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

Abstract

This study investigated the effects of the types and length of nonmetallic inclusions on fatigue properties in rotating bending fatigue testing of Ti-Ni alloy fine wire. It was fabricated to include titanium carbides Ti(C,O) and titanium oxides Ti4Ni2Ox as either single phases or a mixture of both phases as nonmetallic inclusions in Ti-Ni alloy. The fatigue strength of Ti-Ni alloy depended on the number of nonmetallic inclusions of a length of ≥2 μm. Compared with Ti(C,O), Ti4Ni2Ox is coarse. It also exhibited a trend of readily forming particles and void assemblies, which are a defect morphology that originates from nonmetallic inclusions and readily act as crack origins of fatigue fractures.

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

“…8d-f. The stability of Ti(C,O) inclusions with C/O ratios of over 1.5 was also confirmed in our previous report [12]. Only Ti(C,O) inclusions were found in the Ti-Ni fine wires, with C/O ratios of 1.6 and 2.1.…”

“…The reason why these two specimens were selected was as follows. In our previously reported microstructure observations of fine wire [12], the specimens with a C/O ratio of less than 1.5 exhibited a mixed phase of Ti(C,O) and Ti 2 Ni(O), whereas only Ti(C,O) was observed in the as-cast specimens with a C/O ratio of 1.5 or more. Figure 6 shows SEM images of the inclusions electrolytically extracted from the specimens with C/O ratios of 1.0 and 1.5 through each process.…”

“…3a, the following were observed: particle A (Type A), which exhibited a darker contrast than that of the TiNi matrix, and particle D (Type D), which exhibited an intermediate contrast between that of the TiNi matrix and Type A. These differences in contrast between the BSE images depended on the concentration of Ni, which has the largest atomic number among the component elements [12]. That is, the Ni concentration in the TiNi matrix is the highest and that in Type A particles is the lowest.…”

“…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]. The inclusions in the wire can be controlled to single-phase TiC, single-phase Ti 4 Ni 2 O x , or a mixture of both phases by controlling the C/O ratio.…”

“…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.…”