Critical factor and mechanism affecting the tensile ductility of L-PBF fabricated NiTi shape memory alloy

Laser powder-bed fusion (L-PBF) is a promising additive manufacturing technology for the manufacture of NiTi shape memory alloy parts with complex geometries. However, the critical factor and mechanism affecting tensile ductility of NiTi parts produced by L-PBF remain unclear. In this work, it is found and experimentally demonstrated that the tensile ductility of L-PBF NiTi alloys decreases rapidly with increasing apparent yield stress, irrespective of the size, number and type of pore defects that significantly affect the tensile ductility of L-PBF conventional alloys. For L-PBF NiTi alloys, as the porosity increases from 0.05 to 1.68%, their tensile strain at low apparent yield stress is consistently 12-16%, at medium apparent yield stress it is 10-12%, and at high apparent yield stress it is 3-6%. It is revealed that the peculiar relationship derives from the interaction of pore defects and NiTi matrix with different deformation mechanisms by in-situ DIC technique and finite element simulation. When the matrix deforms by the stress-induced transformation or martensitic reorientation, the local excessive deformation around pore defects can be effectively suppressed. When the matrix deforms by dislocation slip, the strain concentration around pore defects will be rapidly intensified. Our work identifies the critical factor and reveals the mechanism, providing a theoretical direction for the development of the L-PBF NiTi parts with large tensile ductility.