Strain localization in polycrystalline material with second phase particles: Numerical modeling with application to ice mixtures

We use a centimeter-scale 2-D numerical model to investigate the effect of the presence of a second phase with various volume percent, shape, and orientation on strain localization in a viscoelastic matrix. In addition, the evolution of bulk rheological behavior of aggregates during uniaxial compression is analyzed. The rheological effect of dynamic recrystallization processes in the matrix is reproduced by viscous strain softening. We show that the presence of hard particles strengthens the aggregate, but also causes strain localization and the formation of ductile shear zones in the matrix. The presence of soft particles weakens the aggregate, while strain localizes within the particles and matrix between particles. The shape and the orientation of second phases control the orientation, geometry, and connectivity of ductile shear zones. We propose an analytical scaling method that translates the bulk stress measurements of our 2-D simulations to 3-D experiments. Comparing our model to the laboratory uniaxial compression experiments on ice cylinders with hard second phases allows the analysis of transient and steady-state strain distribution in ice matrix, and strain partitioning between ice and second phases through empirical calibration of viscous softening parameters. We find that the ice matrix in two-phase aggregates accommodates more strain than the applied bulk strain, while at faster strain rates some of the load is transferred into hard particles. Our study illustrates that dynamic recrystallization processes in the matrix are markedly influenced by the presence of a second phase

Seismic anisotropy of mid crustal orogenic nappes and their bounding structures: An example from the Middle Allochthon (Seve Nappe) of the Central Scandinavian Caledonides

We report compositional, microstructural and seismic properties from 24 samples collected from the Middle Allochthon (Seve Nappe) of the central Scandinavian Caledonides, and its bounding shear zones. The samples stem both from field outcrops and the continental drilling project COSC-1 and include quartzofeldspathic gneisses, hornblende gneisses, amphibolites, marbles, calc-silicates, quartzites and mica schists, of medium to high-strain. Seismic velocities and anisotropy of P (AVp) and S (AVs) waves of these samples were calculated using microstructural and crystal preferred orientation data obtained from Electron Backscatter Diffraction analysis (EBSD). Mica-schist exhibits the highest anisotropy (AVP ~ 31%; max AVs ~34%), followed by hornblende-dominated rocks (AVp ~5–13%; max AVs 5–10%) and quartzites (AVp ~6.5–10.5%; max AVs ~7.5–12%). Lowest anisotropy is found in calc-silicate rocks (AVp ~4%; max AVs 3–4%), where the symmetry of anisotropy is more complex due to the contribution to anisotropy from several phases. Anisotropy is attributed to: 1) modal mineral composition, in particular mica and amphibole content, 2) CPO intensity, 3) crystallization of anisotropic minerals from fluids circulating in the shear zone (calc-silicates and amphibolites), and to a lesser extent 4) compositional banding of minerals with contrasting elastic properties and density. Our results link observed anisotropy to the rock composition and strain in a representative section across the Central Scandinavian Caledonides and indicate that the entire Seve Nappe is seismically anisotropic. Strain has partitioned on the nappe scale, and likely on the microstructural scale. High- strain shear zones that develop at boundaries of the allochthon and internally within the allochthon show higher anisotropy than a more moderately strained interior of the nappe. The Seve Nappe may be considered as a template for deforming, ductile and flowing middle crust, which is in line with general observations of seismic anisotropy in mid-crustal settings

Rheology, microstructure and crystallographic preferred orientation of matrix containing a dispersed second phase: Insight from experimentally deformed ice

We utilize in situ neutron diffraction to continuously track the average grain size and crystal preferred orientation (CPO) development in ice, during uniaxial compression of two-phase and pure ice samples. Two-phase samples are composed of ice matrix and 20 vol.% of second phases of two types: (1) rheologically soft, platy graphite, and (2) rigid, rhomb-shaped calcite. The samples were tested at 10 °C below the ice melting point, ambient pressures, and two strain rates (1×10−5 and 2.5×10−6 s−1), to 10 and 20% strain. The final CPO in the ice matrix, where second phases are present, is significantly weaker, and ice grain size is smaller than in an ice-only sample. The microstructural and rheological data point to dislocation creep as the dominant deformation regime. The evolution and final strength of the CPO in ice depend on the efficiency of the recrystallization processes, namely grain boundary migration and nucleation. These processes are markedly influenced by the strength, shape, and grain size of the second phase. In addition, CPO development in ice is further accentuated by strain partitioning into the soft second phase, and the transfer of stress onto the rigid second phase

Physically Transient Photonics: Random versus Distributed Feedback Lasing Based on Nanoimprinted DNA

The authors report on a room-temperature nanoimprinted, DNA-based distributed feedback (DFB) laser operating at 605 nm. The laser is made of a pure DNA host matrix doped with gain dyes. At high excitation densities, the emission of the untextured dye-doped DNA films is characterized by a broad emission peak with an overall linewidth of 12 nm and superimposed narrow peaks, characteristic of random lasing. Moreover, direct patterning of the DNA films is demonstrated with a resolution down to 100 nm, enabling the realization of both surface-emitting and edge-emitting DFB lasers with a typical linewidth<0.3 nm. The resulting emission is polarized, with a ratio between the TE- and TM-polarized intensities exceeding 30. In addition, the nanopatterned devices dissolve in water within less than two minutes. These results demonstrate the possibility of realizing various physically transient nanophotonics and laser architectures, including random lasing and nanoimprinted devices, based on natural biopolymers.Comment: 20 pages, 5 figures, 31 reference

Deformation behavior of migmatites: insights from microstructural analysis of a garnet–sillimanite–mullite–quartz–feldspar-bearing anatectic migmatite at Rampura–Agucha, Aravalli–Delhi Fold Belt, NW India

In the present study we investigate the microstructural development in mullite, quartz and garnet in an anatectic migmatite hosted within a Grenvillian-age shear zone in the Aravalli–Delhi Fold Belt. The migmatite exhibits three main deformation structures and fabrics (S1, S2, S3). Elongated garnet porphyroblasts are aligned parallel to the metatexite S2 layers and contain crenulation hinges defined by biotite–sillimanite–mullite–quartz (with S1 axial planar foliation). Microstructural evidence and phase equilibrium relations establish the garnet as a peritectic phase of incongruent melting by breakdown of biotite, sillimanite ± mullite and quartz at peak P–T of ~ 8 kbar, 730 °C along a tight-loop, clockwise P–T path. Monazite dating establishes that the partial melting occurred between ~ 1000 and 870 Ma. The absence of subgrains and systematic crystal lattice distortions in these garnets despite their elongation suggests growth pseudomorphing pre-existing 3-D networks of S1 biotite aggregates rather than high-temperature crystal plastic deformation which is noted in the S1 quartz grains that exhibit strong crystallographic preferred orientation (CPO), undulatory extinction and subgrains. Mode-I fractures in these garnet porphyroblasts induced by high melt pressure during late stage of partial melt crystallization are filled by retrograde biotite–sillimanite. Weak CPO and non-systematic crystal lattice distortions in the coarse quartz grains within the S2 leucosome domains indicate these crystallized during melt solidification without later crystal plastic deformation overprint. In the later stages of deformation (D3), strain was mostly accommodated in the mullite–biotite–sillimanite-rich restite domains forming S3 which warps around garnet and leucosome domains; consequently, fine-grained S3 quartz does not exhibit strong CPOs