Microstructure and room temperature deformation of Nbss/Nb5Si3 in situ composites alloyed with Mo

Room temperature deformation behavior of Nb5Si3 based intermetallics in the ternary Nb-Si-Mo alloy system is investigated by compression testing in relation to microstructure and compositional effect. The partial ternary phase diagram of the Nb-Si-Mo alloy system containing phase equilibrium information at 1973 K is determined by metallography, X-ray diffraction and scanning electron microscopy (SEM) equipped with wavelength-dispersive X-ray fluorescence spectroscopy (WDS). A pseudo-binary compound of Nb5Si3-Mo5Si3 is confirmed to form at Nb5Si3-rich compositions by Mo addition, without having a solubility range of Si. The phase transformation from alpha -Nb5Si3 to beta -Nb5Si3 is found to occur at about 5 at.% of Mo content on the Nb5Si3-Mo5Si3 pseudo-binary line. The observed yield stresses are largely dependent on not only the volume fraction and morphology of constituent phases but also the strength of Nb5Si3 phase equilibrating with Nb solid solution. The in situ composites consisting of alpha -Nb5Si3 phase and bcc solid solution exhibit higher yield stress than those consisting of beta -Nb5Si3 phase and bcc solid solution when yield stress is compared at the same volume fraction of bcc phase. It is suggested that room temperature deformability of beta -Nb5Si3 is superior to that of alpha -Nb5Si3 phase. Room temperature deformation behavior of the present composites will be discussed in relation to microstructural evolution. (C) 2001 Elsevier Science Ltd. All rights reserved.