Stress-induced amorphization triggers deformation in the lithospheric mantle

Amorphous shear bands in crystalline materials as drivers of plasticity

Traditionally, the formation of amorphous shear bands in crystalline materials has been undesirable, because shear bands can nucleate voids and act as precursors to fracture. They also form as a final stage of accumulated damage. Only recently were shear bands found to form in undefected crystals, where they serve as the primary driver of plasticity without nucleating voids. Here we have discovered trends in materials properties that determine when amorphous shear bands will form and whether they will drive plasticity or lead to fracture. We have identified the materials systems that exhibit shear-band deformation, and by varying the composition, we were able to switch from ductile to brittle behaviour. Our findings are based on a combination of experimental characterization and atomistic simulations, and they provide a potential strategy for increasing the toughness of nominally brittle materials.

Intracrystalline deformation microstructures in natural olivine with implications for stress estimation

Constraining the stress related to lithospheric deformation in natural rocks is key to develop and test a geodynamic model. However, the cautions of extrapolating piezometers that are established on experimental samples to natural rocks are less addressed. In this study, we investigated the microstructures of a natural harzburgite sample using the electron backscatter diffraction (EBSD) technique. Subgrain boundary (SGB) geometries suggest large percentages of (010)[100] and {0kl}[100] dislocation slip systems in olivines. More importantly, multiple low-angle misorientation boundaries (LAMBs) variants are recognized for the first time in olivine based on their distinctive characteristics with the change of EBSD mapping step size. The LAMBs that exist at a small step size (≤ 1 μm) are mostly equivalent to real SGBs, while other LAMBs that appear only when the step size is larger (> 1 μm) are artificial SGBs. Besides, the former develop mainly in the high LAMB density grains, whereas the latter are mostly found in the low LAMB density grains. This result reinforces the previous knowledge that the stress calculated using subgrain-related piezometers is meaningful only when real SGBs are captured at sufficiently small step size. Furthermore, we provide a proof of concept that SGB density and kernel average misorientation (KAM) are two viable metrics to estimate stress. These two alternative piezometers, which still need calibrations using the experimentally deformed samples, are anticipated to have wide applications in natural rocks.

Real-time in-situ optical detection of fluid viscosity based on the Beer-Lambert law and machine learning

As an important physical quantity to describe the resistance of fluid to flow, viscosity is an essential property of fluids in fluid mechanics, chemistry, medicine, as well as hydraulic engineering. While traditional measurement methods, including the rotating-cylinder method, capillary tube method and falling sphere method, have significant drawbacks especially in terms of accuracy, response time and the sample must be made to move. In this work, a novel Beer-Lambert law-based method was proposed for the viscosity measurement. Specifically, this work demonstrates that viscosity can be quantitatively reflected by spectral line intensity, and castor oil was selected due to its viscous temperature properties (viscosity has been accurately measured under different temperature), and its transmission spectrum at different temperatures ranging from 10 to 50°C was detected firstly. Then, the principal component analysis (PCA) was employed to obtain the intrinsic features of the transmission spectrum. Finally, the processed data was utilized to train and verify the radial basis function (RBF) neural network. As a result, the accuracy of the predictions conducted by means of the RBF reached 98.45%, which indicates the complicated and non-linear relationships between spectra formation and viscosity can be depicted well by RBF. The results show that the real-time in-situ optical detection method adopted in this work represents a great leap forward in the viscosity measurement, which fundamentally reforms the traditional viscosity measurement methods.

Resolving puzzles of the phase-transformation-based mechanism of the strong deep-focus earthquake

Deep-focus earthquakes that occur at 350–660 km are assumed to be caused by olivine → spinel phase transformation (PT). However, there are many existing puzzles: (a) What are the mechanisms for jump from geological 10⁻¹⁷ − 10⁻¹⁵ s⁻¹ to seismic 10 − 10³ s⁻¹ strain rates? Is it possible without PT? (b) How does metastable olivine, which does not completely transform to spinel for over a million years, suddenly transform during seconds? (c) How to connect shear-dominated seismic signals with volume-change-dominated PT strain? Here, we introduce a combination of several novel concepts that resolve the above puzzles quantitatively. We treat the transformation in olivine like plastic strain-induced (instead of pressure/stress-induced) and find an analytical 3D solution for coupled deformation-transformation-heating in a shear band. This solution predicts conditions for severe (singular) transformation-induced plasticity (TRIP) and self-blown-up deformation-transformation-heating process due to positive thermomechanochemical feedback between TRIP and strain-induced transformation. This process leads to temperature in a band, above which the self-blown-up shear-heating process in the shear band occurs after finishing the PT. Our findings change the main concepts in studying the initiation of the deep-focus earthquakes and PTs during plastic flow in geophysics in general.

The developed theory for coupled deformation, plastic strain-induced phase transformation, transformation-induced plasticity, and self-blown-up deformation-transformation-heating in shear band explains the main puzzles of deep-focus earthquakes.

Mechanical Properties and Deformation Behavior of Superhard Lightweight Nanocrystalline Ceramics

Lightweight polycrystalline ceramics possess promising physical, chemical, and mechanical properties, which can be used in a variety of important structural applications. However, these ceramics with coarse-grained structures are brittle and have low fracture toughness due to their rigid covalent bonding (more often consisting of high-angle grain boundaries) that can cause catastrophic failures. Nanocrystalline ceramics with soft interface phases or disordered structures at grain boundaries have been demonstrated to enhance their mechanical properties, such as strength, toughness, and ductility, significantly. In this review, the underlying deformation mechanisms that are contributing to the enhanced mechanical properties of superhard nanocrystalline ceramics, particularly in boron carbide and silicon carbide, are elucidated using state-of-the-art transmission electron microscopy and first-principles simulations. The observations on these superhard ceramics revealed that grain boundary sliding induced amorphization can effectively accommodate local deformation, leading to an outstanding combination of mechanical properties.