PSI - Issue 2_A

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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coupled with a Instron 3119 environmental chamber in three deformation cycle as follow: the first cycle consisted of loading at room temperature (25 °C) up to 2% of strain, then unloading in order to measure the superelastic shape recovery and then heating to 120 °C to measure the shape recovery due to SME. The loading, unloading and heating cycles were repeated for 4% and 6% strain respectively using the same heating temperature. The internal friction tests were conducted by a dynamic mechanical analyzer – DMA (Netzsch, model DMA 242) using the single-cantilever mode at a constant amplitude of 20 µm and a frequency of 0.1 Hz in the temperature range of -60 °C to 140 °C at a heating/cooling rate of 5 °C.min -1 . The MTT was determined by differential scanning calorimetry – DSC (Netzsch, model STA 404C DSC). Elemental depth profile was obtained by glow discharge optical emission spectroscopy (GDOES), the respective profile was attained with the help of a Jobin-Yvon GD profiler. The samples were cut with refrigerated diamond saw with controlled load in order to avoid mechanically induced martensitic transformation. The microstructure of the samples was observed and analyzed in a scanning electron microscopy – SEM (Tescan, model Vega 3 XMU), energy dispersive spectroscopy – EDS (Oxford Instruments, model X-Act SDD EDS detector) and AZtec EDS analysis software (Oxford Instruments). 3. Results and Discussion The Table 2 shows the chemical composition of the NiTi wire used as reference. The content of Ni (%at.), Ti(%at), C (at%) and O (at%) are global values of each sample measured by X-ray fluorescence (XRF). Value of Ni C+O (%at.) refer to the nickel content in the matrix discounted values of carbon and oxygen that are above the solubility limit and theoretically precipitate forming TiC and Ti 4 Ni 2 O (Otubo et al., 2008). Considering the fact that NiTi alloy was melted by VIM furnace using a graphite crucible, the alloy presented medium carbon content and high oxygen content as indicated in Table 2. Table 2. Chemical composition of the NiTi wire.

Ni (%at) 49.42

Ni C+O (%at) 50.25

Ti (%at) 49.92

C (%at)

O (%at)

Sample

0.29

0.37

Reference

The Figure 1 shows the DSC curves upon heating and cooling carried out for reference wire and for PBII treated wire. The Analysis of MTT for both samples (Table 3) show that the samples are in the martensite phase at room temperature. Where, M s (start temperature of direct Martensitic Transformation – MT), M p (temperature peak of direct MT), M f (final temperature of the direct MT), A s (start temperature of reverse MT), A p (temperature peak of reverse MT) and A f (final temperature of reverse MT) are all above room temperature.

Fig. 1. DSC heating and cooling curves for PBII treated wire and NiTi reference wire.