Effect of annealing on the microstructure of Yttrium-doped NiTiCu shape memory alloys

This work is focused on the study of Ni41-xTi50Cu9Yx (x = 1, 3, 5 at%) alloys prepared setting different temperatures of the annealing process: 900°C, 950°C and 1000°C for one hour with quenching at room temperature. Morphology and chemical composition of annealed specimens were investigated by optical and electronic microscopy observations, together with the energy dispersive X-ray analysis. In addition, calorimetry was useful in determining the path and the product of the phase transition as a function of the annealing temperature. These techniques highlight the peculiar multi-phase structure that characterizes the presented alloys, which result to be principally compound by a double-matrix based on NiTiCu and NiTi alloys, by YCu-based areas, and by few Y-based and Cu-based particles. Optical microscopy helped in detecting the presence of the orthorhombic B19 phase in x = 1 at% and x = 3 at% samples, and it revealed to be more precise with respect to the differential scanning calorimetry. Besides, scanning electron microscopy pointed out the polygonal morphology of the Y-based particles that take origin inside the YCu zones; some star-like inclusions, seemingly related to the nucleation points of such Y particles, were also observed. Image processing helped in studying the average size of YCu areas: it was found that these zones have dimension that increases both with Yttrium content and with the annealing temperature. Finally, scanning electron microscopy was also advantageous in detecting some cracks around and across the YCu areas.