Comparison of Microstructure, Ageing Effect and Shape Memory Properties of Additively Manufactured NiTi Alloy using LENS and EBAM Methods

Samples prepared using various additive manufacturing methods were compared in terms of structure, texture, transformation temperature and superelastic properties. Samples manufactured using laser engineered net shaping (LENS) method showed texture several degrees deviated from the <001> build direction, however with composition near to the initial powder composition, enabling superelastic effect. The electron beam additive manufacturing (EBAM) samples showed martensitic structure at room temperature due to a shift of transformation temperatures to the higher range. This shift occurs due to a lower Ni content resulting from different processing conditions. However, EBAM method produced sharper <001> texture in the build direction and made it possible to obtain a good superelastic effect above room temperature. Intermetallic particles of size 0.5-2 mm were identified as Ti2Ni phase using EDS and electron diffraction analyses. This phase was often formed at the grain boundaries. Contrary to the LENS method, the EBAM prepared samples showed Ni-rich primary particles resulted from different processing conditions that reduce the Ni content in the solid solution thus increase the martensitic transformation temperature. Ageing at 500°C allowed for shifting the martensitic transformation temperatures to the higher range in both, LENS and EBAM, samples. It resulted from the formation of Ni rich coherent precipitates. In samples prepared by both methods and aged at 500°C, the presence of martensite B19’ twins was observed mainly on {011} B19’ planes.

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The presented research results are the effect of the project no. UMO2016/23/B/ST8/00754 financed by the National Science Center, Poland.