<i>In Situ</i> Observation of Nonmetallic Inclusion Formation in NiTi Alloys

Abstract

It is well recognized that the fatigue characteristics of NiTi alloys are greatly in uenced by the presence of nonmetallic inclusions. A high fatigue strength is required for the application of these alloys to medical devices, especially stents, to allow them to withstand the repeated processes of pulsation-induced expansion and contraction. In this study, the phases and morphology of the inclusions in commercially produced NiTi alloy were investigated. The inclusions in an as-cast ingot consisted of a single phase of Ti(C, O) and possessed a ber shape with a diameter of about 0.5 µm, and were formed during the solidi cation of the NiTi alloy. In situ observation by confocal laser scanning microscopy revealed that the inclusions formed and grew in the region of residual melt (consisting of concentrated Ti, C, and O) during the solidi cation process.

Effect of Nonmetallic Inclusions on Fatigue Properties of Superelastic Ti-Ni Fine Wire

“…Furthermore, there was a tendency for the number of nonmetallic inclusions to decrease as the sum of the carbon and oxygen concentrations decreased, which is consistent with the results in the literature [3,9]. with oxygen, and the lattice constant is slightly smaller than TiC [16,17]. Thus, the titanium carbides are referred to as Ti(C,O) in this paper.…”

“…Note that the TiC peaks in Figure 3 are slightly shifted to a higher angle compared with the TiC data (00-32-1383) from the International Centre for Diffraction Data. This is because the TiC in the samples has a composition where some of the carbon has been replaced with oxygen, and the lattice constant is slightly smaller than TiC [16,17]. Thus, the titanium carbides are referred to as Ti(C,O) in this paper.…”

Effect of C/O Ratio on Phase Change and Stability of Inclusions in Ti–Ni Alloys Fabricated by a Commercial Production Process

“…The inclusions were electrolytically extracted via the selective potentiostatic etching by electrolytic dissolution method [13,17,[20][21][22], their morphologies were observed by scanning electron microscopy (SEM: Zeiss Ultra 55), and their phases were identified by X-ray diffraction (XRD: Rigaku MiniFlex600) using CuKa radiation. In addition, the phase ratios of inclusions by weight were evaluated quantitatively by using the reference intensity ratio (RIR) method [23,24] from the XRD results for some of the specimens.…”

“…Much attention has been paid to inclusions in Ti-Ni alloy, including their composition and effects on fatigue properties [4][5][6][7][8][9][10][11][12][13][14][15][16][17]. We have recently reported the effect of inclusions on the fatigue properties of superelastic Ti-Ni fine wire [12].…”

“…Commercial Ti-Ni alloy wire is generally fabricated by hot forging, hot rolling, and cold drawing from the cast ingot after high-frequency induction vacuum melting with graphite crucible. There have been many reports individually on the morphology and phases of inclusions in the ascast materials [13,17] and in wire and/or tube as final products [4][5][6][7][8][9][10]12]. Among those reports, Ito et al [17] proposed that the phase of the inclusions in the as-cast Ti-Ni alloys can be controlled by the initial oxygen and carbon content.…”