Creep Mechanisms in High-Temperature In-Situ Composites

Abstract

Abstract : The present work describes three year investigation of creep and toughening mechanisms in high strength, directionally solidified Nb silicides based in-situ composites. These composites consist of a Nb based silicides toughened with a Nb based solid solution. These composites have melting temperatures in excess of 1700 deg. C and densities less than 8 g/cc. The motivation for this study was to derive an understanding of a class of materials that possess a long term high temperature capability in excess of current nickel based superalloys. These studies have shown that room temperature fracture toughness can be increased to above 20 MPa (square root of m) in a range of complex silicides based composites with creep rupture behaviors superior to that of nickel based superalloys. The chemistries of the constituent phases, their volume fractions and their morphologies are critical features that control the balance of high and low temperature properties in this family of composites. During this program a thorough investigation of the dominant creep and toughening mechanisms that can be enhanced by independent modifications of the microstructure and properties of the metal and silicides phases has been completed. This investigation has examined the effect of volume fraction of silicides and strength of the metallic phase on the creep performance and fracture toughness of the composites. In-situ composite alloying concepts have been developed to provide a scheme for optimizing creep performance through control of the constituent phases. A series of phase equilibrium studies have also been completed. These include liquid solid and solid state phase equilibrium in the following systems: Nb-Ti-Si, Hf-Si, Nb-Hf-Si, and Hf-Th-Si. These investigations have been published in the open literature during the present contract.