Microstructures of LENS™ deposited Nb-Si alloys

Nb-Si "in-situ" metal matrix composites consist of Nb3Si and Nb5Si3 intermetallic phases in a body centered cubic Nb solid solution, and show promising potential for elevated temperature structural applications. Cr and Ti have been shown to increase the oxidation resistance and metal loss rate at elevated temperatures compared to the binary Nb-Si system. In this study, the LENS (TM) (Laser Engineered Net Shaping) process is being implemented to construct the Nb-Ti-Cr-Si alloy system from elemental powder blends. Fast cooling rates associated with LENS (TM), processing yield a reduction in microstructural scale over conventional alloy processes such as directional solidification. Other advantages of LENS (TM) processing include the ability to produce near net shaped components with graded compositions as well as a more uniform microstructure resulting from the negative enthalpy of mixing associated with the silicide phases. Processing parameters can also be varied, resulting in distinct microstructural differences. Deposits were made with varying compositions of Nb, Ti, Cr and Si. The as-deposited as well as heat treated rnicrostructures were examined using SEM and TEM techniques. The influence of composition and subsequent heat treatment on microstructure will be discussed.