Effects of oxygen concentration and phase stability on nano-domain structure and thermal expansion behavior of Ti-Nb-Zr-Ta-O alloys

In this study, in order to fully elucidate the origin of nano-domain structure and abnormal thermal expansion behavior of Gum metals, the effects of oxygen concentration, beta phase stability and cold rolling on the microstructure and thermal expansion properties were investigated in (Ti-xNb-2Zr-0.7Ta)-yO (at.%) (x = 21-25; y = 0.3-1.8) alloys. Normal thermal expansion behaviors with positive linear thermal expansion coefficients were observed in all the annealed alloys irrespective of oxygen and Nb concentrations. The thermal expansion behavior of as-rolled alloys was strongly affected by the concentration of oxygen and Nb. A negative thermal expansion coefficient was observed in the as-rolled alloys with lower oxygen and Nb concentrations. The thermal expansion coefficient increased with increasing oxygen and Nb concentrations. The amount of nano-domains increased with increasing oxygen concentration while decreased with increasing Nb concentration. The growth of a preferential nano-domain variant was observed in as-rolled alloys and became more prominent with decreasing oxygen and Nb concentrations. The reverse transformation from the nano-domain structure to the beta phase occurred with increasing temperature. Thermal expansion behavior of as-rolled alloys resulted from a competing relationship between the contribution of lattice distortion of nano-domain structure and the contribution of lattice vibration of the beta phase. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.