Besides the unique shape memory effect and superelasticity, NiTi alloys also show excellent damping properties. However, the high damping effect is highly temperature-dependent, and only exists during cooling or heating over the temperature range where martensitic transformation occurs. As a result, expanding the temperature range of martensite transformation is an effective approach to widen the working temperature window with high damping performance. In this work, layer-structured functionally graded NiTi alloys were produced by laser powder bed fusion (L-PBF) alternating two or three sets of process parameters. The transformation behavior shows that austenite transforms gradually into martensite over a wide temperature range during cooling, and multiple transformation peaks are observed. A microstructure composed of alternating layers of B2/B19′ phases is obtained at room temperature. The functionally graded sample shows high damping performance over a wide temperature range of up to 70 K, which originates from the gradual formation of the martensite phase during cooling. This work proves the potential of L-PBF to create NiTi alloys with high damping properties over a wide temperature range for damping applications.

A stress plateau with a strain of 5–8% normally occurs during the stress-induced martensite transformation (SIM) of NiTi shape memory alloys. Within the stress plateau, the correlation between the stress and strain is lost, which limits their application in certain fields which require accurate control of inelastic deformation. In order to address this limitation, a series of step-like NiTi samples with graded cross-sectional area were designed and fabricated. Multiple stress plateaus were achieved by varying the sample width and adjusting the number of steps; for instance, two and three stress plateaus were obtained in the samples with two and three steps, respectively. Also, linear force–strain response was obtained by changing gradually the width of the samples. The functional stability of the geometrically graded samples was significantly improved by incomplete recrystallization annealing (600 °C) followed by low-temperature (250 °C) aging treatment. The incompletely recrystallized specimens contained many dislocations and grain and sub-grain boundaries, which promoted the uniform precipitation of Ni₄Ti₃ nanoparticles during aging treatment. The homogeneously and densely dispersed Ni₄Ti₃ nanoparticles were able to strengthen the matrix considerably and prevent plastic activities during stress-induced martensite transformation. As a result, the functional stability of the geometrically graded NiTi samples was much improved. After aging at 250 °C for 120 h, all the samples showed a small residual strain of <1.0% after 20 loading–unloading cycles.

Additive manufacturing of Ti6Al4V alloys via laser powder bed fusion (L-PBF) has demonstrated superior tensile strength compared to conventional methods. However, challenges remain in enhancing ductility and tailoring mechanical properties for specific applications. In this work, we show a feasible method to regulate the mechanical properties of additively manufacturing Ti alloys. Ti6Al4V alloys with different Nb content (1, 3, and 10 wt.%) were fabricated through laser powder bed fusion (L-PBF) in situ alloying using the mixture of Ti6Al4V and Nb powders. The powder mixture shows good printability, and dense Ti6Al4V-xNb alloys are obtained. Although the distribution of Nb is highly heterogeneous, no solidification cracks or secondary intermetallics were detected in both the Nb-rich and Nb-lean regions. The microstructure is gradually refined with the increase in Nb addition, mainly due to the heterogeneous nucleation caused by the partially melted Nb particles. The L-PBF-fabricated T6Al4V-xNb alloys are mainly in α’ martensite phase, even with the addition of 10 wt.% Nb, due to the low content of Nb solute in the matrix. The presence of β phase is suggested around the Nb particles, since a small region with graded Nb content is formed around the Nb particles. The ultimate tensile strength increases from 1050 to 1181 MPa with the addition of 3 wt.% Nb, and the total elongation increases slightly from 8.8% to 10.5%. With the addition of 10 wt.% Nb, the total elongation increases largely to 15.6%, while maintaining a high strength of 1135 MPa. Moreover, the elastic modulus decreases from 105 to 80 GPa with the increase in Nb content to 10 wt.%. The results of this work suggest that L-PBF in situ alloying is a promising approach to optimize the mechanical performance of Ti6Al4V alloys.