Investigation of the fracture characteristics of lamellar nanocomposites and in situ-formed composites designed for high temperature service: Volume 1 -- NB-based materials development. Final report, 1 September 1995--31 October 1998

Technical Report · Thu Oct 01 04:00:00 EDT 1998

OSTI ID:339452

Davidson, D L; Chan, K S

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 Cr{sub 2}Nb 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 Cr{sub 2}Nb 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 Nb{sub 5}Si{sub 3} indicated that particles of this intermetallic deform before breaking near a crack tip.

• Research Organization: Southwest Research Inst., San Antonio, TX (United States)
• OSTI ID: 339452
• Report Number(s): AD-A--356569/XAB; SWRI--06-7275; CNN: Contract F49620-95-C-0043
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
ALUMINIUM
CHROMIUM ALLOYS
CRACK PROPAGATION
DEFORMATION
FABRICATION
FRACTURE PROPERTIES
METALLURGICAL EFFECTS
MICROSTRUCTURE
NIOBIUM BASE ALLOYS
SOLIDIFICATION
TEMPERATURE RANGE 0400-1000 K
TITANIUM ALLOYS