A Framework for Analyzing the Microstructure Level Thermomechanical Response Polymer Bonded Explosives

A crystalline orientation dependent transformation model has been developed and finite element simulations clarify the effects of the crystalline orientation on the deformation and transformation behavior of TRIP steels. The results revealed that the deformation and transformation behavior of single crystal substantially depend on the maximum value of the Schmid factors of all slip systems. Decreasing the value of the Schmid factors raises the yield stress and drives the transformation in the early stage of deformation. In the later stage of deformation, the transformation behavior depends on the rotation of crystalline orientation due to the non-uniform deformation in the crystal grain. Meanwhile, the deformation and transformation behaviors of polycrystalline is strongly affected by not only individual crystalline orientation but also the situation of the distribution of crystalline orientation. The increase in the deviation of the distribution of the crystalline orientation causes significant non-uniform deformation due to misalignment of crystalline orientation with respect to the neighboring crystals, which promotes the transformation. Therefore, the transformation and deformation behavior may not be reproduced by the simple superposition of those of single crystals.

Herein, high‐strength medium Mn steel is obtained by intercritical annealing and quenching processes. Experimental steel with a chemical composition of Fe–0.2C–8.0Mn–1.6Si–1.3Al (wt%) is treated. The microstructures of the obtained samples comprise ferrite/martensite and retained austenite (RA) phases. The partitioning behavior of carbon and manganese during the intercritical annealing process is simulated using DICTRA. The sample annealed at 700 °C for 10 min has the best mechanical properties, with a product of strength and elongation as high as 47.25 GPa%. The deformation response of the RA is studied. The γ→α′ transformation mainly occurs in the early stage of deformation, whereas the γ→ε→α′ transformation mainly occurs in the later stage of deformation. The stress is mainly distributed at the deformed martensite and RA, and most of the stress is concentrated at the low‐angle grain boundary. During the deformation process, both the Kurdjumov–Sachs interface and the Nishiyama–Wasserman interface between the RA and deformed martensite decrease, but the Kurdjumov–Sachs interface decreases more obviously.

Profound variations in fold and fault geometries are mapped across a transverse lithofacies boundary in the Upper Devonian (Frasnian) Fairholme Group between hydrocarbon-bearing dolostones of the Southesk and Cairn Formations (Leduc and Nisku reefs) and off-reef shales of the Mount Hawk, Perdrix, and Flume Formations (Cline Channel) in the North Saskatchewan River area. These lithofacies constitute only 10% of the thickness of the stratigraphic package, but, during compression subparallel to the bank margin, they affected the deformational style of the entire supercrustal wedge, including units below them as well as above them. This area is ideal for studying lateral changes in structural style because the transition occurs over a short distance (1.5 km) within individual thrust sheets, where the overlying lithologies, percent shortening, burial depth and thermal history remain constant along strike. The excellent exposures are at the same stratigraphic and structural level, and the Palliser Formation contains peloidal strain markers, making it possible to quantify the changes in structural geometry in cross sections and also to determine the mechanisms of deformation in outcrops and in thin sections. Upper Devonian (Famennian) and Mississippian platform carbonates overlying the reef facies are concentrically folded, and thrust faulting is the primary mechanism of shortening. Fault-bend folding broadly warps strata and the rare folds with wavelengths shorter than 500 m are complexly faulted in their cores. Where the platform carbonates overlie the off-reef shale facies, they are tightly folded above detachment faults. Pressure-solution cleavage is strongly developed and fractures are filled with calcite, thereby significantly reducing the porosity and permeability of reservoir rocks that overlie the incompetent shale facies. Scaled physical models deformed by the centrifuge technique reproduce the variations in structural style seen across transverse lithofacies boundaries and illustrate how a fault-propagation fold can convert into a detachment fold along strike. The position and geometry of the underlying reef margin are evident in plan view at early stages of deformation, but structure contour mapping and detailed structural analysis are required to locate the underlying reef margin at later stages of deformation in the models and real-world examples.

Thermal safety model of HMX-based explosives in diamond turning

Effect of annealing treatment on microstructure and mechanical properties of lightweight steels micro-alloyed with vanadium and niobium

The β→δ phase transition (PT) of cyclotetramethylene tetranitramine (HMX) plays an important role in the safety of explosives when they are exposed to heat. In this work the effect of HMX content on the PT of HMX in Polymer Bonded Explosive (PBX) is reported. Using in situ X‐ray diffraction (XRD) test combined with quantitative analysis, the dependence of the PT temperature and dynamic behavior of HMX in PBX formulations [HMX+triaminotrinitrobenzene (TATB)+Olefin] on the content of HMX is investigated. The results show that the β→δ PT temperature gradually increases with the reduction of HMX content and with the increase of TATB content. Additives of TATB and olefin in high concentration can form compact coatings on the HMX crystals. Such coatings can delay the nucleations of δ‐HMX by inhibiting the volumetric expansion during PT. Moreever, these coatings separate the HMX crystals and build up a heat conduction obstacle. As a result the growths of δ‐HMX are also suppressed by the coatings. In contrast, fewer additives lead to larger free surface area of HMX, which accelerates the PT.

In situ neutron diffraction revealing the achievement of excellent combination of strength and ductility in metastable austenitic steel by grain refinement

Polymer bonded explosives (PBXs) have exhibited localized heating and, in some cases, subsequent reactions in response to ultrasonic excitation. The objectives of this work are to investigate the conditions for, and locations of, hot spot initiation of energetic crystals embedded within a polymer binder subjected to periodic mechanical excitation from a contacting transducer operating at 210.5 kHz. Crystal and binder interactions and events such as delamination, solid-solid phase change, and gas production were observed in real time via optical microscopy. We conclude that there are two main pathways of heat generation which are capable of driving an explosive to decomposition in the systems of interest: frictional heating from a delaminated and moving binder interface and viscoelastic heating in the binder near an embedded crystal. Formulations that address the vibration initiation sensitivity of PBX composites require knowledge of the key internal heat generation mechanisms. The results included here indicate that improving binder adhesion to energetic crystals or improving crystal morphology to reduce heating during cyclic loading may only address one of the available pathways of energy dissipation and that binder and crystal selection should be done concurrently. Furthermore, the results presented herein appear to indicate that rounded particles, in contrast to faceted crystals, with strong adhesion to the binder are expected to result in decreased heating rates under ultrasonic excitation.

Strain hardening behavior of additively manufactured and annealed AlSi3.5Mg2.5 alloy

Extreme conditions involve low or high temperatures (> 1500 K), high pressures (> 30 MPa), high strains or strain rates, high radiation fluxes (> 100 dpa), and high electromagnetic fields (> 15T). Material properties under extreme conditions can be extremely different from those under normal conditions. Understanding material properties and performance under extreme conditions, including their dynamic evolution over time, plays an essential role in improving material properties and developing novel materials with desired properties. To understand material properties under extreme conditions, we use molecular dynamics (MD) simulations with recently developed reactive force fields (ReaxFF) and traditional embedded atom methods (EAM) potentials to examine various materials (e.g., energetic materials and binary liquids) and processes. The key results from the simulations are summarized below. Anisotropic sensitivity of RDX crystals: Based on the compress-and-shear reactive dynamics (CS-RD) simulations of cyclotrimethylene trinitramine (RDX) crystals, we predict that for mechanical shocks between 3 and 7 GPa, RDX is the most sensitive to shocks perpendicular to the (100) and (210) planes, while it is insensitive to those perpendicular to the (120), (111), and (110) planes. The simulations demonstrate that the molecular origin of anisotropic shock sensitivity is the steric hindrance to shearing of adjacent slip planes. Mechanisms of hotspot formation in polymer bonded explosives (PBXs): The simulations of a realistic model of PBXs reveal that hotspots may form at the nonplanar interfaces where shear relaxation leads to a dramatic temperature increase that persists long after the shock front has passed the interface. For energetic materials this temperature increase is coupled to chemical reactions that eventually lead to detonation. We show that decreasing the density of the binder eliminates the hotspots or reduces the sensitivity. Cavitation in binary metallic liquids: We demonstrate the stochastic nature of the cavitation process in binary metallic liquids, and that classical nucleation theory can predict the cavitation rate if we incorporate the Tolman length derived from the MD simulations. Synthesis the single metallic glass on amorphous substrate: We show that single component metallic glasses (SCMGs) can be synthesized by thermal spray coating of nanodroplets onto an amorphous substrate (ND-AS). The key requirements to form the SCMGs are the rapid cooling rates and the amorphous substrates. Carbon and hydrogen phases under extreme conditions: we report on the use of electron force fields (eFF) in characterizing the Hugoniot relationships of carbon, which includes consecutive phase transitions also captured by experiments, as well as the Hugonoit states of hydrogen centered at various initial densities compared to experiments and the predictions of other theories.

Mechanical insult may be able to produce chemical transformations in solids when the energy is released in highly localized regions. This phenomenon is responsible for the nucleation of hot-spots that are responsible for ignition of energetic materials. The concentration of energy at microstructural defects leads to the probabilistic nature of ignition. The effect of the microstructure of the energetic particles, specifically the influence of the initial crack distribution on the sensitivity to ignition, is studied for a particle embedded in a polymeric matrix at impact velocities 100 m/s and 400 m/s with finite element simulations that couple fracture dynamics and heat transport. A phase field damage model that includes heat sources due to frictional heating at the crack surfaces and heat dissipation during crack propagation is developed and verified. These heat sources are compared and, in the range of impact velocities studied, heat generation due to friction is more important than dissipation due to crack propagation. Hot-spots nucleated at 100 m/s do not reach the critical temperature while conditions consistent with the Lee-Tarver criterion for ignition are observed at 400 m/s impact velocity. The variability observed due to the stochasticity of the initial crack distribution is studied and it increases with a higher impact velocity. In particular, regions of high temperature develop close to cracks intersecting the particle polymer interface. Therefore, controlling the surface quality of the energetic particles may lead to a reduction on the sensitivity uncertainty in polymer-bonded explosives.

Characterization of low cycle fatigue damage in 9Cr–1Mo ferritic steel using X-ray diffraction technique

Deformation heterogeneity in copper oligocrystals using high-resolution stereo DIC

Toughening alpha-Ti by dislocation-induced phase transformation at crack tips

Activating profuse pyramidal slips in magnesium alloys via raising strain rate to dynamic level