Influence of local thermal cycle on the lattice and microstructure evolution of twin-wire directed energy deposition-arc fabricated equiatomic NiTi alloy

Abstract

In the present work, the authors wish to investigate the effect of repeated heating cycles on the phase and crystal structure few layers at a time. Equiatomic NiTi alloy is additively manufactured using twin-wire directed energy deposition-arc (TW-DED-arc) in an in-situ alloying process from pure Ni and Ti wire feedstocks. The central region is wide enough for the extraction of several metallographic and tensile specimens away from dilution zones. Microstructure characterization reveals the incorporation of silicon preventing the formation of NiTi2 and the existence of two types of B2 austenite in the matrix, which has a composition of 50.2 at.%Ni-49.8 at.%Ti. The lattice is observed to dilate near the top regions under tensile thermal stress with no contribution from Ni-rich precipitates. Mechanical test gives a maximum ultimate tensile stress of 782.5 MPa with a fracture surface indicative of quasi-cleavage failure. The results show TW-DED-arc is able to produce fully dense, chemically homogenous NiTi alloys without detrimental phases. In addition, the local thermal cycles experienced by the samples gradually promote the nucleation of Ti5Si3 precipitates and lead to a preferential growth of NiTi grains along the (0 0 1) direction.