Nickel Titanium Alloy Bar

The nickel-titanium alloy bar is a binary alloy rod composed of nickel and titanium. The proportion of the two components is about 50 atomic percent (the weight of nickel is about 55%). However, the fine adjustment of the proportion of the two elements can make the performance of NiTi alloy greatly different, especially its transformation temperature, that is, the temperature at which the crystal structure of the alloy changes from austenite to martensite or vice versa. If the nickel content exceeds 50/50, the transformation temperature will drop sharply and the yield strength of austenite will increase.

(Active Af) phase change temperature

If the Ni-Ti ratio is increased to 51/49, the transition temperature will decrease by more than 100 ° C. The sensitivity of this property to a slight increase in the percentage of nickel makes it difficult to manufacture Nitinol with uniform and repeatable properties. However, at the same time, this provides manufacturers with a powerful way to control performance and manufacture ingots with the required transition temperature.

Austenite phase and martensite phase

Due to the change in temperature and mechanical pressure, there are two different crystal structure phases, namely the austenite phase and the martensite phase.

The phase transformation sequence of Ni-Ti alloy during cooling is the parent phase (austenite phase) - R phase - martensite phase. The R-phase is rhombohedral, and the austenite is in the state when the temperature is higher (greater than the same: the temperature at which the austenite starts) or when the load is removed (external force is removed from the activation), cubic and hard. The shape is relatively stable. The martensite phase is in the state when the temperature is relatively low (less than Mf: the temperature at which the martensite ends) or loaded (activated by an external force). It is hexagonal, ductile, repetitive, unstable, and easy to deform.

Superelastic NiTi alloys use stress-induced martensitic transformation to achieve incredible flexibility and anti-kinking. For example, alloys designed to have hyperelasticity at room temperature are usually produced at a temperature of slightly lower than room temperature, such as 0 to 20 ° C. This material will also show good hyperelasticity at body temperature (37 ° C). The superelastic alloy constitutes the largest volume of Nitinol material.

The shape memory NiTi alloy utilizes the ability of the material to recover the training shape when it is heated above its transformation temperature. Therefore, the most critical characteristic to be specified is the transition temperature. Active Af indicates the completion of transformation from martensite to austenite during heating, so it indicates the temperature at which the shape recovery is completed.

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