  Proposed as blanket structural materials for fusion power reactors, reduced
activation ferritic/martensitic (RAFM) steel undergoes volume expanding and
contracting in a cyclic mode under service environment. Particularly, being
subjected to significant fluxes of fusion neutrons RAFM steel suffers
considerable local volume variations in the radiation damage involved regions.
It is necessary to study the structure properties of the alloying elements in
contraction and expansion states. In this paper we studied local substitution
structures of thirteen alloying elements Al, Co, Cr, Cu, Mn, Mo, Nb, Ni, Si,
Ta, Ti, V, and W in bcc Fe and calculated their substitutional energies in the
volume variation range from -1.0% to 1.0%. From the structure relaxation
results of the first five neighbor shells around the substitutional atom we
find the relaxation in each neighbor shell keeps approximately uniform within
the volume variation from -1.0% to 1.0% except those of Mn and the relaxation
of the fifth neighbor shell is stronger than that of the third and forth,
indicating that the lattice distortion due to the substitution atom is easier
to spread in <111> direction than in other direction. The relaxation pattern
and intensity are related to the size and electron structure of the
substitutional atom. For some alloying elements, such as Mo, Nb, Ni, Ta, Ti and
W, the substitutional energy decreases noticeably when the volume increases.
Further analysis show that the substitutional energy comprises the energy
variation originated from local structure relaxation and the chemical potential
difference of the substitutional atom between its elemental crystalline state
and the solid solution phase in bcc Fe. We think the approximately uniform
relaxation of each neighbor shell around a substitutional atom give rise to a
linear decrease in the substitutional energy with the increasing volume.
