Superelastic behavior:

For fix temperatures above the austenite finish temperature Af (follow this link for definition), the austenite to martensite transformation can be induced by applying stress:

Figure 1: Superelastic behavior.

In Figure 1, at the left is a schematic of the evolution of microstructure with increasing and decreasing load, where the light color is the austenite and the gray color is the martensite. At the right in Figure 1, a load verses displacement diagram is shown, where the nonlinearity in the strain and the hysteresis loop characteristic of a loading cycle are depicted.

An example of superelastic behavior in a Cu-Al-Ni shape memory alloy - the experimental data is courtesy of Tom Shield.

Figure 2: Superelastic behavior in a Cu-Al-Ni shape memory alloy. Courtesy of Tom Shield.

Notice that the strain due to the transformation is about 8.4% and that all of the strain is recovered upon unloading!

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