The Effect Of Dislocation And Nanovoild On The Shock-induced Phase Transition In UN

In this paper,the phase transition of UN crystal under impact pressure was studied by molecular dynamics simulations.Firstly,the lattice parameters,elastic properties and sound velocities of UN crystal under hydrostatic pressure are investigated.The calculated lattice constants of the Fm-3m and R-3m structure agree well with the experimental and other theoretical values.The phase transition from Fm-3m phase to R-3m structure under 35GPa hydrostatic pressure is verified by the discontinuities of the variation of calculated elastic constants and sound velocities with hydrostatic pressure.The impact pressures were applied to<100>,<110>and<111>crystal directions respectively,under a certain shock pressure,it was found that the sound velocity US and particle velocity UP along close-packed<100>crystal direction have a maximum values.For<100>,<110>and<111>orientations,the impact pressures at phase transformation are 42GPa,32GPa and 29GPa,respectively.Due to the different shock loading crystal directions,the lattice constants of R-3m structure obtained under shock pressure are also slightly differentConsidering the effect of dislocation on the phase transition as the shock pressure loaded along<110>direction,it is found that the phase transition pressure decreases with the increasing dislocation density.The phase transition pressure has a rapidly decline when the dislocation density increases initially,and then the decline trend becomes flattening and smoothing with the increasing dislocation density.When the dislocation density increases to 8.45×1011cm-2,the phase transition pressure decreases from the initial 32GPa to 25GPa.In the process of shock loading.the R-3m phase are emitted preferentially from dislocation to extra half-plane along<100>crystal direction and forms banded phase transition zone,then extend to the non-transition regions.The compress stress of R-3m phase in formed transition zones is larger than that of Fm-3m structure in non-transition regions.With the loading of shock pressure,the R-3m phase are also initiated from dislocation to lower regions of dislocation along<100>crystal direction.The phase transition are observed to preferentially propagates along the close-packed<100>direction.It may due to that UN have a minimum atomic distance along the close-packed<100>direction,the atoms in phase transition regions are more likely to drive the phase transition in non-transition regions along<100>direction.As the formed banded transition zones widen and spread to non-transition regions,some smaller banded transition zones are also initiated in non-transition regions.Finally the widening and spreading of these banded transition zones makes UN crystal compete the phase transition from Fm-3m to R-3m structure.Similar to the effect of dislocation,the phase transition pressure decreases with the increasing nanovoid diameter.The phase transition pressure has a rapidly decline when the nanovoid diameter increases initially,and then the decline trend becomes flattening and smoothing with the increasing nanovoid diameter.The R-3m phase are emitted firstly from nanovoid to non-transition regions along close-packed<100>direction.These formed banded transition zones spread and extend out until the Fm-3m phase finishes the transformation to R-3m structure.The atoms around nanovoid have a larger energy than that of other atoms and are prone to have an occurrence of phase transition,this induced role makes the phase transition emitted preferentially around nanovoid.In addition,the case of two nanovoids also illustrates the induced effect on phase transition.