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The goal of this work is to investigate the effect of varying phase fractions on the overall spall strength and damage behavior of a material. Specifically, two plain carbon, ferrite–pearlite steels (1045 and A283) were subjected to spall recovery experiments to investigate the effect of pearlite fraction on spall strength and total damage. The A283 (20% pearlite) alloy exhibited a higher Hugoniot elastic limit and spall strength compared with 1045 (60% pearlite). Discontinuous and continuous yielding behaviors were observed at quasi-static and dynamic rates for A283 and 1045, respectively. The yielding behavior was connected to pearlite fraction and the prevalence of dislocation-emitting, ferrite/cementite interfaces. Postmortem characterization revealed cementite lamellae cracking within pearlite of 1045, suggesting that pearlite reduces spall strength by providing low-energy damage nucleation sites. The rate of damage growth and coalescence was similar between the two alloys; however, 1045 exhibited more continuous cracks than A283, which exhibited a greater prevalence of discrete voids.

Certification and product qualification of an engineering component generally involves meeting engineering and physics requirements tied to its functional requirements.. In this paper, the results of a study quantifying the microstructure and the dynamic damage evolution of Tantalum (Ta) fabricated using an EOS laser-powder-bed machine are presented. The microstructure of the AM-Ta is detailed and compared / contrast to wrought Ta. The dynamic damage evolution and failure response of the AM-Ta material, as well as wrought Ta, was probed using flyer-plate impact driven spallation experiments. The differences in the spallation response between the AM and wrought Ta were measured using in-situ velocimetry as well as post-mortem quantification of damage in “soft-recovered” samples. The damage evolution of the AM and wrought Ta were characterized using optical metallography.

For ductile metals, the process of dynamic fracture occurs through nucleation, growth and coalescence of voids. For high purity single-phase metals, it has been observed by numerous investigators that voids tend to heterogeneously nucleate at grain boundaries and all grain boundaries are not equally susceptible to void nucleation. However, for materials of engineering significance, especially those with second phase particles, it is less clear if the type of bi-metal interface between the two phases will affect void nucleation and growth. To approach this problem in a systematic manner two bi-metal interfaces between Cu and Pb have been investigated: {111} and {100}. Qualitative and quantitative analysis of the collected data from molecular dynamics shock and spall simulations suggests that Pb becomes disordered during shock compression and is the preferred location for void nucleation under tension. Despite the interfaces being aligned with the spall plane (by design), they are not the preferred location...

With PBX9501 we explosively loaded fully annealed OFHC-Cu and an OFHC-CuPb (extruded with 1% Pb that aggregates at the Cu grain boundaries) to study the effects of the 1% Pb on the elastic-plastic yield Y of Cu. The yield-stress Y was studied through observation of surface velocimetry and total ejected mass ρA from periodic surface perturbations machined onto the sample surfaces. The perturbation’s wavelengths were λ ≈ 65 µm, and their amplitudes h were varied to determine the wavenumber (2π/λ) amplitude product kh at which ejecta production for the Cu and CuPb begins, which relates to Y. The Y of the two materials is apparently different.

We investigate the responses of four representative grain boundaries in face-centered cubic Cu bicrystals to shock compression as a function of the loading direction. Two loading directions are considered, either parallel or perpendicular to the grain boundary plane, representing the extremes that a polycrystalline sample will ordinarily experience under the uniaxial strain conditions of planar shock loading. Using molecular dynamics simulations, we demonstrate that the deformation processes during shock compression of the same boundary are altered measurably by changing the loading direction. The Majority of the differences in the nanoscale deformation processes were related to the activation of varying slip systems in the same boundary under the two loading conditions. This change in deformation processes, and hence the plastic response, might eventually affect the failure stress for a grain boundary.

We present a series of experiments on the response of additive manufactured (AM) tantalum to dynamic loading, specifically the spall strength. Rectangular plates of AM tantalum were produced, with subsequent characterization revealing a highly anisotropic microstructure. Samples were taken from these plates to investigate the effect of anisotropy on the spall strength: the resistance to high strain-rate tensile damage. A conventional, wrought tantalum sample, possessing an equiaxed microstructure, was also tested to serve as a control. Shock loading was performed via light gas-gun flyer-plate impact experiments, with laser velocimetry on the rear of the samples to record the shock wave profiles and soft-recovery techniques to allow post-mortem analysis. In general, the AM samples were found to have a higher Hugoniot elastic limit, the dynamic yield strength under shock loading, while having a reduced spall strength, when compared to the wrought tantalum samples.

Materials subjected to dynamic environments experience a complex and wide range of stress, strain, and strain-rate conditions. To have confidence in material models, an accurate, predictive capability is required. In this study, we present a series of flyer-plate impact tests on well characterized, high purity tantalum. The shock-waves generated at impact release from the free-surfaces, reflect, and interact to produce incipient spall fracture. By varying the flyer-plate material and impact velocity, both the peak stress and the strain-rate in the samples were controlled independently. Velocimetry was used on the rear free-surface of the samples to measure the shock-response and the spall strength. While this measurement provided the same spall strength for all cases, at approximately 5.1 GPa, when the samples were sectioned during post-mortem, the quantity and distribution of internal damage was markedly different. For the high-strain rate cases, voids remained small and isolated, whereas in the lower strain-rate experiments, the spall damage was far more localized, with a well-defined continuous spall plane. With the use of hydrocode simulations, this was discovered to result from how the different release rates affect the interaction volume inside the sample. These results highlight the importance of careful sample recovery, and the risks of relying solely on free-surface velocity measurements.

For additive manufacturing (AM), the certification and qualification paradigm needs to evolve as there exists no “ASTM-type” additive manufacturing certified process or AM-material produced specifications. Accordingly, utilization of AM materials to meet engineering applications requires quantification of the constitutive properties of these evolving materials in comparison to conventionally-manufactured metals and alloys. Cylinders 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 is detailed in both the as-built condition and following heat-treatments designed to obtain full recrystallization. The constitutive behavior as a function of strain rate and temperature is presented and compared to that of nominal annealed wrought 316L SS plate. The dynamic damage evolution and failure response of all three materials was probed using flyer-plate impact driven spallation experiments at a peak stress of 4.5 GPa to examine incipient spallation response. The spall strength of AM-produced 316L SS was found to be very similar for the peak shock stress studied to that of annealed wrought or AM-316L SS following recrystallization. The damage evolution as a function of microstructure was characterized using optical metallography.

Previous experimental research has shown that microstructural features such as interfaces, inclusions, vacancies and heterogeneities can all act as void nucleation sites. However, it is not well understood how important these interfaces are to the damage evolution as a function of the surrounding parent materials. In this work, we present results on three different materials: 1) Cu, 2) Cu-10 wt%Ag, and 3) Cu-15 wt%Nb examined to probe the influence of bi-metal interfaces on void nucleation and evolution. These materials were chosen due to the differences in the stacking fault energy between the two phases. The initial results suggest that when there are significant differences between the bulk properties (for example: stacking fault energy and melting temperature etc) the type of interface between the two parent materials does not influence the damage process. Rather, it is the “weaker” material that dictates the dynamic spall strength of the material.

At high pressures zirconium is known to undergo a phase transformation from the hexagonal close packed (HCP) alpha phase to the simple hexagonal omega phase. Under conditions of shock loading, a significant volume fraction of high-pressure omega phase is retained upon release. However, the hysteresis in this transformation is not well represented by equilibrium phase diagrams and the multi-phase plasticity under shock conditions is not well understood. For these reasons, the influence of peak shock stress and temperature on the retention of omega phase in Zr has been explored. VISAR and PDV measurements along with post-mortem metallographic and neutron diffraction characterization of soft recovered specimens have been utilized to quantify the volume fraction of retained omega phase and qualitatively understand the kinetics of this transformation. In turn, soft recovered specimens with varying volume fractions of retained omega phase have been utilized to understand the contribution of omega and alpha phases to strength in shock loaded Zr.

Several factors can affect the failure stress of a grain boundary, such as grain boundary structure, energy and excess volume, in addition to its interactions with dislocations. In this paper, we focus on the influence of grain boundary energy, excess volume and plasticity at the boundary on the failure stress of a grain boundary, in copper from molecular-dynamics simulations. Flyer plate simulations were carried out for four boundary types with different energies and excess volumes. These boundaries were chosen as model systems to represent various boundaries observed in real materials. Simulations indicate that there is no direct correlation between the void nucleation stress of a boundary and either its energy and excess volume. This result suggests that average properties of grain boundaries alone are not sufficient indicators of the failure strength of a boundary. However, local boundary properties related to the ability of a grain boundary to undergo plastic deformation are better markers of its strength.

When BaZrO(3) is doped with Y in 12.5% of Zr sites, density functional theory with the PBE functional predicts octahedral distortions within a cubic phase yielding a greater variety of proton binding sites than undoped BaZrO(3). Proton binding sites, transition states, and normal modes are found and used to calculate transition state theory rate constants. The binding sites are used to represent vertices in a graph. The rate constants connecting binding sites are used to provide weights for graph edges. Vertex and color coding are used to find proton conduction pathways in BaZr(0.875)Y(0.125)O(3). Many similarly probable proton conduction pathways which can be periodically replicated to yield long range proton conduction are found. The average limiting barriers at 600 K for seven step and eight step periodic pathways are 0.29 and 0.30 eV, respectively. Inclusion of a lattice reorganization barrier raises these to 0.42 and 0.33 eV, respectively. The majority of the seven step pathways have an interoctahedral rate limiting step while the majority of the eight step pathways have an intraoctahedral rate limiting step. While the average limiting barrier of the seven step periodic pathway including a lattice reorganization barrier is closer to experiment, how to appropriately weight different length periodic pathways is not clear. Likely, conduction is influenced by combinations of different length pathways. Vertex and color coding provide useful ways of finding the wide variety of long range proton conduction pathways that contribute to long range proton conduction. They complement more traditional serial methods such as molecular dynamics and kinetic Monte Carlo.