Investigation of some thermomechanical processing effects on the structure and properties of a TiNiCu shape memory alloy

Abstract

A Ti50Ni45Cu5 shape memory alloy (SMA) was cast, homogenized and hot rolled into 0.5 mm-thick lamellas. After homogenization, the intercrystalline Ti2(Ni, Cu) based solid solution, gradually decomposed with the increase of hold temperature and time, as pointed out by optical, scanning electron and atomic force microscopy. After hot rolling, by wire spark erosion, rectangular specimens with the dimensions 0.5 mm × 2 mm × 6 mm were cut each one with a mass of 0.44 g. Since the specimens experienced a thermoelastic martensitic transformation, while being martensitic at room temperature (RT) they were soft and could be easily bent against cylindrical calibers, in order to induce their “cold” (martensitic) shape. When heated, up to 100 °C with a hot air gun, they readily recovered, by shape memory effect (SME) their initial straight form, also known as “hot shape”. Thermomechanical training cycles were subsequently applied, consisting in: (i) RT bending; (ii) hot air gun heating and (iii) air cooling. These cycles were repeated up to 50 times. The effects of the thermomechanical training cycles were subsequently analyzed, by differential scanning calorimetry, on fragments cut from the active sections of the specimens. It was noticed that training basically increased thermal hysteresis. Finally, experiments were performed which demonstrated the capacity of Ti50Ni45Cu5 SMA elements to develop.