Thermally-Induced Transformation with Applied Mechanical Load

If mechanical load is applied to the material in the state of twinned martensite (at low temperature) it is possible to detwin the martensite. Upon releasing of the load, the material remains deformed. A subsequent heating of the material to a temperature above A0f will result in reverse phase transformation (martensite to austenite) and will lead to complete shape recovery, as shown in Figure 3. The above described process results in manifestation of the Shape Memory Effect (SME).

Shape Memory Effect of an SMA

Figure 3. Shape Memory Effect of an SMA.

It is also possible to induce a martensitic transformation which would lead directly to detwinned martensite. If load is applied in the austenitic phase and the material is cooled, the phase transformation will result in detwinned martensite. Thus, very large strains (on the order of 5-8%) will be observed. Reheating the material will result in complete shape recovery. The above-described loading path is shown in Figure 4.

Temperature-induced phase transformation with applied load Temperature-induced phase transformation with applied load

Figure 4. Temperature-induced phase transformation with applied load.

The transformation temperatures in this case strongly depend on the magnitude of the applied load. Higher values of the applied load will lead to higher values of the transformation temperatures. Usually a linear relationship between the applied load and the transformation temperatures is assumed, as shown in Figure 4.