Volcanic forces inferred from EBSD and µXRD analyses of Yellowstone quartz

<p>The magnitude of forces at play in active magmatic systems is poorly constrained because direct observation is difficult. Additional complications include short time scales and the likelihood of overprinting signatures of deeper processes by the catastrophic nature of eruption. Deformation of crystal lattices is one signature of magmatic force common to all crystals that survive eruption. Quartz crystals have documented residual elastic stresses in the hundreds of MPa measured using synchrotron µXRD. These stresses may be caused by several processes: crystal-crystal impingement in a crystal mush, explosive fragmentation, or shear in flowing lavas. To better unravel when these stresses were imparted relative to the ultimate eruption, we combine µXRD with new EBSD measurements. EBSD helps constrain subgrain and twin boundary relationships, geometrically-necessary dislocation density (GND), and plastic deformation.</p><p>We target quartz grains from a violent Yellowstone super-eruption and from a large-volume rhyolitic obsidian lava flow (Huckleberry Ridge Tuff and Summit Lake lava, respectively). We use ‘Herkimer diamonds’ as a comparative baseline for deformation. Herkimer diamonds are quartz crystals, famous in the mineral specimen community, that grew into vugs and have experienced no tectonic or volcanic stresses. Samples from both Yellowstone eruptions preserve roughly the indistinguishable amounts of elastic residual stresses, ranging from 100 to 150 MPa. EBSD indicates a GND density of ca. 4E12, with slightly higher values in the Summit Lake Lava. Diffraction peak broadening provides a record of plastic deformation using µXRD. Diffraction peaks are significantly more smeared in Summit Lake lava (0 to 0.15 degrees) than in Huckleberry Ridge Tuff (~0.06 degrees). Subgrain formation in both samples is documented by both µXRD and EBSD. By isolating processes we conclude that elastic residual stresses record pre-eruptive magmatic environment. Viscous shear during lava emplacement generates the majority of plastic deformation, which swamps the signal of lesser amounts of plastic deformation produced in the reservoir or conduit. Pre-eruption processes are likely the source of elevated elastic residual stresses, and we favor an interpretation where the stresses arise from force-chain impingements within crystal mushes prior to eruption.</p><p>Finally, EBSD and µXRD provide complementary and overlapping results. Because µXRD peak smearing is sampling both geometrically-necessary and statistically-stored dislocations (SSD; dislocations which contribute no net lattice bending but do contribute to strain hardening), and EBSD only GNDs via relative lattice curvature, the relative proportion of both types of dislocation may be calculated. Huckleberry Ridge Tuff grains preserve up to 20% GNDs, and Summit Lake lavas less than 10%, potentially reflecting the greater total stresses of the Summit Lake samples.</p>

The grain structure refinement of metals and alloys under severe plastic deformation: experimental data and analysis of mechanisms

Wide opportunities of using fine-grained materials as structural and functional materials with advanced physical and mechanical properties have proved the importance of improving the existing technology and creating new processing methods and treatment conditions for such ma-terials. At the same time, a preliminary theoretical analysis using mathematical models gives an opportunity to significantly reduce the cost of such experimental studies. Thus, it is necessary to develop multilevel models of polycrystalline metals and alloys based on crystal plasticity with the description of structure, deformation mechanisms and refinement at various scale levels. To con-struct a correct model of such a class, it is necessary to analyze information and arrange a large amount of experimental data about grain structure refinement. The article presents a review of the experimental studies describing and analyzing the grain structure refinement during severe plastic deformations of various metal alloys. The refine-ment mainly occurs at low temperatures that are a priori lower than the temperatures at which re-crystallization becomes an important factor and the solid-state phase transitions may take place. We have summarized the significant physical mechanisms of the grain refinement during cold deformation based on the arranged experimental data from the review. All the considered articles pay attention to the local accumulation of lattice dislocations inside the grains (in the form of flat clusters), which leads to the lattice curvature and separation of grains into cells. As a result of a further accumulation of dislocations in the walls, there comes an increase in misorientation of the neighboring cells. The curved lattice is unstable (it seems clear that the flat clusters become a source of such curvatures) and relaxes with the formation and movement of the partial disclina-tions, which leads to the rotation of the adjacent grain regions and creation of new grain bounda-ries. In addition, the mesoscale defects located at the junctions of the grains (including the boundary intersection disclinations), flat clusters of the dislocations of the orientational mismatch at the grain boundaries and partial dislocations in the grains have a significant effect on the frag-mentation. The articles about the severe plastic deformation at high temperatures are briefly de-scribed here. It is noted that recrystallization is the main mechanism of the fine-grained structure formation under these conditions. We suggest including the description of the discussed mechanisms in the multilevel con-stitutive material models. When new experimental data appear for a specific process of the severe plastic deformation, the considered refinement mechanisms can be added.

Influence of Lattice Curvature and Nanoscale Mesoscopic Structural States on the Wear Resistance and Fatigue Life of Austenitic Steel

The gauge dynamic theory of defects in a heterogeneous medium predicts the nonlinearity of plastic flow at low lattice curvatureand structural turbulence with the formation of individual dynamic rotations at high curvature of the deformed medium. The present work is devoted to the experimental verification of the theoretical predictions. Experimentally studied are the influence of high-temperature radial shear rolling and subsequent cold rolling on the internal structure of metastable Fe–Cr–Mn austenitic stainless steel, formation of nonequilibrium ε- and α′-martensite phases, appearance of dynamic rotations on fracture surfaces, fatigue life in alternating bending, and wear resistance of the material. Scratch testing reveals a strong increase in the damping effect in the formed hierarchical mesosubstructure. The latter is responsible for a nanocrystalline grain structure in the material, hcp ε martensite and bcc α′ martensite in grains, a vortical filamentary substructure on the fracture surface as well as for improved high-cycle fatigue and wear resistance of the material. This is related to a high concentration of nanoscale mesoscopic structural states, which arise in lattice curvature zones during high-temperature radial shear rolling combined with smooth-roll cold rolling. These effects are explained by the self-consistent mechanical behavior of hcp ε-martensite laths in fcc austenite grains and bcc α′-martensite laths that form during cold rolling of the steel subjected to high-temperature radial shear rolling.