Experimental characterization of shape memory alloys

The techniques used for the characterization of the alloys properties are reported and discussed for a better understanding of the material behavior as well as for an appropriate exploitation of the material properties for specific engineering applications and practical implementations. Proper determination of transformation temperatures is the preliminary characterization test that has to be performed on the alloys by calorimetric analysis. On the basis of calorimetric results, further investigations, such as thermomechanical tests, can be set-up 1-9. These tests are of fundamental importance for the use of the alloys in structural or actuation devices, since for any kind of application a quantitative knowledge of the material properties is required. To identify the internal structure of the alloys and determine transformation temperatures values, another useful experimental test is the electrical resistance measurement10. This technique is not a conventional one and it has been investigated since few years. It is useful to assess the dependence of electrical resistance upon temperature and mechanical loads and tracking down the phase composition and microstructure changes induced by external thermomechanical conditions. In the next paragraphs a complete characterization of an SMA will be illustrated. The tests described in the followings will set up and their results analyzed by an engineering approach. The following experiments can be used as a sort of “protocol” for the determination of the basic properties of a new shape memory alloy, however further characterization experiments can be carried out on the material in order to investigate specific properties. Several kinds of characterization techniques are used for the determination of the internal structure of shape memory alloys, such as neutron diffraction technique and microscopic observations. These tests are used for detecting the crystallographic changes upon phase transformation and can be carried out during heating or during the appliance of a magnetic field, in order to in situ monitor the structural changes. The description of these characterization techniques is not the aim of this chapter. For a more comprehensive illustration of all the characterization methods used for the identification of SMA structures during phase transformations, neutron diffraction and other morphology techniques have been also described.