Structure, Martensitic Transformation, and Damping Properties of Functionally Graded NiTi Shape Memory Alloys Fabricated by Laser Powder Bed Fusion.

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.