The electron beam freeform fabrication of NiTi shape memory alloys. Part II: Influence of the heat treatment on the superelastic behaviour

In a previous study (part I), the deposition of functional single-track and multi-layer Ni-rich NiTi using the electron beam freeform fabrication (EBF3) technique was demonstrated. The processed microstructure consists of columnar millimetre range grains that grow parallel to the build direction. Based on the same parameters of part I, this work successfully deposited a stable multi-track and multi-layer NiTi block. Different heat treatments were applied to modify the superelastic properties of the deposited material. Initially, the solution treatment reduces the chemical inhomogeneity and the thermal stresses that originated during the deposition and cooling. Subsequently, due to ageing, Ni₄Ti₃ particles precipitate at 350 °C and 450 °C for 1, 6 and 12 h, modified the superelastic behaviour of the deposited NiTi. The differential scanning calorimetry results did not reveal any difference in the martensitic starting temperature between as-built and solution-treated samples. However, a remarkable influence of the ageing temperature was detected: while ageing at 350 °C impacted R-phase starting temperatures, 450 °C altered the martensitic starting one. The superelastic behaviour was evaluated by cycling compression. As-built and solution heat-treated samples presented poor mechanical performance, with about 40% of strain recovered after 10 cycles. The ageing at 350 °C for 1 h led to a maximum measured strain recovery of 72.5% after 10 uniaxial compression cycles up to 860 MPa. This condition also resulted in 3.3% permanent deformation out of 11.3% of the total. This study addressed for the first time the fabrication by the EBF3 technique of a stable multi-track and multi-layer NiTi block besides the importance of heat treating it, demonstrating that this post-treatment improves the superelastic performance of EBF3 fabricated parts.