Effect of the surface oxide layer on transformation behavior and shape memory characteristics of Ti-Ni and Ti-Ni-Mo alloys

摘要

Oxidation behaviors of Ti-49Ni and Ti-48.3Ni-0.7Mo (at.%) alloys in dry air from 723 K to 1273 K have been investigated by means of scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction, and then effects of oxidation on transformation behavior and shape memory characteristics of the alloys were studied by means of differential scanning calorimetry and thermal cycling tests under constant load. Three-layered surface scale was formed in both alloys oxidized at temperatures higher than 1023 K, consisting of an outer TiO2 layer, an intermediate layer of mixture of TiO2 and Ni and an inner TiNi3 layer. Thickness of the surface oxide layer increased from 1 mum to 50 mum with raising oxidation temperature from 923 K to 1273 K. The surface oxide layer raised transformation temperatures associated with the B2-B19' and the R-B19' transformation, while it did not almost change transformation temperatures associated with the B2-R transformation. Recoverable elongation was not changed in the alloys oxidized at temperatures below 823 K with raising oxidation temperature, whereas it decreased in the alloys oxidized at temperatures above 923 K. Transformation hysteresis was not almost changed by oxidation in a Ti-49Ni alloy, but it decreased largely in a Ti-48.3Ni-0.7Mo alloy. (C) 2003 Kluwer Academic Publishers. [References: 16]