NiTiCu Shape Memory Alloy Characterization Through Microhardness Tests

NiTiCu alloys are one of the foremost investigated shape memory alloys (SMA) because of its better performance as SMA actuators in a variety of industrial and engineering applications. Its advantages in front of other SMA are: lower functional fatigue, higher deformation recovery, higher transformation temperatures stability and lower hysteresis range. Although NiTiCu shows these properties, it is strongly influenced by thermomechanical cycling (TMC), which generates a degradation that depends on the load and the deformation applied.

Since hardness, a measure of a material’s resistance to localized plastic deformation, is one of important material properties, which strongly influences its mechanical performance, microhardness tests can be used to obtain a relation between hardness evolution and functional properties degradation with TMC.

In this work, a special in-situ method has been developed to evaluate the microvickers hardness at different levels of TMC. With this configuration, hardness can be tested at different stages of cycling because specimens are not destroyed and there are no affectations from sample preparation as they are in the typical hardness preparation methods.

TMC influence has been analysed with different tests conditions:  thermal cycling at constant stress (evaluated at different stress levels from low levels to above critical stress), isothermal tensile tests at constant deformation (evaluated at different deformation levels) and a combination of both (thermal cycling at constant stress + isothermal tensile tests at constant deformation).

To evaluate the change of hardness as a result of the heat treatment done for shape-setting, all tests have been done on specimens with four different heat treatments (400, 450, 500 and 550 °C followed by water quenching). Finally, to contrast the results and have a better understating of which are the intrinsic mechanisms that modifies the hardness through TMC, metallographic analysis have been also carried out.