 6 wt pct Al alloys showed additional strengthening at strains greater than 0.10. This additional strengthening was attributed to deformation twins and the presence of twins was confirmed by optical microscopy. The strengthening contribution of deformation twins was incorporated in a modified Hall-Petch equation (using intertwin spacing as the “effective” grain size), and the calculated strength was in agreement with the observed quasi-static flow stresses. While the work-hardening rate of the low SFE Cu-Al alloys was found to be independent of the strain rate, the work-hardening rate of Cu and the high SFE Cu-Al alloys (low Al content) increased with increasing strain rate. The different trends in the dependence of work-hardening rate on strain rate was attributed to the difference in the ease of cross-slip (and, hence, the ease of dynamic recovery) in Cu and Cu-Al alloys. more...

The relationships between the stress state and anisotropic mechanical response for a drawing-quality, special-killed (DQSK) mild sheet steel has been analyzed. The strain rate and temperature sensitivity of the flow stress and the insensitivity of the strain hardening to strain rate are shown to be consistent with thermal activation over a Peierls barrier as the rate-controlling mechanism for deformation in DQSK. A calculated yield surface, using the quadratic Hill criterion, is shown to produce an accurate correlation with the experimental results as a function of stress state. Annealing of the DQSK sheet steel at 773 K for 1 hour reduced some of the residual stresses developed during the forming process, but had little effect on the texture. The R values derived from computed yield surfaces suggested very little difference between in-plane (IP) and transverse tensile tests, consistent with the experimental results. A comparison of the stress-strain response with the calculated yield-surface and texture measurements correlates well with the relationships between the IP and through-thickness (TT) deformation. more...

Plate impact experiments were conducted on a γ-titanium aluminide (TiAl) based ordered intermetallic alloy. Stress measurements were recorded using manganin stress gauges supported on the back of TiAl targets using polymethylmethacrylate windows. The Hugoniot in stress-particle velocity space for this TiAl alloy was deduced using impedance matching techniques. The results in this study are compared to the known Hugoniot data of the common alpha-beta engineering Ti-based alloy Ti-6Al–4V. The results of the current study on the intermetallic alloy TiAl support that TiAl possesses a significantly higher stress for a given particle velocity than the two-phase Ti–6Al–4V alloy. more...

Manganin stress gauges have been placed in samples of a γ-titanium aluminide alloy in such orientation that renders them sensitive to the lateral component of stress during one-dimensional shock loading. The impact stresses were in the range 1.8 to 6 GPa. The resultant stress histories show clear evidence of a transition from elastic to plastic behaviour, which, through the elastic relations correlates with the Hugoniot Elastic Limit of this material. In combination with the longitudinal stress data, it has been possible to use the lateral stresses from this series of experiments to calculate the shear strength during shock loading. The Hugoniot of the γ-titanium aluminide studied is seen to be significantly different than a standard γ-β Ti-alloy such as Ti-6Al-4V. The results show this titanium aluminide alloy displays a significant degree of hardening with increasing shock stress. more...

The influence of temperature and strain rate on the flow stress and work hardening rate of a 7075 aluminum alloy was studied under compressive loading over the temperature range from 23 °C to 470 °C, and strain rates from 0.001 s -1 and 2100 s -1 . While the temperature dependence of the flow stress was found to be most significant at temperatures below 300 °C, the strain rate dependence of the flow stress was found to be pronounced at temperatures above 23 °C. Concurrently, the work hardening rate decreases significantly with increasing temperature between 23 °C and 300 °C and increases slightly at higher temperatures. The minimum work hardening rate is observed to occur at temperatures between 200 °C and 300 °C and shift to higher temperatures with increasing strain rate. A negative strain rate dependence of work hardening rate was observed at 23 °C, although a positive strain rate dependence of work hardening rate occurs at higher temperatures. Analysis of the experimental data revealed three deformation regimes. more...

The mechanical behavior of a fine-grained duplex γ-TiAl alloy was studied in compression at strain rates ranging from 0.001 to 2000 s−1 and temperatures from −196°C to 1200°C. The temperature dependence of the yield and flow stresses is found to depend on the strain rate. At strain rates of 0.001 and 0.1 s−1, the yield stress decreases as the temperature increases, with a plateau between 600°C and 800°C. At strain rates of 35 and 2000 s−1, the yield stress exhibits a positive temperature dependence at temperatures above 600°C; however, postyield flow stresses exhibit a reduced temperature dependency. The work-hardening rate decreases dramatically with temperature at low and high temperatures, with a plateau occurring at intermediate temperatures for all strain rates. The work-hardening-rate plateau is seen to extend to higher temperatures as the strain rate increases. The strain-rate sensitivity at strain rates of 0.1 s−1 and greater is lower than 0.1, although it increases slightly with temperature. At 0.001 s−1, the strain-rate sensitivity increases dramatically at high temperatures (equal to 4.5 at 1200°C). The anomalous (positive) temperature dependence of the yield stress at high strain rates (>1 s−1) and high temperatures (>600°C) is explained via a dislocation-jog pinning mechanism. The negative temperature dependence of the yield stress at low strain rates ( 900°C) is though to be due to a thermally activated dislocation-jog climb process in the grain interiors and/or deformation and recovery processes at/near grain boundaries. The decreased anomalous temperature dependence of the flow stress at high strain rates and high temperatures is ascribed to dynamic recovery promoted by adiabatic heating. more...

Extensive research over the past two decades has led to the development of alloys such as Ti-46.5Al-3Nb-2Cr-0.2W ({gamma}-TiAl) and Ti-25Al-10Nb-3V-1Mo (super {alpha}{sup 2}-Ti{sub 3}Al), which offer a good balance of formability and high temperature strength. Although the mechanical behavior of these intermetallic alloys at low strain rates has been extensively studied, fewer studies have probed the systematic mechanical behavior of these alloys over a wide range of strain rates and temperatures. The purpose of this study is to compare and contrast the behavior of these two intermetallic alloys over a wide range of strain rates and temperatures. Variations in the mechanical responses of these two structurally different Ti-Al based intermetallics are documented. more...