Misfit strain affecting the lamellar microstructure in NbSi2/MoSi2 duplex crystals

The effect of including an additional element, like Re, Cr, V, Zr or Ir, in (Mo0.85Nb0.15)Si-2 crystals, carried out with the aim of improving the thermal stability of the fine lamellar microstructures of the crystals, was examined with the help of first-principles calculations. The addition improved the thermal stability of the microstructures at 1400 degrees C in all cases, and the effect was especially significant in crystals containing Cr and Zr additives. Quantitative evaluation by Moire fringe analysis revealed that the misfit strain at the interfaces in crystals with added Cr and Zr was smaller than those in crystals with other elements added. The reduced misfit strain resulted in the suppression of the rapid growth of the C11(b) phase precipitates that did not possess the variant orientation relationship with the C40 matrix, and hence the thermal stability of the fine lamellar microstructures improved. The results further revealed that the ratio of the atomic size of the additive element to that of Mo, which is the main constituent element in the C11(b) phase, is an important factor that controlled the misfit strain at the lamellar interface by the result of Cr addition. However, the addition of Zr was found to result in a significant improvement in the thermal stability of the lamellar microstructure, despite the markedly larger atomic radius of Zr compared with that of Mo. This indicates that there may be other factors, in addition to the atomic radius of the additive, contributing to the thermal stability. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.