Effects of annealing time on the microstructures and tensile properties of formed laminated composites in Ti-Ni system

The effects of annealing time on the microstructures and tensile properties of formed laminated composites in Ti-Ni system were investigated. The results showed that Ti₂Ni, TiNi and Ni₃Ti intermetallics were formed between the Ti and Ni foils after 1 h annealing. These intermetallics grew thicker with the increase of annealing time until the complete consumption of Ti layers. TiNi with two different crystal structures (B2 and B19′) were identified in the composites after 1 h and 6 h annealing. In the composites after 12 h, 18 h, 24 h and 36 h annealing, Ni₄Ti₃phase in different shapes precipitated in different regions (interfaces and the interior) of TiNi layers, resulting in the multi-step martensite transformation behavior of TiNi layers. The tensile test results revealed that high TiNi content in the composite after 12 h annealing had the highest yield strength (391 MPa), ultimate tensile strength (912 MPa) and elongation (6.3%) of all the composites. With the increase of annealing time, the thickness of TiNi layers decreased and that of Ni₃Ti layers increased, leading to the decrease of strength and elongation of the composites after18, 24 h and 36 h annealing, which indicated that TiNi layers had great influence on the strength and ductility of these composites.