Shape Memory Nickel Titanium Alloy

The shape is "programmed" through a process called Shape Setting. The alloy is physically constrained in the desired final geometry (e.g., bent, coiled) using a fixture. It is then heated to a high temperature (typically 450-550°C) for several minutes and cooled. This heat treatment aligns the crystal structure of the high-temperature austenite phase with the constrained shape, which then becomes the "remembered" shape.

Yes, the memorized shape can be erased and reprogrammed. By heating the alloy above its shape setting temperature without any constraint, the previous memory is erased. A new shape can then be set by constraining it in a new geometry and repeating the shape setting heat treatment cycle.

In a one-way shape memory actuator, the Nitinol element only moves when heated. To return it to its starting position upon cooling, a "bias" spring (often a conventional steel spring) is used to apply a constant force opposing the Nitinol's recovery direction. This creates a cyclic actuator: heat = Nitinol overcomes spring, cool = spring returns Nitinol to deformed state.

The actuation speed is limited by heat transfer. For thin wires resistively heated, actuation can occur in less than a second. For larger components or ambient heating, it may take seconds to minutes. Cooling (reset) is generally slower as it relies on passive heat dissipation, unless active cooling is employed.

Cycle life is primarily limited by functional fatigue—gradual changes in the transformation temperatures and strains due to microstructural evolution (dislocation accumulation, precipitate coarsening) with repeated cycling. Proper alloy processing, avoiding excessive strain per cycle, and operating within a controlled temperature range maximize cycle life, which can range from thousands to millions of cycles.