The effects of Ti and Mo additions on the microstructure of Nb-silicide based in situ composites

The effects of Ti and Mo additions on phase selection, phase transformations and microstructure development in as cast and heat-treated Nb-Si-Cr-Al in situ composites have been investigated. The βNb5Si3 was formed in all the as cast alloys, which are classified as hyper-eutectic alloys. After heat treatment at 1500 °C, the βNb5Si3 transformed to αNb5Si3 completely or partially. The lattice parameter of the bcc Nb solid solution (Nbss) decreased with the addition of Ti and with increasing Mo concentration. The C14-Cr2Nb Laves phase was absent in the alloys with Ti addition at 24at.% in the presence of Mo (≤5 at.%). The eutectoid decomposition of Nb3Si to Nbss and αNb5Si3 was very sluggish in the alloy without Ti but it was enhanced with the addition of Ti. The partitioning of Ti between the Nbss and (Nb,Ti)5Si3 led to the formation of Ti-rich (Nb,Ti)5Si3, where the concentration of Ti was about 30.3 at.% in the heat treated microstructure. Nbss particles precipitated inside αNb5Si3 in the heat treated alloys.