 Award ID(s):
 1904830
 Publication Date:
 NSFPAR ID:
 10146386
 Journal Name:
 ArXivorg
 ISSN:
 23318422
 Sponsoring Org:
 National Science Foundation
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Study of the plastic flow and straininduced 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 pressuredependence of the yield strength and proves that ωZr behaves like perfectly plastic, isotropic, and strain pathindependent immediately after PT. Record minimum pressure for αω PT was identified. Kinetics of straininduced PT depends on plastic strain and time. Crystallite size and dislocation density in ωZr during PT depend solely on the volume fraction of ωZr.

Abstract Deepfocus 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 − 10^{3} 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 sheardominated seismic signals with volumechangedominated 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 straininduced (instead of pressure/stressinduced) and find an analytical 3D solution for coupled deformationtransformationheating in a shear band. This solution predicts conditions for severe (singular) transformationinduced plasticity (TRIP) and selfblownup deformationtransformationheating process due to positive thermomechanochemical feedback between TRIP and straininduced transformation. This process leads to temperature in a band, above which the selfblownup shearheating process in the shear band occurs after finishing the PT. Our findings change the main concepts in studying the initiation of the deepfocus earthquakes and PTs during plastic flow in geophysics in general.

Deepfocus earthquakes that occur at 350–660 km, where pressures p =1223 GPa and temperature T =18002000 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 [3]) 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 volumechange 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 straininduced (instead of pressure/stressinduced) and find an analytical 3D solution for coupled deformationtransformationheating processes in a shear band. This solution predicts conditions for severe (singular) transformationinduced plasticity (TRIP) and selfblownup deformationtransformationheating process due to positive thermomechanochemical feedback between TRIP and straininduced transformation. In nature, this process leads to temperature in a band exceedingmore »

The PM4Silt plasticity model for representing lowplasticity silts and clays in geotechnical earthquake engineering applications is presented herein. The PM4Silt model builds on the framework of the stressratio 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 lowplasticity 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 poststrongshaking shear strength reduction factor.more »

The 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 Xray diffraction employing platinum as an internal pressure standard. The deformation behavior of HfB2 was studied by radial Xray 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 Xray diffraction measurements yielded an ambientpressure volume V0 = 29.73 Å3/atom and a bulk modulus K0 = 282 GPa. Density functional theory calculations showed ambientpressure 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 strainstress calculation shows a maximum shear stress τmax~39 GPa along the (1−10) [110] direction of the hexagonal lattice of HfB2, which thereby can be an incompressible high strength material for extremeenvironment applications.