High-Temperature Deformation of Nb-18 Al

ABSTRACTSpecimens of Nb-18 at. % Al were bend-tested at 20° C, 1200° C, and 1600° C, and the deformed microstructures were characterized using TEM to determine phase distributions and slip systems. Material deformed at 20° C showed brittle fracture and was characterized by large grains of a Nb-Al phase showing B2 ordering. Lamellar colonies of heavily dislocated massively transformed Nb3Al (A15) were present at grain boundaries. The slip system for dislocations in Nb3Al, was determined to be {100}/<001>, with defects typically extending in <011> directions. Fracture at 1200° C was ductile, and TEM observations revealed a bcc Nb-Al(ss) matrix with second phase growth of Nb3Al and α -Al2O3. The Nb3Al phase contained total dislocations and slip-induced planar defects on {100} planes terminating in partial dislocations. In tilting experiments, the planar defects exhibited stacking fault contrast consistent with a displacement vector of R=a/4<100>. Fracture at 1600° C was also ductile, and the deformed material consisted of a niobium matrix with equiaxed inclusions of alumina resulting from internal oxidation during deformation. The creep resistance reported previously was attributed to a combination of dispersion hardening of the refractory metal matrix and inherent creep resistance of Nb3Al.