PSI - Issue 2_A

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Osmar de Sousa Santosa et al. / Procedia Structural Integrity 2 (2016) 1443–1450 Osmar de Sousa Santos/ Structural Integrity Procedia 00 (2016) 000–000

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The DSC results showed that PBII treatment practically does not alter the MTT compared to reference NiTi wire, where the maximum MTT change was for A s values, where ΔA s = 2.4 °C (71.7 – 69.3). The peaks temperatures differences (M p and A p ) between two conditions were less than 1 °C, which is very important result considering the fact that the NiTi wire was heat treated for 2 hours in a temperature of 741 °C. As shown in table 2, the alloy presents a medium amount of carbon and a reasonable amount of oxygen that could react with titanium forming TiC and/or Ti 4 Ni 2 O. These two types of precipitates make the matrix richer in nickel content lowering the MTT as shown in earlier work (Otubo et al., 2008). However, even with the heat treatment at 741 °C for 2 h of the PBII wire, there was no change in MTT (Table 3) indicating that no new amount of precipitates was formed as the consequence of heat treatment. Table 3. MTT of the reference wire and PBII treated wire.

M s (°C)

M p (°C)

M f (°C)

A s (°C)

A p (°C)

A f (°C)

Sample

53.2 48.1 43.1 69.3 82.2 84.6 53.2 48.3 43.1 71.7 82.8 85.2

Reference PBII wire

The atomic concentration profile of nitrogen implanted onto NiTi wire surface is shown in Figure 2. It reveals that the thicknesses of the modified layer were about 100 nm for the sample treated at 741 °C. The nitrogen (N) reaches a peak concentration of about 37% at 25 nm in depth, then to 28% at 78 nm in depth, it steep decreases afterwards, until it reaches 0 % at approximately 100 nm in depth, where the nickel (Ni) concentration starts to rise. Despite the presence of carbon and oxygen in the alloy, the major contribution for these elements (measured by GDOES) come from the impurities present in the PBII discharge. Even though a reasonable concentration of carbon was detected in the very near surface, its content decreases to 0 at.% at 200 nm. The oxi-nitrided phase is due to the diffusion and the higher oxygen reactivity with titanium.

Fig. 2. Depth element profile for NiTi sample, implanted at 741°C.

The Figure 3 shows the SEM micrograph of the PBII treated wire surface, where one can see certain roughness mainly due to the etching before PBII treatment and the contribution brought by the growth of an oxi-nitrided phase, underneath the nitride implanted layer. By the other hand, roughness is desirable in biomaterials application in order to facilitate cell adhesion, proliferation and migration (Wirth et al., 2005).