2007 Volume 48 Issue 3 Pages 414-421
Shape memory characteristics of Ti-18 mol%Nb alloys containing 3, 4, 5 and 6 mol%Cu (termed Ti18Nb3Cu, Ti18Nb4Cu, Ti18Nb5Cu and Ti18Nb6Cu, respectively) were investigated and effects of Cu addition on shape memory behavior was clarified. The alloys fabricated by Ar arc-melting method were cold-rolled with 98% reduction thickness and solution-treated at 1273 K for 1.8 ks followed by quenching into water. It was found by θ-2θ X-ray diffraction analysis (XRD) at room temperature (RT) that Ti18Nb3Cu is composed of α″ (c-centered tetragonal)martensite phase and β (bcc) parent phase. The other alloys with higher Cu contents are β single phase. These results indicate that 6 mol%Cu is completely dissolved in Ti-18 mol%Nb alloys. Besides, the lattice parameter of β phase is decreased by Cu addition with a rate of 0.2×10−3 nm/mol%Cu, and then, the atomic radius of Cu in Ti-18 mol%Nb alloys is estimated to be 0.130 nm. By tensile test it was found that (1) shape recovery strain of Ti18Nb3Cu reaches 3% by heating after deformation, (2) Ti18Nb4Cu exhibits superelasticity at RT, and (3) either shape memory effect or superelasticity does not appear at RT for Ti18Nb5Cu and Ti18Nb6Cu. Besides, the stress for slip deformation is increased by Cu addition with a rate of 50 MPa/mol%Cu. By tensile tests at cryogenic temperatures, the martensitic transformation start temperature (Ms) of Ti18Nb5Cu is determined to be 75 K, and the Cu addition to Ti-18 mol%Nb alloys decreases Ms with a rate of 100 K/mol%Cu. Moreover, more than 5% in transformation strain is observed for Ti18Nb5Cu at 173 K. It was concluded that Cu is an effective additional element in order to improve shape memory and superelastic properties of Ti-Nb alloys.