- [ Shape-memory alloy and superelastic alloy ]
- The shape-memory effect is expressed as thermo-elastic martensitic transformation; the transformed martensite phase is reversely transformed into the parent phase (austenite) when heating. The superelasticity is expressed as stress-induced martensitic transformation; the martensite phase induced by the loaded stress is transformed into the parent phase by relief of the loaded stress.
Its physical behavior is explained on the stress-strain curve as follows:
| (1) |
Normally, on metallic materials, the strain is proportional to the stress in the elastic area, but, if a given strain exceeds the elasticity range, it enters the yielding area. If the stress is removed at the yielding area, the deformation permanently remains after removal of the stress. |
| (2) |
With regard to the shape-memory effect, if any deformation is applied to the martensite phase at the low temperature phase, the deformation in the shape remains after relieving the load in the yielding area in the same manner as normal metals, but the strain is completely removed by heating up to higher than the transformation temperature, at which the alloy recovers to the mother phase. |
| (3) |
With regard to superelasticity, yielding occurs in the same manner as normal metals, but when the load is removed, the phenomenon occurs reversely, recovering to the original condition. The NiTi superelastic alloy recovers to its original shape from an 8% deformation in appearance. |
- [ Principle properties of KSM-SM alloy ]
-
| Density |
6.4 ~ 6.5 [ g/cm3 ] |
| Specific heat |
0.3 [ kJ/(kg K) ] |
| Electric resistance |
0.5 ~ 1.1 [ 10-6 m ] |
| Heat conductivity |
21 [ W/(mK) ] |
| Linear coefficient of expansion |
10 [ 10-6/K ] |
| Modules of rigidity |
Martensite phase |
5 ~ 10 [ GPa ] |
| Austenite phase |
15 ~ 25 [ GPa ] |
| Young’s modules |
Martensite phase |
15 ~ 30 [ GP ] |
| Austenite phase |
50 ~ 70 [ GPa ] |
| Hardness |
Martensite phase |
180 ~ 300 [ Hv ] |
| Austenite phase |
300 ~ 400 [ Hv ] |
| Rupture stress |
Martensite phase |
700 ~ 1000 [ MPa ] |
| Austenite phase |
1200 ~ 1500 [ MPa ] |
| Elongation |
Martensite phase |
~ 30 [ % ] |
| Austenite phase |
~ 20 [ % ] |
- [ Alloy series ]
- We have prepared various series of KTS-SM alloys depending on shape-memory and superelasticity characteristics.
| Effect |
Alloy series |
Transforming temperature
(°C) |
Recoverable stress
(%) |
Temperature hysteresis
(°C) |
Temperature cyclic characteristic |
Application example |
| Shape-memory effect |
NiTi
NiTi-Co |
- 20 ~ 70 |
1 |
2 ~ 3 |
> 1,000,000 |
Sensor actuator (long life) |
| NiTi-Cu |
40 ~ 80 |
5 ~ 6 |
10 ~ 15 |
< 50,000 |
Sensor actuator (large stroke) |
| NiTi |
- 10 ~ 100 |
6 ~ 8 |
20 = 40 |
< 100 |
Connector joint |
| Superelasticity |
NiTi
NiTi-Co,V,Cr |
- |
6 ~ 8 |
- |
- |
Various spring elements |
|
Features
0.002 mm and up), ribbons, coil springs, 3-dimensional shapes, and so on.