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The influences of doping elements (Ti, Zr, Hf, V, Ta, Cr, Mo, Wand Al) on the heats of formation of Nb, Nb(5)Si(3) and Nb(3)Si have been studied using first-principles pseudopotential plane-wave method based on density functional theory. Site preferences of dopants in Nb(5)Si(3) and Nb(3)Si 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 Nb(5)Si(3) 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 Nb(3)Si phase, we conclude that Ta is a Nb(3)Si stabiliser while other dopants destabilise Nb(3)Si at the investigated concentration.