Microstructure, mechanical properties and phase transformation behaviour of directed energy deposition developed NiTixCu shape memory alloys

Abstract

This paper studies microstructure, mechanical properties, and phase transformation behaviour of NiTixCu shape memory alloys (SMAs) developed by adding copper in wire and powder form by twin-wire arc additive manufacturing (T-WAAM) and µ-plasma arc metal additive manufacturing (µ-PAMAM) process respectively. Microstructure of µ-PAMAM developed NiTi5Cu showed formation of Cu(Ni, Ti) intermetallic compounds (IMCs) suggesting Cu segregation and altered segregation pattern of Ni and Ti. However, addition of 20 at% of Cu powder by µ-PAMAM process significantly suppressed formation of dendritic structure and led to formation of Cu-rich and Ti-rich phases in the NiTiCu matrix. Microstructure of T-WAAM developed NiTi20Cu SMA revealed formation of elongated well defined coarse grains along with porosity indicating slower solidification rate. Elemental distribution found that at% of Ti is more than at% of Ni in all the developed SMAs and that T-WAAM developed NiTi20Cu SMA has slightly more at% of Cu than µ-PAMAM developed NiTi20Cu SMA. All the developed SMAs showed evolution of austenitic B2 phase and Ti2Ni and Ti2(Ni, Cu) intermetallic. Additionally, evolution of martensitic B19' phase in µ-PAMAM developed NiTi and NiTi5Cu SMAs, R phase in µ-PAMAM developed NiTi5Cu SMA, and B19 phase in µ-PAMAM and T-WAAM developed NiTi20Cu SMA are also observed. A notable shift in the peak of austenitic B2 phase towards lower values of 2θ is observed in the µ-PAMAM and T-WAAM developed NiTi20Cu SMA. Microhardness values of all the developed SMAs increase along their build direction. The µ-PAMAM developed NiTi20Cu SMA has maximum microhardness and tensile strength but minimum % elongation whereas the T-WAAM developed NiTi20Cu SMA has minimum microhardness and tensile strength. The µ-PAMAM and T-WAAM developed NiTi20Cu SMAs showed different start and finish temperatures for the martensitic and austenitic phases unlike NiTi and NiTi5Cu SMA. This enabled µ-PAMAM developed NiTi20Cu SMA to possess minimum thermal hysteresis of 7.6ºC among all the developed SMAs which will make it useful in the applications requiring precise actuation and high reliability i.e., aerospace, biomedical devices, and robotics applications. © 2025 Elsevier Ltd