The SMA has two types of special behaviors, which in reality are two expressions of the same phenomenon: The shape memory and super elasticity . In both cases, the behavior is the product of a phase transformation without diffusion, martensitic type, in which the order to nearest neighbors is not lost. Strictly speaking, the order itself is lost, not lost are the neighboring atoms. If an atom has been a first group of neighbors, their position in the second state is different, but the neighbors do not change.
One way to see the shape memory effect is in principle the material is in its high temperature phase. By lowering the temperature the transformation to martensite. If such material is now deformed by twinning deformation occurs, ie, again the short-range order is not lost and the first neighbors are the same as shown in Fig. When the material temperature rises again, the material returns to the high temperature phase with the way it was before the deformation. If this same process looking at a graph of stress-strain after cooling, in a first stage, linear behavior is aware of an elastic behavior. Reached a critical stress, deformation continues without increasing voltage or a very smooth voltage rise.
In a third stage, after reaching a critical strain, the strain begins to increase again linearly with a slope similar to that showed in the beginning.
If before reaching the critical strain, the tension relaxes the specimen retains a permanent deformation. Now if the specimen is heated to over a certain temperature characteristic of the alloy (As), begins to recover the deformation that remained until, when the temperature exceeds another critical temperature (Af) which also depends on the alloy and heat treatments, we found that there is no longer the specimen deformation and is in its original dimensions. This behavior is known as "Shape Memory."
In the same alloy but with different mechanical or thermal treatments, with the addition of very small amounts of alloying elements, it may happen that in the tensile test at room temperature, the behavior of the specimen is as shown in Fig. As in the case of shape memory behavior, also seen here as part of achieving a certain tension, the continuous deformation no significant increase in tension. But in this case, to ease the tension, we see a hysteresis loop that ends with almost no permanent deformation. In this seemingly plastic deformation behavior then full recovery is called "super elastic" or "Pseudo plasticity."
Maria Linares 19881179 EES
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