Microstructure and shape memory behavior of a bi-layer fabricated by NiTi deposition on NiTiCu substrate using laser powder bed fusion

Today, two-way shape memory alloys have become increasingly popular in various industries. In this study, innovative nitinol-based bi-layers were fabricated using the additive approach. The purpose of the study was to examine the characteristics of a bi-layer sample made of Ni_50.8Ti_49.2 that was deposited using Laser Powder Bed Fusion on a Ni₄₅Ti₅₀Cu₅ substrate made by vacuum arc remelting. Transmission electron microscopy with high resolution was used to study the bi-layer's microstructure. The as-printed bi-layer exhibits dislocation, stacking faults, precipitates such Ni₄Ti₃ and Ti₂Ni, compound twins, and Type Ⅱ twins, according to the transmission electron microscopy investigation. After going through annealing at a temperature of 1000 °C for a duration of 16 h, it was observed that Type Ⅰ and compound twins were the prevailing characteristics. Furthermore, it has been demonstrated that subjecting the material to heat treatment leads to enhanced elongation and reduced fracture stress. The sample aged at 500 °C for 30 min exhibited the most favorable properties, including a maximum elongation of 10 ± 0.1 % after 12 incremental cycles. In addition, the bi-layer samples exhibited both pseudoelasticity and shape memory effect, with approximately 80 % intrinsic reversible two-way shape memory behavior. The findings of this study can be applied in the production of various actuators.