Structure formation during thermomechanical processing of Ti-Nb-(Zr, Ta) alloys and the manifestation of the shape-memory effect

The formation of structure during thermomechanical processing by the regime of cold plastic deformation by rolling and postdeformation annealing (PDA) and its influence on the mechanical properties of Ti-Nb-(Zr, Ta) shape-memory alloys (SMAs) have been investigated. A moderate strain (e a parts per thousand 0.3) leads to the formation of a developed dislocation substructure in the beta phase. With going to severe plastic deformation (e a parts per thousand 2), a nanocrystalline structure can locally be formed without the amorphization of the structure. There are also present alpha aEuro(3)-, alpha-, omega phases in the deformed alloys. When the PDA (1 h) is performed below 450A degrees C, the structure of the beta phase changes only slightly. Above 450A degrees C, a polygonized substructure is formed in the beta phase, which is nanosubgrained at an annealing temperature of 500A degrees C and transforms completely into a submicron one at 600A degrees C. In the case of severe plastic deformation to e a parts per thousand 2, in this range of annealing temperatures, high-angle misorientations of blocks are also observed. The recrystallization of the beta phase in the Ti-Nb-(Zr, Ta) SMAs develops at temperatures above 600A degrees C. The presence of the omega phase is detected at temperatures of up to 550A degrees C. The lattice parameters of the strain-induced alpha aEuro(3) martensite formed in the Ti-Nb-Ta alloy are independent of the PDA temperature in the range from 600 to 900A degrees C, where the polygonized substructure transforms into the recrystallized structure of the beta phase. The range of PDA temperatures that are most favorable for the manifestation of the effect of superelasticity in the Ti-Nb-(Zr, Ta) alloys is in the vicinity of 600A degrees C.