Study of Surface State Influence on Functional Properties of Ti-Ni Alloys

摘要

Binary Ti-Ni alloys containing 50.0 at.% Ni and 50.7 at.% Ni were tested under bending. Shape memory effect (SME) and two-way SME (TWSME) parameters were measured. Low-temperature thermomechanical treatment (LTMT) by drawing with true strain e ≈ 0.6 and post-deformation annealing (PDA) was performed in the temperature range 300-600 ℃ for 0.3-1 h and permitted different types of initial austenite structure to be obtained. Wires with a diameter of 0.3 and 0.45 mm were studied. The curves of element distribution through the surface layer were depicted graphically. Thickness, structure and element composition of the oxide film are all strongly dependent on PDA regimes. LTMT permits decreasing the film thickness by more than 10 times. Surface state strongly affects all studied parameters: characteristic temperatures, recovery strain and TWSME value. Increasing the diameter of the wire from 0.3 to 0.45 mm brings significant changes in the studied parameters. Forecasting of the said influence is difficult because of its ambiguity and therefore, the oxide film must be eliminated by etching in order to obtain reliable results.