Due to unique functional and mechanical properties, NiTi shape memory alloys are one of the most promising metallic functional materials. However, the poor workability limits the extensive utilization of NiTi alloys as components of complex shapes. The emerging additive manufacturing techniques provide high degrees of freedom to fabricate complex structures. A freeform fabrication of complex structures by additive manufacturing combined with the unique functional properties (e.g., shape memory effect and superelasticity) provide great potential for material and structure design, and thus should lead to numerous applications. In this review, the unique microstructure that is generated by selective laser melting (SLM) is discussed first. Afterwards, the previously reported transformation behavior and mechanical properties of NiTi alloys produced under various SLM conditions are summarized.

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.

Application of a new method—high-sensitivity measurements of periodic stress-induced induction (reversible Villari effect, RVE)—allows us to uncover new effects in the low-field behaviour of different magnetic phases of polycrystalline Dy. A loss of magnetoelastic coupling (Villari critical point) is found around 166 K, close to the temperature which was supposed to separate the helical structure from the possible vortex one. We show that the low-field magnetic hysteresis emerges immediately below the Néel temperature and, below the Villari critical point, demonstrates a qualitative difference for cooling and heating from the ferromagnetic state. It has been found that, below the Villari critical point, in the helical phase, polycrystalline Dy is in essentially non-equilibrium state, revealed as a time-dependent relaxation of RVE. We relate the effects observed in the helical phase with thermal internal stresses and existence of lattice defects which inherit ferromagnetic order upon heating from the ferromagnetic state.