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).
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.
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.