A study of ductile second phase toughening of chromium-silicon alloys for high temperature aerospace applications

Cr-Si alloys offer important advantages as future aerospace materials, including high service temperature and good oxidation resistance. The alloys form in-situ composite microstructures through an eutectic reaction. The composite structure combines the properties of the hard, brittle silicide phase with the softer more ductile chromium phase. The silicide phase has a high specific stiffness from room temperature up to 1400°C. The softer chromium phase adds damage tolerance by bridging cracks and stopping them from propagating at low temperatures. Different volume percents of the two phases were evaluated and the bend strength measured as a function of temperature. Alloys with high volume percents of the silicide phase had good strength values at temperatures up to 1200°C. Although standard KIC fracture toughness tests were not performed during this initial study, microhardness indents were used to demonstrate that the chromium phase could act as a crack bridging material. In addition, the cracks surrounding the microhardness indents were used to approximate the KIC fracture toughness of the microstructures. The microstructural stability of these materials were studied up to 1600°C with encouraging results. Due to the promising results, further study of these alloys is recommended and specific areas of study are included --Abstract, page iii