Microstructures and Mechanical Properties of Nb/Nb-Silicide <I>in-situ</I> Composites Synthesized by Reactive Hot Pressing of Ball Milled Powders

Abstract

To develop advanced structural materials at very high temperatures, ball milling followed by reactive hot pressing was employed to fabricate binary Nb/Nb-silicide in-situ composites with 3.5 to 16 mol% silicon content. Changes in morphology during milling of niobium and silicon elemental powders were studied, and microstructures and mechanical properties of reactively hot pressed composites were evaluated by OM, SEM, XRD, DTA and EPMA. No detectable niobium silicide was formed during milling. Reactive hot pressing of the milled powders yields two kinds of silicides, Nb3Si and Nb5Si3, through in-situ reaction between heavily deformed niobium particles and fragmented silicon particles. DTA revealed that silicide formation occurs at 940 K for Nb3Si and 1050 K for Nb5Si3. Fully dense Nb/Nb-silicide in-situ composites with equiaxed grain microstructures are produced by the reactive hot pressing of milled powders. All of the composites exhibit considerable compressive ductility without cracking even at room temperature. Annealing at high temperature results in growth of equiaxed grains of niobium and silicide, thereby increasing high temperature strength.