Tailoring damping properties of a friction stir processed Cu-Al-Ni shape memory alloy: Effects of annealing time on microstructure and damping performance

Shape memory alloys exhibiting high damping capacity after grain refinement are critical for vibration attenuation in structural and precision instrument applications. This study reveals the evolution of damping capacity and the associated martensitic structure, as regulated by annealing time, in a Cu–13.2Al–4Ni shape memory alloy processed by friction stir processing for grain refinement. As annealing time progresses, the damping capacity of the alloy exhibits an initial increase followed by a slight decrease. This is ascribed to the evolution of grain size and martensite configuration in the alloy. The average grain size first decreases to 30 μm and then increases to 70 μm, which results in a corresponding change in the width of martensite bands. More importantly, the martensite morphology of the alloy transforms from large self-accommodated variants to fine parallel bands with nearly defect‑free interfaces, further grows into crossover bands. The original low-mobility variants vanish firstly. Further annealing then leads to new internal twins exhibiting step‑like structure. These results are expected to further expand the possibilities for designing high-damping materials.