FRACTURE-TOUGHNESS AND FATIGUE-CRACK GROWTH IN RAPIDLY QUENCHED NB-CR-TI IN-SITU COMPOSITES

In situ composites based on the Nb-Cr-Ti ternary system were processed by rapid solidification in order to reduce the size of the reinforcin g intermetallic phase. Two-phase microstructures with small Cr2Nb part icles in a Nb(Cr, Ti) solid solution alloy matrix were produced for se veral compositions that previous work showed to produce high toughness composites in cast materials. The fracture and fatigue behaviors of t hese composites were characterized at ambient temperature. The results indicate that the fracture resistance increases with a decreasing vol ume of Cr2Nb particles. Fracture toughnesses of the rapidly solidified materials with their smaller particle sizes were lower than for conve ntionally; processed composites with larger particles of the intermeta llic compound. The fatigue crack growth rate curves exhibit steep slop es' and a low critical stress intensity factor at fracture. The lack o f fracture and fatigue resistance is attributed to the contiguity of t he intermetallic particles and the absence of plastic flow in the Nb s olid solution matrix. The matrix alloy appears to be embrittled by (1) the rapid solidification processing that prevented plastic relaxation of residual stresses, (2) a high oxygen content, and (3) the constrai nt caused by the hard Cr2Nb particles.