  Reduced activation ferritic/martensitic (RAFM) steels are structural
materials with potential application in Generation-IV fission and fusion
reactors. We use density-functional theory to scrutinize the micro-mechanical
properties of the main alloy phases of three RAFM steels based on the
body-centered cubic FeCrWVMn solid solution. We assess the lattice parameters
and elastic properties of ferromagnetic $\alpha$-Fe and Fe$_{91}$Cr$_{9}$,
which are the main building blocks of the RAFM steels, and present a detailed
analysis of the calculated alloying effects of V, Cr, Mn, and W on the
mechanical properties of Fe$_{91}$Cr$_{9}$. The composition dependence of the
elastic parameters is decomposed into electronic and volumetric contributions
and studied for alloying levels that cover the typical intervals in RAFM
steels. A linear superposition of the individual solute effects on the
properties of Fe$_{91}$Cr$_{9}$ is shown to provide an excellent approximation
for the \emph{ab initio} values obtained for the RAFM steels. The intrinsic
ductility is evaluated through Rice's phenomenological theory using the surface
and unstable stacking fault energies, and the predictions are contrasted with
those obtained by empirical criteria. Alloying with V or W is found to enhance
the ductility, whereas additional Cr or Mn turns the RAFM base alloys more
brittle.
