Investigation of Fracture Resistance in Microstructures of Intermetallic Materials for High Temperature Service.
Abstract
Smooth specimens of a fully lamellar TiAl-based alloy were cycled in reverse bending in order to study the initiation and growth of fatigue cracks. Cyclic deformation prior to crack initiation was studied with Atomic Force Microscopy. Deformation was found to occur mainly within the boundary region between Iamellae, with strains as high as 50%, but fatigue cracks did not form in these boundaries. The slope of the stress-lifetime curve was 0.17. Stresses within =15% of ultimate stress were required to cause fracture in several million cycles. Small fatigue cracks formed within peculiar looking regions of microstructure where the larnellae were curved. These cracks would not continue to grow unless the stress amplitude was stepped up with continued cycling. A "small crack effect" was identified. Other deformation and fracture characteristics of the TiAl-based alloy recently identified include the strain rate sensitivity of the tensile ductility, which is governed by the size of the colonies of lamellae, and the strain rate insensitivity of the fracture toughness at 25 degrees C. However, at 800 degrees C, fracture toughness is rate sensitive. The relative high fracture toughness of these alloys has been found to arise from a resistance curve behavior caused from ligaments that form behind the tearing crack tip. These ligaments result from the non-coplanar nature of microcracks that form during crack growth. Composites of Nb-Cr alloys formed in situ showed fracture toughness values up to = 8 MPa4m, which is = 4X better than Cr2Nb. Fatigue crack growth rate slopes for two composites were in the range of 20 to 70. Further work on these composites is in progress.
Document Details
- Document Type
- Technical Report
- Publication Date
- Mar 31, 1993
- Accession Number
- ADA379767
Entities
People
- David L. Davidson
- James Lankford
- Kwai S. Chan
Organizations
- Southwest Research Institute