On the origin of large interstitial clusters in displacement cascades

Abstract

Displacement cascades with wide ranges of primary knock-on atom (PKA) energy and mass in iron were simulated using molecular dynamics. New visualisation techniques are introduced to show how the shock-front dynamics and internal structure of a cascade develop over time. These reveal that the nature of the final damage is determined early on in the cascade process. We define a zone (termed 'spaghetti') in which atoms are moved to new lattice sites and show how it is created by a supersonic shockfront expanding from the primary recoil event. A large cluster of selfinterstitial atoms can form on the periphery of the spaghetti if a hypersonic recoil creates damage with a supersonic shock ahead of the main supersonic front. When the two fronts meet, the main one injects atoms into the lowdensity core of the other: these become interstitial atoms during the rapid recovery of the surrounding crystal. The hypersonic recoil occurs in less than 0.1 ps after the primary recoil and the interstitial cluster is formed before the onset of the thermal spike phase of the cascade process. The corresponding number of vacancies is then formed in the spaghetti core as the crystal cools, i.e. at times one to two orders of magnitude longer. By using the spaghetti zone to define cascade volume, the energy density of a cascade is shown to be almost independent of the PKA mass. This throws into doubt the conventional energy-density interpretation of an increased defect yield with increasing PKA mass in ion irradiation.

Cascade morphology transition in bcc metals

“…Calder et al investigated defect clustering in Fe [2]. Using a single Fe PKA, they did not observe interacting multiple shocks.…”

“…This correlation suggests three cascade morphologies as a function of energy, To reveal the cascade morphology, a colormap of atom density (number of neighbors within 1.4a 0 , a 0 is the lattice constant) is analyzed as in Ref. [2] using the OVITO code [19]. Figure 4d shows the number of displaced atoms, N DA , vs. t from a typical high-energy (75 keV) cascade in W. Displaced atoms are those beyond 0.3a 0 from any lattice site, and are useful to reveal the pressure wave progression (note that displaced atoms create pressure variation within the materials).…”

Displacement cascades and defects annealing in tungsten, Part I: Defect database from molecular dynamics simulations

“…The formation of such clusters is presumed to be an athermal process. Such clusters typically form within the first 0.2 ps of the cascade development when the destructive phase occurs [11,19]. During this phase, the cascade energy is transferred from atom to atom much faster than the phonon-mediated dissipation allows, hence supersonic shock waves form.…”

Molecular dynamics and object kinetic Monte Carlo study of radiation-induced motion of voids and He bubbles in bcc iron

“…An interesting mechanism for interstitial cluster formation during cascades in pure α-iron has been observed by Calder et al 6 They find that some cascades emit particles moving faster than the cascade wave front, which causes secondary cascades ahead of the main one. Collision of the high-density primary wave with the core of the secondary cascades can lead to nucleation sites for interstitial clusters.…”

“…Additionally, it provides an effective method for visualizing the cascade by joining a line from the initial to final location of each atom (cf. Calder et al 6 ). The region covered by these lines is the part of the simulation "melted" by the cascade and the lines tend to form connected paths due to replacement-collision events.…”