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

1444

2

Otsuka and Ren, 2005; Otsuka and Wayman, 1998; Wu et al., 1996). The shape memory effect (SME) is related to the fact that after deformation at low temperature, the material recovers its original shape after heating and the superelastic effect (SE) denotes an unusual flexibility of the material that is far higher, up to 8% of recovery, than the flexibility of usual metallic materials. Due to their unique and superior properties the commercial and development of these Shape Memory Alloys (SMAs) has been supported by fundamental and applied research studies (Jani et al., 2014). Despite the fact of these alloys have generally excellent corrosion resistance, there are some applications which require Ni-free surface, such as biocompatible devices (stents for the vascular, urological and gastroenterological fields, staples for the orthopedic field, etc.) and others that require resistance to atomic diffusion, chemical stability, and increasing surface hardness, through elimination of Ni from the surface. This chemical element is known to be allergenic and toxic, though essential for the human body (Poon et al., 2005; Shabalovskaya et al., 2008). In order to have a Ni-free surface and improve the mechanical, tribological or corrosion resistances of NiTi SMA, it is possible to nitriding the surface of the alloy via plasma based ion implantation (PBII) technique. The PBII is an advanced technique that allows three-dimensional ion implantation in complex shape work-pieces, with no dimensional change of treated components. As no film is deposited, delamination is avoided (Chollet et al., 2013; Silva et al., 2012). Sometimes surface modification techniques are combined with procedures employed for the design of optimal shape memory and superelasticity. The consequence is that not only is the surface composition modified, but so also is the bulk of NiTi (Shabalovskaya et al., 2008). However, to ensure a proper shape memory effect (SME) according to desired application, the thermomechanical history of NiTi SMA should be taking to account when processing a Ni free surface wire via PBII technique. Previous studies undertaken by the ITASMART ( ITA S hape M emory A lloys R esearch and T echnology) and others researchers showed that martensitic transformation temperatures (MTT) is very dependent on the chemical composition of the alloy, where the presence of contamination in the NiTi alloy (e.g., carbon and oxygen) change the MTT (Frenzel et al., 2007; et al., 2004; Mazzolai et al., 2007; Nayan et al., 2007; Otubo et al., 2008; Rigo et al., 2005). The heat treatments such as annealing, solution treatment, and aging may have significant effects on the microstructural phases in the final products and also on thermomechanical properties of NiTi devices (Elahinia et al., 2012). Since the PBII is undertaken in relatively high temperature, it is important to know what are the consequences on the MTT and mechanical properties of the PBII treated NiTi wire. 2. Experimental Procedure The starting material was a Vacuum Induction Melting (VIM) NiTi ingot with controlled chemical composition. The ingot was hot-formed and then wire drawn to a wire of 2.00 mm in diameter at the ITASMART facilities. The wire fabrication can be seeing elsewhere (Souza et al., 2014). The samples were divided in two categories: PBII treated wire and the reference wire (with no implanted surface). Before the PBII treatment the wires were etched for 3 seconds in a solution of 50 mL HCl + 50 mL HNO 3 . Nitrogen PBII experiments were performed in the home-made TAPIIR set-up (thermally assisted plasma ion implantation reactor), whose scheme was previously described in details (Marot et al., 2002; Pichon et al., 2010). The Table 1 shows the parameters used in the PBII treated specimen. The working temperature was reached after 1 h heating under vacuum (base pressure below 10 −4 Pa) with the external furnace surrounding the quartz tube reactor and also complemented by additional energy due to plasma excitation and, mainly by the ions implantation. The system stabilized in about 22 min of operation of PBII.

Table 1. Treatment conditions of the PBII.

PBII high voltage pulses (kV)

Frequency (Hz)

Pulse (µs)

Temperature (°C)

Time (h)

Sample

16

200

35

741 ± 18

2

PBII wire

The PBII high voltage pulses frequency and length were manually adjusted to maintain a constant operating temperature during the whole treatment. After the PBII treatment, the samples were cooled down to room temperature under vacuum. In order to analyze the shape memory properties, the samples were tested in a Instron 5500R tensile testing machine