TEM Characterization of the microstructures in shape memory alloys

FRANCISCO C. LOVEY; ADRIANA M. CONDÓ; MARCOS SADE; VICENÇ TORRA

Cusco, Perú

Congreso; IX Interamerican Congress of Electron Microscopy; 2007

The mechanical properties and the phase stabilities of the shape memory alloys (SMA) are strongly influenced and controlled by the microstructure of the alloys. Important differences are observed whether the material is in the form of a single crystal, a polycrystal, a multiphase, a thin film or showing any other microstructure. The capability of shape memory alloys to produce important forces and displacement in small volumes makes them very suitable for the development of micro and even nano scale devices. In this work thin films of Cu- 26.9at% Al-5.5at% Ni were grown using a sputtering magnetron DC, from the alloy previously melted in an induction furnace. The sputtered alloy was deposited on glass or crystalline Si at room temperature. Films of about 3mm thick were removed from the substrate for further studies. The structures and microstructures of the as grown films were analyzed by transmission electron microscopy. A nanometric mixture of BCC and 2H phases have been found irrespective of the substrate. The film appears preferentially textured with the 111 direction of the BCC phase perpendicular to the film surface. On the other hand polycrystalline CuAlBe alloys are suitable for damping applications particularly for short time events (earthquakes). The hysteresis depends on the microstructure, i.e., the grain size. The smaller the grain size the larger the hysteresis of the martensitic transformation. However, strong evolution of the mechanical stresses occur on cycling, producing a degradation of the damping capacities. In this work the evolution of the specimen microstructure during the pseudoelastic cycling was investigated by TEM. It was found that dislocation bands and retained martensite plates are left at the grain boundaries. These defects arise as a mechanism for the grain accommodation during the transformation. The energy spent in the creation of these defects is responsible of the changes in the stress-strain curves on cycling.