Author: Carl Boehlert

Publisher:

Published: 1997

Total Pages: 466

ISBN-13:

The phase evolution, creep, and tensile behavior were studied for near Ti2AlNb and Ti12Al-38Nb O+BCC alloys. Monolithic materials were produced through conventional thermomechanical processing techniques. Heat treatment and TEM studies estimated the temperature ranges for the respective phase fields and a pseudobinary diagram based on Ti=50at.% was constructed. The aging-transformation behavior was studied in detail. O-phase precipitation within BCC-dominated microstructures resulted in significant room temperature (RT) strengthening. The BCC phase was vital for imparting RT ductility. The deformation observations and calculated creep exponents and activation energies suggested that three creep mechanisms are dominating the secondary creep behavior. For low applied stress, Coble creep characteristics were exhibited. For intermediate stresses, the minimum creep rates were proportional to square delta/GS and fiducial-line experiments revealed grain boundary sliding and grain boundary cavitation. For high stresses, the stress exponents were greater than or equal to 3.5 and a high density of dislocations were observed, indicative of a dislocation climb mechanism. Overall, the sub-transus processed and heat-treated microstructures contained much smaller grain sizes than super-transus microstructures and this resulted in worse creep resistance. For targeted low-to-intermediate stress and intermediate temperature applications, grain size is the dominant microstructural feature influencing the creep behavior of O+BCC alloys.