Virtual melting as a new mechanism of stress relaxation under high strain rate loading

Abstract

Generation and motion of dislocations and twinning are the main mechanisms of plastic deformation. A new mechanism of plastic deformation and stress relaxation at high strain rates (10 9 -10 12 s −1 ) is proposed, under which virtual melting occurs at temperatures much below the melting temperature. Virtual melting is predicted using a developed, advanced thermodynamic approach and confirmed by large-scale molecular dynamics simulations of shockwave propagation and quasi-isentropic compression in both single and defective crystals. The work and energy of nonhydrostatic stresses at the shock front drastically increase the driving force for melting from the uniaxially compressed solid state, reducing the melting temperature by 80% or 4,000 K. After melting, the relaxation of nonhydrostatic stresses leads to an undercooled and unstable liquid, which recrystallizes in picosecond time scales to a hydrostatically loaded crystal. Characteristic parameters for virtual melting are determined from molecular dynamics simulations of Cu shocked/compressed along the h110i and h111i directions and Al shocked/compressed along the h110i direction.high strain-rate plasticity | relaxation of non-hydrostatic stresses | thermodynamics under uniaxial straining G eneration and motion of dislocations, twinning, and crystalcrystal phase transformations are the main mechanisms of plastic deformation and relaxation of nonhydrostatic stresses that are reflected in the deformation-mechanism maps (1, 2). Recent large-scale nonequilibrium molecular dynamics (NEMD) simulations of shockwave propagation in fcc metallic single crystals have revealed that for wave propagation along the h110i and h111i directions, melting occurs at temperatures below the equilibrium melt temperature T m ðpÞ at the corresponding shock pressure p-e.g., for Cu by 20% in ref. 3 and by 7-8% (AE4%) in ref. 4. However, for shockwaves along the h001i direction, superheating is observed. A possible mechanism for the observed melting below T m ðpÞ has been attributed to solid-state disordering due to high defect densities. Indeed, a small reduction in the melt temperature due to defects has been reported (5). It has also been argued that anomalous plastic flow rather than bulk melt can explain the experimentally observed low melt temperatures of Ta under pressure (6). Here, we propose a new deformation mechanism, under which melting can occur at temperatures much below T m ðpÞ in materials subjected to high deviatoric stresses (such as those produced under shock loading and isentropic deformation) and compete with defect nucleation mechanisms, as will be shown here in the case of metals. We have developed a thermomechanical theory of melting under uniaxial deformation that predicts extremely large driving forces. The thermodynamic driving force for melting is due to the energy and work of nonhydrostatic stresses. The theory is quite general and not materialspecific, and it is most applicable at deformation rates sufficiently high, where melting may proceed at rates faster th...

Shock-induced plasticity in tantalum single crystals: Interatomic potentials and large-scale molecular-dynamics simulations

“…This premelting phenomenon has been observed by several authors in simulations of directional melting in fcc single crystals. [73][74][75][76] However, unlike fcc crystals where only the (110) and (111) directions exhibit premelting and the (100) orientation exhibits overheating, here the shocked crystal premelts in all three directions at about the same pressure-temperature point in the Hugoniot. The thickness of the metastable liquid region behind the shock increases with shock temperature and/or pressure.…”

Molecular dynamics simulation of shock-wave loading of copper and titanium

“…The fact that it is the BCC phase rather than the FCC phase that is stable in MD simulations at pressures above 100 GPa and temperatures above 2000 K was not mentioned in previous publications, though the calculations [7,8] were also performed for the shock wave propagation regime close to the regime of complete melting with the use of the same interatomic interaction potential [1]. A possible reason is the use of only paired correlation function and the central symmetry parameter [3] in analyzing the resultant phase state in [7,8].…”

“…The calculation of structure factor (1) clearly shows an absence of any long-range order in that atomic configurations in a section of simulation cell behind shock front ( 30 nm X ). In this case we observe the effect which was called virtual melting in [8]. The mechanism of this crystal structure damage is related to the fact that, due to the nonhydrostatic character of loading in bulk behind the shock front, high tangential stresses arise, which are related to the deviator part of the stress tensor [5,6,8].…”

“…In this case we observe the effect which was called virtual melting in [8]. The mechanism of this crystal structure damage is related to the fact that, due to the nonhydrostatic character of loading in bulk behind the shock front, high tangential stresses arise, which are related to the deviator part of the stress tensor [5,6,8]. During compression, the tangential stresses achieve the maximum about 0.2 GPa for a calculation presented on Fig.…”

“…A possible reason is the use of only paired correlation function and the central symmetry parameter [3] in analyzing the resultant phase state in [7,8]. However, these parameters are insufficient for the structural analysis at extremely high temperatures.…”