| Customization: | Available |
|---|---|
| Type: | Titanium Bars |
| Application: | Medical |
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Ti-Ni alloy, consisting of nickel and titanium alloy in nearly equiatomic proportions, is known for its unique mechanical properties of shape memory and superelasticity. These properties occur in association with thermoelastic martensitic transformation. Superelasticity is a phenomenon at a temperature above the reverse transformation temperature range, associated with stress-induced martensitic and reverse transformations. The alloy exhibits high flexibility owing to its wide recoverable strain range, in which the stress is kept low. Since Ti-Ni alloy shows high corrosion resistance and good biocompatibility, it has been applied in the medical and dental fields.
Composition
The chemical composition of NiTi alloy is outlined in the following table.
|
Element |
Value % |
Element |
Value % |
|
Ni |
40~50 |
Mo |
<0.01 |
|
Ti |
50~60 |
Si |
<0.01 |
|
Al |
<0.1 |
Fe |
<0.05 |
|
Cu |
<0.03 |
Mg |
<0.02 |
The physical properties of NiTi alloy are tabulated below.
| Properties | Unit | Ni-Ti | Cu-Zn-Al | Cu-Al-Ni | Cu-Al-Be |
| Composition | Nickel titanium | Copper zinc aluminium | Copper aluminium nickel | Copper aluminium beryllium | |
| Melting point | °C | 1260-1310 | 950-1020 | 1000-1050 | 970-990 |
| Density | Kg/m3 | 6400-6500 | 7800-8000 | 7100-7200 | 7300 |
| Young's modulus | Gpa | 95 | 70-100 | 80-100 | 90 |
| Tensile strength (in martensite domain) | Mpa | 800-1000 | 800-900 | 1000 | 900-1000 |
| Breaking strength elongation | % | 30-50 | 15 | 8-10 | 15 |
| Fatigue limit in austenite domain | Mpa | 350 | 270 | 350 | − |
Since the Ti-Ni alloy is an intermetallic compound, cold workability is not good and there are many problems with using this method of fabrication. In cold working, it is necessary to keep the cold work rate at around 40% and then soften by annealing. Figure 5.8 shows the work hardening curve. Because the work hardening rate of Ti-Ni alloy is high, this makes the decrease in workability worse.
The workability of the Ti-Ni alloys is excellent compared with other SMAs, although it is inferior to that of steel; workability is especially good at high temperatures over 700 °C, where the Ti-Ni alloy becomes more stretchable due to its reduced strength as shown in Fig. 4.6. However, if excessive titanium is present locally, for example, due to segregation, a liquid phase appears at temperatures over 950 °C (Murray, 1996) and cracks may form during processing; thus it is advisable to use a processing temperature of 700 to 950 °C. In particular, since working speed is relatively fast in hot rolling, attention must be paid to avoid unregulated, local temperature increases due to the heat of processing.
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