Effect of Cr addition on microstructure and mechanical properties in Nb-Si-Mo base multiphase alloys

Abstract

The effects of Cr addition to Nb-22Si-5Mo alloy on phase equilibria, microstructures and mechanical properties are investigated by metallography. X-ray diffraction, scanning electron microscopy equipped with wavelength dispersive X-ray fluorescence spectroscopy and compression test at temperatures from room temperature to 1773 K. With increasing Cr content a duplex microstructure consisting of Nb5Si3 and Nbss (niobium solid solution) is changed to three-phase microstructure consisting of Nb5Si3, Nbss and NbCr2. Chromium additions does not change the volume fraction of constituent phases in the two-phase alloys, whereas the volume fraction of NbCr2, C14 Laves phase, at the expensed of mostly Nbss in the three-phase alloys. The c/a axis ratio of α-Nb5Si3 phase increases with increasing (Cr + Mo) content. It is found that Cr alloying in Nbss increases the room temperature strength due to atomic size misfit and decreases high temperature strength due to accelerated diffusion. The existence of C14 Laves phase increases high temperature strength, but degrades room temperature deformability. High temperature strength is found to be sensitive to the volume fraction and crystal structure of constituent phases as well as microstructure.