Fabricating TiNiCu Ternary Shape Memory Alloy by Directed Energy Deposition via Elemental Metal Powders  
[PERSON]  
1 Department of Mechanical and Aerospace Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA; [EMAIL_ADDRESS] (X.Z.); [EMAIL_ADDRESS] (M.M.P.); [EMAIL_ADDRESS] (FL.)  
[PERSON]  
1 Department of Mechanical and Aerospace Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA; [EMAIL_ADDRESS] (X.Z.); [EMAIL_ADDRESS] (M.M.P.); [EMAIL_ADDRESS] (FL.)  
[PERSON]  
1 Department of Mechanical and Aerospace Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA; [EMAIL_ADDRESS] (X.Z.); [EMAIL_ADDRESS] (M.M.P.); [EMAIL_ADDRESS] (FL.)  
[PERSON]  
3 Department of Materials Science and Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA; [EMAIL_ADDRESS] (Correspondence: [EMAIL_ADDRESS])  
[PERSON]  
1 Department of Mechanical and Aerospace Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA; [EMAIL_ADDRESS] (X.Z.); [EMAIL_ADDRESS] (M.M.P.); [EMAIL_ADDRESS] (FL.)  
Footnote 1: email: {cheng202122012  
and high entropy alloys [19; 20; 21; 22; 23; 24]. A schematic diagram for the mechanism of DED using laser and pre-mixed powders is demonstrated in Figure 1.  
TiNi alloy has attracted much more attention in applications in various engineering areas due to its unique shape memory effect and superelastic behavior [25; 26]. Researchers have found AM to be a good choice to fabricate TiNi since conventional machining is challenging [27]. Several previous works attempted to fabricate TiNi alloys via pre-alloyed TiNi powders [28; 29]. Due to the high cost of producing pre-alloyed TiNi powders, blending Ti and Ni elemental powders has become a novel approach to fabricate TiNi alloys with more flexibility and low cost [27]. It was reported that a small amount of a third metal element could be added to design new ternary shape memory alloys [30]. A good example of metal elements that can act as the third element is Cu. Adding Cu in TiNi alloy to replace a small portion of Ni attracts interest due to its capability to shorten thermal hysteresis, which can be more welcomed in the field of actuator fabrication [30].  
According to the high material flexibility of the DED process, adjusting material properties by adding a third element in the Ti-Ni system can be carried out in a more flexible way [31]. However, manufacturing ternary shape memory alloys by DED has not been extensively reported. [PERSON] et al. [32] attempted to fabricate TiNiCu ternary alloys via the DED process with elemental powders, and they compared material properties among TiNiCu parts with different Cu compositions. However, some special aspects were not reported in detail in [32]. For DED using elemental powders as feedstocks, it is also worth mentioning the inhomogeneity of composition, microstructure, phase formation, and material properties reported in the works of elemental powder-based DED [31]. The DED process can cause anisotropic material behaviors due to its layer-wise fashion [33], and for parts fabricated by DED with an elemental powder mixture, the interaction between the powder mixture and energy is more complex than using pre-alloyed powders [31]. Thus, the composition of the final as-fabricated part can be location dependent with compositional deviations [31]. In [32], the location information and the correlation among chemical compositions, phase, and material behaviors at various locations within an as-fabricated TiNiCu alloy part were not fully described. Therefore, in this work, we use a Ti, Ni, and Cu elemental powder mixture to fabricate a TiNiCu ternary shape memory alloy to find out more about the correlation among the composition, phase, mechanical properties, and functional properties, which could provide a more comprehensive understanding of the structure-property relationship in the TiNiCu ternary shape memory alloy, especially when fabricated by the elemental powder-based DED process.