Investigation of the Fracture Characteristics of Lamellar Nanocomposites and in Situ-Formed Composites Designed for High Temperature Service. Volume 1: Nb-Based Materials Development

摘要

There were three objectives: (1) Determine the origin of high fracture toughness for the 50Nb-13Cr-37Ti alloy. (2) Increase the fracture toughness of composites containing 40 v/o Cr2Nb. (3) Determine the influence of Al on the fracture resistance of the 50Nb-13Cr-37Ti alloy. The high fracture toughness of the alloy was explained by finding that: (1) alloying Nb-Cr with Ti has little effect on the emission from the crack tip. (2) Adding Ti decreased the P-N stress, especially for edge dislocations. (3) The detailed crack tip analysis confirmed the ease of deformation at the crack tip, and TEM confirmed that edge dislocations were moving easier than screws. The low fracture toughness of the in situ composites were found to be caused by: (1) fracture of the large Cr2Nb particles, and (2) constraint of deformation in the matrix by the particles. The microstructure of the in situ composites was modified by rapid solidification and mechanical alloying to reduce particle size. Both these fabrication methods resulted in reduction of the size of the Cr2Nb particles, but the magnitude of the fracture toughness was not increased. Constraint was measured and found to be high in these composites. Modeling indicated that the highest fracture toughness that could be expected was 25 MPa square root of m. When all these results are considered together, it appears that high fracture toughness values at ambient temperatures cannot be obtained in composites with Cr2Nb particles, because they do not deform. If the particles are small enough to prevent their fracture, then they provides so much constraint that plasticity in the matrix is limited to values that prevent the achievement of high fracture toughness. Fracture studies of in situ composites containing Nb5Si3 indicated that particles of this intermetallic deform before breaking near a crack tip. Therefore, composites based on this intermetallic ma.