Duality in dislocation density-superelasticity correlation in a TNTZ bio alloy processed by cold rolling and subsequent annealing

The effect of restoration mechanisms on the superelastic behavior of a Ti-Nb-Ta-Zr (TNTZ) bio-alloy was investigated in terms of dislocation density-superelasticity correlation. For this purpose, the cold-rolled material (by 70% reduction) was subjected to various annealing temperatures ranging from 500 to 900 degrees C (30 min) to obtain microstructures with different restoration conditions. The characterization of microstructures was mainly performed by SEM/EBSD, OM, and TEM analysis, although X-ray analysis was also employed to calculate the dislocation densities of the obtained microstructures. The superelasticity of specimens was evaluated by loading-unloading tensile tests. A dual correlation between dislocation density and pseudoelastic (S) ratio was observed in the studying microstructures. A very low superelasticity was observed in dislocation dense structures of cold-rolled and 500 degrees C annealed specimens. The S value significantly increased in the 600 degrees C annealed specimen (S = 49%) as dislocation density decreased in the structure. However, further reduction of dislocation density degraded the superelasticity in the specimens annealed at the higher temperature range of 700-900 degrees C. This nonlinearity behavior was explained by the dissimilar effect of microstructure dislocation density on the direct and reverse transformation of martensite.