This paper studies the estimation of titanium oxide (TiO2) thickness on a thermally oxidized nickel titanium (NiTi) alloy based on its surface color. The NiTi plate specimens were subjected to oxidation at temperatures from 450 °C to 900 °C for different duration to induce the formation of oxide layers of different thicknesses. The oxide thickness of the specimens oxidized from 650 °C to 900 °C was measured by means of scanning electron microscopy. A fitting curve was generated from the measured thicknesses, and further extrapolated to predict the oxide thickness for specimens heated at lower temperature of 450 °C to 600 °C. The oxide thickness estimation was done using thin‐film optical interference, with reference to the observed color of the oxidized surface. The thin‐film optical interference approach showed accurate measurement of oxide thickness as compared to the extrapolated data, highlighting the applicability of this measurement technique.

NiTi arch wires are susceptible to hydrogen embrittlement upon contact with dental brackets in oral cavity during orthodontic treatment. This study investigated the effect of hydrogen absorption and diffusion over time towards the thermal and mechanical properties of NiTi shape memory wire. The hydrogen absorption process was carried out via electrolytic charging at constant current density for 16 hours in 1.0 weight percent (wt.%) sodium sulphate solution. The hydrogen charged wires were aged at room temperature in air for different durations to allow further inward diffusion of the hydrogen into the specimens. The results show that after hydrogen charging, the latent heat of forward and reverse martensitic phase transformation of the NiTi wire changed from 19.96 to 11.98 J/g, and 19.21 to 13.42 J/g, respectively. Further suppression and disappearance of thermal transformation peaks were observed as the charged specimen aged for 7 days. The transformation stress level on tensile deformation increased by almost 90 MPa after hydrogen charging, and exhibited non-flat stress plateau after further aging.

The tensile deformation of NiTi alloy can proceeds in either homogeneous manner, or localized deformation via formation and propagation of macroscopic shear bands, that is commonly known as Lüders-like deformation. The high deformation strain within the localized deformed regions can result in the changes of surface characteristics of NiTi specimen. This paper studies the surface roughening effect associated with Lüders-like deformation of martensitic NiTi alloy, via surface characterization of polished surface and localized deformed region that consists of Lüders bands on tensile specimens, respectively. The surface roughness profile and roughness parameters of surface with Lüders bands are significantly different and higher as compared to the polished surface.

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Abstract. Heat treatment of Nickel Titanium (NiTi) shape memory alloy at high temperature will result in formation of titanium oxide (TiO 2 ) layer at the surface. TiO 2 is a ceramic with higher value of hardness and does not exhibit martensitic phase transformation, thus affects the shape memory behaviour of the NiTi core. This work studies the effect of formation of TiO 2 layer on the surface towards the shape memory behaviour of near-equiatomic NiTi alloy. The NiTi plate with composition of Ti-50.2at%Ni was subjected to heat treatment at temperature of 723K to 1173K for 30 and 60 minutes. The formation of TiO 2 layer was observed using scanning electron microscope. The change in surface hardness of heat treated NiTi plate was characterized by Vickers hardness test and nano-indentation. The results showed that surface hardness increased as the thickness of the TiO 2 layer increased. Subsequently, the austenite finish and martensite start temperatures were also shifted to higher temperature to correspond to the effect of TiO 2 presence at the surface.