Energetic effects of dopants on the eutectoid decomposition of Nb-Si in situ composites.

The influences of doping elements (Ti, Zr, Hf, V, Ta, Cr, Mo, W and Al) on the heats of formation of Nb, Nb5Si3 and Nb3Si have been studied using first-principles pseudopotential plane-wave method based on density functional theory. Site preferences of dopants in Nb5Si3 and Nb3Si intermetallics are first determined by comparing the heats of formation of the systems with different site occupations of the doping atoms. The partitionings of dopants between the equilibrium phases Nb and Nb5Si3 of Nb-Si in situ composites are then discussed and compared with experimental results. Phase partitioning behaviours of the alloying elements are found to depend strongly on the number of their valence electrons. By calculating the enthalpy of reaction regarding the eutectoid decomposition of Nb3Si phase, we conclude that Ta is a Nb3Si stabiliser while other dopants destabilise Nb3Si at the investigated concentration. [ABSTRACT FROM AUTHOR]