Effects of short-time heat treatment and subsequent chemical surface treatment on the mechanical properties, low-cycle fatigue behavior and corrosion resistance of a Ni-Ti (50.9 at.% Ni) biomedical alloy wire used for the manufacture of stents
Abstract
Cold-drawn and straight-annealed NiTi wires (50.9% Ni)with a tensile strength of 1650 MPa were subjected to heat treatments at 450, 510 and 600oC for 10 min in air to simulate the shape-setting process in the manufacture of stents. Afterwards, the wires were chemically etched in acidic baths conaining HF, HNO3 and H2O, followed by boilign in water.
Variations in the internal structure, surface state and chemistry and trasformation behavior of the wires due to these treatments were examined in detail by scanning and transmission electron microscopy, energy dispersion spectrometry, glow discharge spectrometry, X-ray photoelectron spectroscopy and differetnial scanning calorimetry. Mechanical properties were determined by tensile tests, and low-cycle fatigue gehavior was measured by bend-type cyclic loading tests.
Corrosion behavior was assessed by immersion tests and potentiodynamic measurements. A hign tensile strength of the wire was shown to be atributable to a very fine-grained structure and work hardening.
Heat treatment at 450-510oC/10 min did not significantly affect the tensile strength of the wire. At 600oC/10 min, the strength decreased by about 600 MPa due to recrystallization.The transformation temperatures first slightly increase after heat treatment at 40 oC and then reduced arter treatments at higher temeperatures due to changes in the composition of the B2 phase.
The fatigue life was observed to prolong with both heat treatment and chemical etching. In contrast, the corrosion resistance worsened with heat treatment, but in improved significantly upon chemical etching.
The observed behaviors are discussed in relation to the structural and chemical characterstics of the wires subjected to various treatment regimes.