Investigation of Fracture Resistance in Microstructures of IntermetallicMaterials for High Temperature Service

摘要

In situ composites based on the intermetallic Cr2Nb were formulated, fabricated,and tested to determine the effects of alloy design on fatigue and fracture toughness. Studies of Nb-Cr in situ composites indicated that these materials were not sufficienfly damage tolerant to warrant detailed fracture studies. Alloying with titanium increased the fracture toughness of the matrix to approximately 80 MPam and the composites to = 20 MPam, but the toughness of Cr2Nb remained unchanged at = 1.5 MPam. However, fracture toughness of the composites is still lower than anticipated. Fatigue cracks were grown in the best of these materials and detailed micromechanics measurements were made. The fatigue crack growth characteristics of the high toughness matrix alloy were similar to pure Nb at higher values of delta(K) where cracks in the new alloy grew much more slowly. Crack growth through the Nb-Cr-Ti composite were faster and at lower values of delta(K). Similar measurements were made in a Nb-10 at % Si composite. Reasons for the fatigue crack growth behavior observed were that the crack tip strains for the composites were lower than matrix alloy under comparable conditions; thus, the number of cycles delat(N) required for crack advanced was decreased. Also, the average increment of crack advance A a was increased by fracture of the intermetallic. The decrease in fracture strain was attributed to an increase in the level of constraint caused by presence of the reinforcing intermetallic particles.