Effect of TiN nanoparticle on shape memory properties of additively manufactured ferrous high entropy shape memory alloy

Emerging 4D printing is an innovative additive manufacturing (AM) technique that incorporates an extra dimension of time into AM. 4D printed objects can change their shapes or properties in response to external stimuli. However, the intrinsic microstructural anisotropy also affects the shape memory performance of AM printed shape memory alloys (SMAs). This work investigated the effect of TiN nanoparticle addition on the mechanical and shape-memory properties of a high entropy shape memory alloy (HESMA) Fe 50 Mn 20 Co 10 Cr 10 Si 10 (at.%) fabricated by laser powder bed fusion (LPBF). Results show that the TiN addition enhances the yield strength (YS) of the AM HESMA at the expense of ductility. Meanwhile, the as-built shape memory performance is reduced, with the maximum bending recovery strain of the vertical sample decreasing from 6.3% to 4.7%. Moreover, it is shown that introducing TiN nanoparticles can significantly alleviate the 4D-printing anisotropy. The YS anisotropy ratio is reduced from 40.0% to 20.5%, and the shape memory anisotropy ratio decreases from 56.3% to 14.9%. The underlying reason is attributed to a columnar-to-equiaxed microstructure transition induced by the TiN addition, which results in similar amount of grain boundaries during deformation in different directions. To restore the shape memory ability, the TiN/HESMA is subjected to further heat treatment. The maximum recovery strain is improved greatly and approaching that of HESMA matrix, and the shape memory anisotropy ratio further decreases to 3.4%. The underlying mechanism of the heat treatment is revealed. The synergy of TiN addition and heat treatment provides a novel approach to balance strength enhancement and functional anisotropy of 4D printing.