Effects of oversized tungsten on the primary damage behavior in Fe-W alloys

Tungsten (W) is proposed to replace molybdenum (Mo) as an alternative minor alloying element in iron (Fe) based ferritic alloys to reduce the activation time upon irradiation. Tungsten, which is about 10.5% larger in atomic radius or 34.8% larger in atomic volume than Fe, can induce both global volume expansion and local lattice distortion in the Fe matrix. To understand how oversized W influences the defect production behavior in Fe-based alloys, molecular dynamics simulations are conducted to study the primary damage in three systems at 300 K: (a) unstrained pure Fe, (b) Fe-5at.%W (Fe-5W) alloy, and (c) strained pure Fe with the same volume expansion as the Fe-5W. Compared to the unstrained pure Fe, both Fe-5W and strained pure Fe result in producing more Frenkel pairs at high irradiation energies, indicating that the global volume expansion may lead to enhanced defect production. Meanwhile, the damaged Fe-5W alloy contains many C15 Laves-phase-like interstitial clusters, while the interstitial clusters in the unstrained and strained pure Fe systems are mainly crowdion bundles or dislocation loops. The average size of vacancy clusters in the Fe-5W alloy is much smaller than its counterparts in the two pure Fe systems. The average size of interstitial clusters in the Fe-5W alloy is also smaller than those in the two pure Fe systems. These results indicate that the local lattice distortion induced by oversized W can significantly influence the morphologies and size distributions of defect structures. Finally, defect formation energies are calculated to interpret the different defect production behaviors in these systems.