Researchers atShenzhen Universityand partner institutions showed a way to reduce functional anisotropy in a 4D-printed high-entropy shape memory alloy (HESMA) by adding TiN nanoparticles during laser powder bed fusion (LPBF). Reported inAdditive Manufacturing Letters, the study shows that nanoparticle-driven grain refinement lowers both mechanical and shape-memory anisotropy, and that a short heat treatment after printing restores recovery performance.
The team investigated an Fe50Mn20Co10Cr10Si10 (at.%) alloy produced by LPBF and compared samples with and without 1.0 wt.% TiN nanoparticles. LPBF typically produces strong columnar grains aligned with the build direction. However, the addition of TiN triggered a columnar-to-equiaxed transition (CET), resulting in a more isotropic microstructure.
Reducing anisotropy in 4D printed alloys
Metal additive manufacturing often produces anisotropic properties because steep thermal gradients and directional solidification create direction-dependent microstructures. In shape memory alloys, this anisotropy changes martensitic transformation behavior, causing differences in recovery strain and yield strength between directions.
In the as-built HESMA matrix without TiN, yield strength differed significantly between build orientations, with the horizontal sample reaching 582.5 MPa and the vertical sample 417.4 MPa. Shape memory performance also varied, with maximum recovery strain reaching 6.3% in the vertical direction compared to 4.0% horizontally.
After adding TiN nanoparticles, yield strength rose to 802.4 MPa (horizontal) and 665.9 MPa (vertical). The yield strength anisotropy ratio decreased from 39.6% to 20.5%. Shape memory anisotropy was reduced even more, with the maximum recovery strain difference dropping from 56.3% to 14.9%.
Grain refinement drives microstructural transition
Electron backscatter diffraction analysis showed that TiN nanoparticles promoted heterogeneous nucleation during solidification. This led to strong grain refinement, reducing the average grain size from about 15.8 μm in the base alloy to 1.68 μm in the TiN-modified material. The strong <100> texture typical of LPBF-processed metals weakened, resulting in a more uniform distribution of grain orientations.
While grain refinement increased strength and reduced anisotropy, it initially reduced shape memory performance. The higher grain boundary density restricted stress-induced martensitic transformation, lowering maximum recovery strain to 4.7% in the vertical direction.
Heat treatment restores shape memory performance
Source: 3D Printing Industry
