NiTi alloys with simultaneous equiaxed-grain strength and columnar-grain ductility manipulated by direct energy deposition

NiTi alloys prepared by additive manufacturing (AM) often have coarse columnar grains showing poor tensile superelasticity, strength, and ductility, significantly impairing part qualification and targeted applications. In this work, we used laser-engineered net shaping (LENS) to fabricate NiTi alloys with lamellar structures which consist of fine nearly equiaxed grains, columnar grains, and high-density dislocations, through periodically alternating processing parameters. This strategy can partially keep the equiaxed grains produced by the transformation from columnar grains to equiaxed grains (CET) at the top of the deposited layer, and interrupt the epitaxial growth of columnar grains in AM-ed NiTi alloys. The results show that NiTi alloys with lamellar structures exhibit an excellent combination of strength and ductility with a tensile strength of 649 MPa strain of 17.5 %, a maximum recovery strain of 5.3 %, and a stable recovery strain of 3.1 % after 20 cycles. Our findings may increase the range of adjustable microstructures and combinations of properties in NiTi alloys and provide a new idea for microstructure modification and reinforcement of AM shape memory alloy.