The performance of complex concentrated alloys (or high entropy alloys) with widely varying microstructure is evaluated by ballistically impacting targets with spheres fired at normal incidence. By changing alloy composition in the Al-Co-Cr-Fe-Ni multi-principal system, the variation in microstructure included single-phase equiaxed grains, single-phase with bimodal grain-size distribution, and eutectic two-phase lamellar microstructure. Rigorous characterization in the form of bulk mechanical testing, scanning electron microscopy, and spatially resolved nano-indentation on initial and ballistically impacted plates was used to connect microstructural details to aspects of ballistic behavior governing performance. Based on the results, it was shown that although the addition of a harder secondary phase improves strength, cracks that initiate and propagate within the harder phase and ultimately across the target plate drastically reduce the ballistic performance of the two-phase material. The single-phase alloy with bimodal grain-size distribution exhibited superior ballistic performance compared to the other high entropy alloys, although none of these materials exceeded the performance of conventional rolled homogeneous armor steel. These results pave the way for development of high-performance concentrated alloys for ballistic applications by appropriate microstructural design.

The interconnected phenomena of dynamic recrystallization (DRX) and adiabatic shear bands (ASBs) are critical features of dynamic deformation and fracture in metals. Despite their importance, observations of these phenomena have largely been restricted to post-mortem analyses. Here we report on the first in situ X-ray diffraction (XRD) observations of DRX. By combining high strain rate loading via Kolsky bar with high speed X-ray diffraction, time resolved observations of microstructural evolution in Ti–7Al are made. XRD measurements are coupled with explicit finite element simulations to assess the stress, strain, and temperature dependence of the microstructural evolution. These observations reveal a continuous evolution of the microstructure with increasing plastic strain up to a maximum stress. At the maximum stress, the structure ceases evolving and the subsequent softening suggests the onset of shear banding. As a whole, these unique experiments definitively reveal the onset of DRX to be driven by accumulation of plastic strain and not the temperature rise associated with plastic deformation. The observed sequence of events further suggests DRX results in microstructural softening and precedes ASB formation.

Multi-principal element alloys represent a new paradigm in structural alloy design with superior mechanical properties and promising ballistic performance. Here, the mechanical response of Al 0.3 CoCrFeNi alloy, with unique bimodal microstructure, was evaluated at quasistatic, dynamic, and ballistic strain rates. The microstructure after quasistatic deformation was dominated by highly deformed grains. High density of deformation bands was observed at dynamic strain rates but there was no indication of adiabatic shear bands, cracks, or twinning. The ballistic response was evaluated by impacting a 12 mm thick plate with 6.35 mm WC projectiles at velocities ranging from 1066 to 1465 m/s. The deformed microstructure after ballistic impact was dominated by adiabatic shear bands, shear band induced cracks, microbands, and dynamic recrystallization. The superior ballistic response of this alloy compared with similar Al x CoCrFeNi alloys was attributed to its bimodal microstructure, nano-scale L1 2 precipitation, and grain boundary B2 precipitates. Deformation mechanisms at quasistatic and dynamic strain rates were primarily characterized by extensive dislocation slip and low density of stacking faults. Deformation mechanisms at ballistic strain rates were characterized by grain rotation, disordering of the L1 2 phase, and high density of stacking faults.

Due to their unique combination of properties, tungsten (W)-based composites are desirable alternatives to other high-density materials used as projectiles in defense applications. As such, heterogeneous tri-layer sandwich structures, consisting of W/metal (M)/W layers, have been produced by joining the cold-rolled individual W to the M layers using a diffusion bonding process. Under uniaxial high strain rate compression, these composite materials showed flow-softening and failed via adiabatic shear localization. Herein, we described for the first time the experiments designed to scale up the tri-layer, esp. W/Fe/W fabrication process to create multilayer structures that were formed into much thicker subscale projectiles. Results of instrumented ballistic testing into semi-infinite mild-steel blocks showed that the projectiles survived the high velocity launch process and penetrated the target. Detailed post-mortem examinations of the penetrator residuals showed clear evidence that the layered composites retained their integrity and deformed by extensive flow and shear localization of the individual W layers. The fabrication methodology, resultant characteristics, and the ballistic test results were discussed.

Rolled magnesium alloys exhibit pronounced tension-compression asymmetry as well as anisotropy in yield and strain-hardening behavior. Although differences in the mechanical response are reasonably well-understood, it is not clear to what extent anisotropy alters the deformation and failure of plates subjected to ballistic loading conditions. In this work, the role of texture and anisotropy in the ballistic response is investigated using a combined experimental–computational approach. Sphere impact experiments are performed on rolled magnesium plates cut from orientations that exhibit differing mechanical responses. The complex failure process is characterized by in situ diagnostics, including ultra-high-speed Digital Image Correlation and Photonic Doppler Velocimetry, and compared with simulations performed on polycrystalline aggregates using a polycrystal plasticity model for hcp metals. The anisotropic deformation behavior arises from deformation mechanisms with disparate strength and strain hardening behavior, which is well captured in the model. The occurrence of low-strength extension twinning is shown to govern the initial anisotropic deformation and bulging of the plate. When compared with post-mortem 3D X-ray microscopy, regions that experience intense basal slip are shown to be sites for damage initiation that leads to eventual fracture. The combination of experiments and simulations suggest that at low to intermediate ballistic loading rates, material orientation plays a crucial role in dictating eventual fracture and failure in strongly anisotropic metals such as in rolled magnesium alloys.

It has been shown that tungsten (W) and other refractory metals with body-centered cubic (bcc) structures with a grain size d in the ultrafine grain (UFG, 100 nm < d < 1000 nm) or nanocrystalline (NC, d < 100 nm) regimes exhibit adiabatic shear localization under uniaxial high strain rate compression. This is especially true if the UFG/NC microstructure is produced by top-down methods such as severe plastic deformation (SPD). Such adiabatic shear localization is a long-sought-after behavior for certain applications. But limitations in the physical dimensions of bulk SPD products remain a hurdle to practical applications of such UFG/NC bcc metals and alloys. In this work, cold-rolled W sheets are joined by diffusion bonding into a bulk sample that retains the plastic deformation mode of monolithic cold-rolled W. The microstructure and mechanical properties, particularly at high strain rate, are evaluated and discussed. Because the as-rolled microstructure survived the diffusion bonding process, the propensity for adiabatic shear localization in the bonded heterogeneous multilayer structure is retained as well. It is envisioned that diffusion bonding could serve as a promising and scalable fabrication approach for UFG/NC bcc refractory metals in the desired applications.

The plastic deformation of polycrystalline metals at high strain rates is controlled by the way defects (dislocations and twins) nucleate, propagate, and interact in the microstructure. To-date, the role of these defects has been estimated based on dynamic mechanical measurements coupled with ex situ investigations of the deformed microstructure. However, such investigations are fundamentally limited in their ability to characterize transient mechanisms. Here, we present for the first time direct, experimental observations of the nucleation, motion, and interaction of defects and cracks during deformation of pure copper at strain rates between 103 and 104 s−1. These observations are enabled by coupling a custom-built in situ high-rate straining stage with nanosecond-resolution dynamic transmission electron microscopy. The results show that while twins play only a minor role in the deformation of copper at quasi-static strain rates, the twin nucleation rate increases markedly at high strain rates. The preferred nucleation sites for twins also change, and the new twin interfaces become preferential paths for crack propagation, facilitating fracture through the original grains.

High‐entropy alloys, consisting of multiple principal elements, represent a new paradigm in structural alloy design with excellent mechanical properties and potentially promising ballistic performance. Herein, the ballistic response of a single‐phase Al0.1CoCrFeNi high‐entropy alloy is evaluated with spherical E52100 steel (RC60) projectiles at velocities ranging from 500 to 1000 m s−1 at normal obliquity, indicating failure by ductile–hole growth. A wide range of microstructural features are observed corresponding to varying degrees of deformation and the corresponding hardness maps are obtained. The microstructure in the partially penetrated condition is dominated by microbanding and microtwinning close to the crater wall. With striking velocity that result in plugging but not complete penetration, the deformation is dominated by twinning and crack initiation around adiabatic shear bands close to the exit hole. A high density of localized adiabatic shear bands and recrystallized grains are observed at impact velocities corresponding to full penetration. Highly deformed areas near the crater wall and narrow zones around shear bands show the maximum hardness, indicating significant work hardening of the material during penetration.

The deformation-induced transformation of metastable austenite to martensite can contribute to improved performance of many steel alloys in a range of applications. For example, one class of Ni-containing steels that has undergone consecutive heat treatments of quenching (Q), lamellarization (L), and tempering (T) exhibits improved ballistic resistance and low-temperature impact toughness. To better understand the origin of this improvement, we tracked the volume fraction of austenite present in a QLT 10 wt pct Ni steel during compression at low and high strain rates ( $$\dot{\varepsilon }={0.001}\,{{\text{s}}^{-1}}$$ and $$\dot{\varepsilon }\simeq {2500}\,{{\text{s}}^{-1}}$$ , respectively) using ex situ vibrating sample magnetometry measurements and in situ time-resolved X-ray diffraction measurements. We observe that the austenite-to-martensite transformation occurs more readily during quasi-static loading than during dynamic loading, even at small values of applied strain, which is qualitatively different from the behavior of steels known to undergo a strain-induced martensitic transformation mechanism. We propose that the strain-rate dependence of transformation in the QLT 10 pct Ni steel is dominated by the transformation in small austenite particles, where stress-assisted martensitic transformation is likely to be the dominant mechanism. Indirect evidence for this hypothesis is provided by electron backscatter diffraction measurements of deformed specimens.

In this paper, we detail a study involving the characterization of two grades of alumina (CoorsTelc Engineered Ceramics AD-85 and AD-995) and subsequent impact experiments with tungsten carbide spheres at 400 m/s. Through this work, we hope to understand the relationship between microstructure and the high strain rate behaviors of advanced ceramics. Initial characterization of microstructure with scanning electron microscopy, electron backscatter diffraction, and energy dispersive X-ray spectroscopy determined grain size, and the elemental character of the materials and the inclusions present. AD-85 alumina was found to have smaller alumina grains and more non-alumina inclusions than AD-995 alumina. Compression testing with in-situ visualization for crack speeds demonstrated that the strength of AD-85 increased from 2.0 +/- 0.1 GPa at 10(-3) s(-1) to 2.7 +/- 0.3 GPa at 500 s(-1) (crack speeds of 1800 +/- 600 m/s). The AD-995 had strength increases of 2.4 +/- 0.2 GPa at 10(-3) s(-1) to 3.6 +/- 0.5 GPa at 500 s(-1) (crack speeds of 2200 +/- 400 m/s). The greater rate sensitivity and higher crack speed of the AD-995 is related to microstructure and mechanical properties. Impact experiments used spherical tungsten carbide projectiles fired from a smoothbore powder gun into alumina discs held in place with polycarbonate holders that provided no lateral confinement and minimal back face confinement. The use of simultaneous high-speed video, photon-doppler velocimetry, and flash X-ray during these impact experiments produced information on the cracking and microbending of the target in the first 10 mu s of impact, with a number of significant events happening within the first 5 mu s. At 400 m/s, AD-85 alumina suffered penetration by the projectile while AD-995 alumina defeated the projectile. The data allows for comparison of event timing such as peak back face velocity and the onset of radial cracking, demonstrating that during an impact event, interface defeat occurs within the first 5 mu s. The data collected and analysis done also suggests that the failure of the projectile can couple with the target via the transmission of shear waves and contribute to failure, which has implications for future modelling and design of improved protection systems.

The tensile strength of ceramic materials are well known to be highly size dependent, arising from the sampling of intrinsic flaws. As such, determining the true tensile strength of ceramics remains a challenge. Here, we investigate the distribution of tensile properties in armor ceramics (SiC, B4C, and SiC-B4C composite) at the microscale. By employing high throughput femtosecond laser based machining, tensile bars with critical dimensions less than 100 mu m are fabricated. The specimens are tested in a custom micromechanical testing apparatus in order to create a statistically significant failure distribution. The fracture surfaces are further characterized to assess the failure mechanisms. Together these observations provide a clear picture of the intrinsic tensile response of armor ceramics.

Pressure-shear plate impact experiments generate normal and transverse particle velocities during high strain rate deformations. Traditionally, freespace lenscoupled tabletop laser interferometry techniques are used together with diffraction gratings to interrogate the evolving velocity vector at the back face of the target plate. Recently, fiberoptic velocimetry (photon Doppler velocimetry or PDV) has become commonplace for measuring normal particle velocities above 200m/sec. In this work, we demonstrate transverse velocity detection using a modified PDV system where we subtract the measured normal velocity history from a concurrent velocity history measured at a canted angle to the target surface to obtain the transverse velocity component. This modified system is time-multiplexed to reduce the number of components, and uses an erbium doped fiber amplifier (EDFA) to boost the angled signal intensity while maintaining low noise. The system operates as a heterodyne interferometer, but features a frequency upshifted reference leg to improve data analysis at the particle velocities expected in the experiment. We demonstrate by direct comparison that this inexpensive and simple approach is as effective as traditional grating methods.

This work investigates the importance of the microstructure of boron carbide for initiating inelastic deformation under impact conditions. Simple loading resulting from a flyer plate impact geometry is used to illustrate the importance of microstructure for the well-controlled and easily instrumented experimental geometry. A second set of simulations is performed on a miniaturized impact geometry to investigate the importance of the microstructure for the early stages of semi-infinite penetration for impact velocities between 0.9 km/s and 1.9 km/s. The effect of the microstructure is more pronounced for the flyer plate impact geometry.

Growing interest in characterizing the fracture response in non-bulk systems, such as second phase precipitates or microscale structural materials, necessitates the development of accurate fracture methodologies at relevant length scales. Here, femtosecond laser based micromachining is employed to fabricate microscale fracture specimens. Femtosecond laser machining enables rapid production of fractures specimens with sharp initial notches. Fracture tests are subsequently performed in a custom micromechanical testing apparatus. Initially, the femtosecond laser based fracture methodology is applied to fused quartz to validate the accuracy of the measurements. Further measurements are made on polycrystalline silicon carbide to demonstrate the broader applicability to higher fracture toughness materials.

Tensile strength is a critical design parameter to ensure reliability of silicon carbide devices. However, SiC's tensile strength depends on the dominant flaw subjected to maximum tensile stress. Typically, processing related flaws (i.e. voids or inclusions) dominate the tensile response thereby masking role of the underlying microstructure. To probe the intrinsic microstructural flaws in a hot-pressed SiC microscale tensile bars were machined using a custom femtosecond laser micromachining apparatus from thin sections. When tested in tension, the fracture strength was dramatically higher compared to bulk tensile strengths. An analysis of strength-size scaling suggested a transition in the dominant strength-determining flaw at the microscale. Analysis of fracture surfaces and critical flaw sizes suggest failure is governed by isolated large grains. These observations have important implications for parameterizing ceramic failure models.

An AlCoCrFeNi2.1 eutectic high entropy alloy (EHEA), comprising of FCC phase and a high volume fraction of plate and irregularly shaped BCC-ordered intermetallic B2 domains, was subjected to normal impact by spherical Tungsten–Carbide projectiles. Depending on projectile velocity, the impacted AlCoCrFeNi2.1 plates were partially penetrated (at the lowest velocity of 803 m/s), plugged (intermediate velocity of 1159 m/s), and fully penetrated (highest velocity of 1388 m/s). Electron microscopy was utilized to characterize the residual damage or deformation features in the recovered specimens. Failure in the partially penetrated conditions was dominated by interfacial decohesion at the plate-like B2 and FCC interfaces. At higher velocities where plugging occurred, failure was dominated by crack formation in regions containing adiabatic shear band. These results indicated a transition in failure modes as a function of projectile velocities, where the FCC-B2 microstructure dominate failure at lower velocities, while such microstructural features do not influence dynamic failure at higher velocities.

Ceramics are important materials due to their high strength and hardness, particularly in armor systems such as personnel body armor where they are used extensively. Understanding the failure process for these types of systems is key to improving their performance. To better understand the process of failure in ceramic materials subjected to ballistic impacts, we planned and executed reverse ballistic experiments to study material failure during impact on a silicon carbide target. The primary diagnostic tool we used was proton radiography conducted at Los Alamos National Laboratory Neutron Science Center (LANSCE) using their 800 mega-electron-volt (MeV) linear accelerator. Proton radiography at this facility is capable of excellent spatial and temporal resolution with up to 31 frames of data captured with variable frame spacing and gate time. We report and discuss some of the results of these experiments.

The compressive stress-strain response of a commercially pure (99.98%) Ta was investigated at strain-rates ranging from 0.001/s to 500 k/s. Strain-rates up to 20 k/s were obtained using conventional load frames and Kolsky bar methods. The higher strain-rates were obtained using optically instrumented miniature Kolsky bars. Because these experiments require sample sizes as small as ∼30 um, a fine grain structure was desired. To achieve this, we study a Ta billet that was processed by ECAE to produce an ultrafine grain structure. The billets were subsequently annealed at 1203 K under high vacuum for 2 h to coarsen the grain size to approximately 2 um. The as-worked and annealed microstructures were investigated by electron backscatter diffraction to verify the grain structure. A strong rate dependence is observed over this range of strain-rates, although there is a discrepancy between data at similar strain-rates using different sample sizes. This discrepancy is the subject of on-going investigation.