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Study of the plastic flow and strain-induced phase transformations (PTs) under high pressure with diamond anvils is important for material and geophysics. We introduce rough diamond anvils and apply them to Zr, which drastically change the plastic flow, microstructure, and PTs. Multiple steady microstructures independent of pressure, plastic strain, and strain path are reached. Maximum friction equal to the yield strength in shear is achieved. This allows determination of the pressure-dependence of the yield strength and proves that ω-Zr behaves like perfectly plastic, isotropic, and strain path-independent immediately after PT. Record minimum pressure for α-ω PT was identified. Kinetics of strain-induced PT depends on plastic strain and time. Crystallite size and dislocation density in ω-Zr during PT depend solely on the volume fraction of ω-Zr.
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−1to seismic 10 − 103 s−1strain 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.
Deep-focus earthquakes that occur at 350–660 km, where pressures p =12-23 GPa and temperature T =1800-2000 K, are generally assumed to be caused by olivine→spinel phase transformation, see pioneering works [1–10]. However, there are many existing puzzles: (a) What are the mechanisms for jump from geological 10−17−10−15 s−1 to seismic 10−103s−1(see ) strain rates? Is it possible without phase transformation? (b) How does metastable olivine, which does not completely transform to spinel at high temperature and deeply in the region of stability of spinel for over the million years, suddenly transforms during seconds and generates seismic strain rates 10−103s−1 that produce strong seismic waves? (c) How to connect deviatorically dominated seismic signals with volume-change dominated transformation strain during phase transformations [9,11]? Here we introduce a combination of several novel concepts that allow us to 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 processes 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. In nature, this process leads to temperature in a band exceedingmore »
The PM4Silt plasticity model for representing low-plasticity silts and clays in geotechnical earthquake engineering applications is presented herein. The PM4Silt model builds on the framework of the stress-ratio controlled, critical state compatible, bounding surface plasticity PM4Sand model (version 3) described in Boulanger and Ziotopoulou (2015) and Ziotopoulou and Boulanger (2016). Modifications to the model were developed and implemented to improve its ability to approximate undrained monotonic and cyclic loading responses of low-plasticity silts and clays, as opposed to those for purely nonplastic silts or sands. Emphasis was given to obtaining reasonable approximations of undrained monotonic shear strengths, undrained cyclic shear strengths, and shear modulus reduction and hysteretic damping responses across a range of initial static shear stress and overburden stress conditions. The model does not include a cap, and therefore is not suited for simulating consolidation settlements or strength evolution with consolidation stress history. The model is cast in terms of the state parameter relative to a linear critical state line in void ratio versus logarithm of mean effective stress. The primary input parameters are the undrained shear strength ratio (or undrained shear strength), the shear modulus coefficient, the contraction rate parameter, and an optional post-strong-shaking shear strength reduction factor.more »
Experimental and Computational Studies of Compression and Deformation Behavior of Hafnium Diboride to 208 GPaThe compression behavior of the hexagonal AlB2 phase of Hafnium Diboride (HfB2) was studied in a diamond anvil cell to a pressure of 208 GPa by axial X-ray diffraction employing platinum as an internal pressure standard. The deformation behavior of HfB2 was studied by radial X-ray diffraction technique to 50 GPa, which allows for measurement of maximum differential stress or compressive yield strength at high pressures. The hydrostatic compression curve deduced from radial X-ray diffraction measurements yielded an ambient-pressure volume V0 = 29.73 Å3/atom and a bulk modulus K0 = 282 GPa. Density functional theory calculations showed ambient-pressure volume V0 = 29.84 Å3/atom and bulk modulus K0 = 262 GPa, which are in good agreement with the hydrostatic experimental values. The measured compressive yield strength approaches 3% of the shear modulus at a pressure of 50 GPa. The theoretical strain-stress calculation shows a maximum shear stress τmax~39 GPa along the (1−10)  direction of the hexagonal lattice of HfB2, which thereby can be an incompressible high strength material for extreme-environment applications.