MATERIALS TRANSACTIONS
Online ISSN : 1347-5320
Print ISSN : 1345-9678
ISSN-L : 1345-9678
High-Pressure Phase Transformations under Severe Plastic Deformation by Torsion in Rotational Anvils
Valery I. Levitas
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2019 Volume 60 Issue 7 Pages 1294-1301

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Abstract

Numerous experiments have documented that combination of severe plastic deformation and high mean pressure during high-pressure torsion in rotational metallic, ceramic, or diamond anvils produces various important mechanochemical effects. We will focus here on four of these: plastic deformation (a) significantly reduces pressure for initiation and completion of phase transformations (PTs), (b) leads to discovery of hidden metastable phases and compounds, (c) reduces PT pressure hysteresis, and (d) substitutes a reversible PT with irreversible PT. The goal of this review is to summarize our current understanding of the underlying phenomena based on multiscale atomistic and continuum theories and computational modeling. Recent atomistic simulations provide conditions for initiation of PTs in a defect-free lattice as a function of the general stress tensor. These conditions (a) allow one to determine stress states that significantly decrease the transformation pressure and (b) determine whether the given phase can, in principle, be preserved at ambient pressure. Nanoscale mechanisms of phase nucleation at plastic-strain-induced defects are studied analytically and by utilizing advanced phase field theory and simulations. It is demonstrated that the concentration of all components of the stress tensor near the tip of the dislocation pileup may decrease nucleation pressure by a factor of ten or more. These results are incorporated into the microscale analytical kinetic equation for strain-induced PTs. The kinetic equation is part of a macroscale geometrically-nonlinear model for combined plastic flow and PT. This model is used for finite-element simulations of plastic deformations and PT in a sample under torsion in a rotational anvil device. Numerous experimentally-observed phenomena are reproduced, and new effects are predicted and then confirmed experimentally. Combination of the results on all four scales suggests novel synthetic routes for new or known high-pressure phases (HPPs), experimental characterization of strain-induced PTs under high-pressure during torsion under elevated pressure.

Fig. 3 Stationary solutions for interaction of the phase transformation to a HPP (red color) and dislocations in a bicrystalline sample subjected to normal stress and shear strain, without (a) and with (b) plasticity in the right grain. Dislocation pileup causes PT at pressure (averaged over the grain) significantly smaller than under hydrostatic loading with a single dislocation. Dislocations promote PT by producing stress-tensor concentrators (a) but also suppress PT through relaxation of stresses near the tip of the dislocation pileup (b). The contour lines of the equilibrium PT work are presented in black and, in most cases, coincide with the stationary phase interfaces. This figure is reproduced with permission from Ref. 39). Fullsize Image
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© 2019 The Japan Institute of Metals and Materials
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