Homogeneous phase separation in binary alloys under ion irradiation conditions: Role of interstitial atoms

Abstract

Kinetic Monte Carlo simulations have been performed to investigate homogeneous precipitation in binary metal alloys under ion irradiation conditions. The kinetic model includes defect production, recombination, defect trapping, the formation of defect-solute complexes, and atomic mixing. While no special assumptions are made about vacancy diffusion, interstitial diffusion is assumed fast compared to the characteristic time scales of all other processes. The diffusion path of an interstitial, moreover, ends with one of the three possible outcomes: recombination, trapping in a solute-rich location, or clustering with other interstitials. Interstitials promote solute segregation in this model by the formation of mobile interstitial-solute complexes. Several unusual features of radiation-induced precipitation are revealed in this model. For ideal alloys, we find at a given temperature that for every trapping number [i.e., the minimum number of nearest-neighbor solute (type $B$) atoms required to trap a migrating interstitial, ${N}_{BT}$], there exists a corresponding defect pair production rate ${K}_{0}$, below which the alloy becomes a random solution state and above which macroscopic phase separation occurs. ${K}_{0}$ depends on the length scale of trapping sites $L$ as ${L}^{4}$. Solute-rich precipitates have the composition ${c}_{B}$ approaching ${c}_{B}={N}_{BT}∕z$, where $z$ is the lattice coordination number. This feature results in ``swelling'' of precipitates, i.e., dilution of initially pure $({c}_{B}=1)$ precipitates located in the matrix prior to ion irradiation. Ballistic mixing is observed to erode precipitates, and above some critical rate the system reverts to a random solution. For ideal solutions, and even those with a small tendency for ordering, phase separation occurs due to the interstitial interaction with solutes. At sufficiently high positive values of the heats of mixing, the usual thermal vacancy-driven precipitation prevails. Between these low and high limits, the alloy unexpectedly enters a field of solid solutions. Finally, it is shown that even in the absence of the interstitial trapping $({N}_{BT}>z)$, alloys with a small positive ordering energy can undergo nonequilibrium phase separation at a composition below its solubility limit due to an effective trapping of vacancies in solute-rich locations. The significance of these findings for real alloy systems is discussed.