2426 K.NORDLUND AND R.S.AVERBACK 56 y (a) X FIG.4.As Fig.3,but for vacancies.Circles show vacancies whose final position is in the former liquid zone,squares other vacancies. The qualitatively similar behavior of the defects in this event and the cascade events indicates that the movement of interstitials adjacent to a collision cascade outside the liquid core can be understood as a thermal migration process.The pressure gradient induced by the nonhomogeneous defect distribution appears to slightly bias the random-walk-like movement of the interstitials. In conclusion,although the pressure wave emanating from a cascade appears to have little effect on preexisting damage,the high temperatures induced by the cascade can cause some defect migration and annealing outside the liquid core of the cascade. 2.Defects in the liquid core (b) X In the cascade events with spherical defect distributions FIG.3.Movement of interstitials during one 5 keV cascade in (Au 2-4),between 10 and 20 initial vacancies and 0 and 5 gold,projected onto the xy and x=planes.Each marker shows the position of one interstitial (which may contain several atoms). Shown in the figure are all interstitials which have existed for more 5 keV Au -0.5ps than 10 ps.Circles show interstitials whose final position is in the 口 104 …10p8 zone which at some point of the simulation was liquid,squares ---20p8 ◇ 50 ps other interstitials.The open markers denote the initial positions of the interstitials and the solid markers the final positions.The lines 0 connect the initial and final position of each interstitial.The dashed 103 curve shows the maximum extent of the liquid region. 人 102 -日--p-0-8-8 0…00…0…00 cascade runs,this average movement was relatively small compared to the random movement in the cell.Since the calculation for T includes the liquid core temperature,the number of jumps per interstitial (about 40)and amount of 10 clustering seen in the cell were clearly larger than for the defects outside the liquid core in the cascade events.This 20 40 60 80 100 indicates that the local temperature has a large effect on the Distance from center(A) interstitial motion. During the 50 ps run at 205 K,11 vacancies annihilated FIG.5.Temperature at different times in a 5 keV gold cascade with interstitials.Due to their high migration energy,the rest as a function of the distance from the cascade center.The tempera- of the vacancies did not move at all from their initial lattice ture at a certain distance is the temperature of all atoms within this positions. distance from the center of the cell.cascade runs, this average movement was relatively small compared to the random movement in the cell. Since the calculation for Tm includes the liquid core temperature, the number of jumps per interstitial ~about 40! and amount of clustering seen in the cell were clearly larger than for the defects outside the liquid core in the cascade events. This indicates that the local temperature has a large effect on the interstitial motion. During the 50 ps run at 205 K, 11 vacancies annihilated with interstitials. Due to their high migration energy, the rest of the vacancies did not move at all from their initial lattice positions. The qualitatively similar behavior of the defects in this event and the cascade events indicates that the movement of interstitials adjacent to a collision cascade outside the liquid core can be understood as a thermal migration process. The pressure gradient induced by the nonhomogeneous defect distribution appears to slightly bias the random-walk-like movement of the interstitials. In conclusion, although the pressure wave emanating from a cascade appears to have little effect on preexisting damage, the high temperatures induced by the cascade can cause some defect migration and annealing outside the liquid core of the cascade. 2. Defects in the liquid core In the cascade events with spherical defect distributions FIG. 3. Movement of interstitials during one 5 keV cascade in ~Au 2–4!, between 10 and 20 initial vacancies and 0 and 5 gold, projected onto the xy and xz planes. Each marker shows the position of one interstitial ~which may contain several atoms!. Shown in the figure are all interstitials which have existed for more than 10 ps. Circles show interstitials whose final position is in the zone which at some point of the simulation was liquid, squares other interstitials. The open markers denote the initial positions of the interstitials and the solid markers the final positions. The lines connect the initial and final position of each interstitial. The dashed curve shows the maximum extent of the liquid region. FIG. 4. As Fig. 3, but for vacancies. Circles show vacancies whose final position is in the former liquid zone, squares other vacancies. FIG. 5. Temperature at different times in a 5 keV gold cascade as a function of the distance from the cascade center. The tempera￾ture at a certain distance is the temperature of all atoms within this distance from the center of the cell. 2426 K. NORDLUND AND R. S. AVERBACK 56