Effects of Nb and Al on the microstructures and mechanical properties of high Nb containing TiAl base alloys

The influence of Nb and Al contents on the microstructure and yield strength of high Nb containing TiAl base alloys was investigated. The experimental results show that the yield strength at 900 degreesC of the alloys with the same type of microstructure, such as fully lamellar (FL), nearly lamellar (NL) and degraded fully lamellar (DFL), increases with increasing Nb content and decreasing Al content in the composition range of 0-10 at.% Nb and 44-49 at.% Al. DFL is the degraded form of FL microstructure after exposure at 1050 degreesC for 30 h. It is shown that the Nb addition in the alloys increases the value of the sigma(0) term in the Hall-Petch relation of yield stress vs. lamellar spacing. This result has been related to TEM observations of dislocation structure in deformed specimens. The observations indicated that high level of Nb solute in the gamma-TiAl matrix leads to a high critical resolved shear stress (CRSS) of dislocation loops. High Nb addition also reduces the degradation rate of FL microstructure after exposure at 1050 degreesC for 30 h. Both effects of high Nb addition are related to the change of the directionality of Ti-Ti (Nb) and Nb-Al bonds in the lattice. The decrease in Al content results in an increase in the volume fraction of 12 phase, which leads to a decrease in the lamellar spacing of the lamellar structure. The high temperature strength of the alloys is determined by the lamellar spacing through the Hall-Petch equation k(lambda)lambda(-1/2). (C) 2002 Elsevier Science Ltd. All rights reserved.