Microstructural origin of ultrahigh damping capacity in Ni50.8 Ti49.2 alloy containing nanodomains induced by insufficient annealing and low-temperature aging

We report a Ni50.8Ti49.2 alloy that exhibits an ultrahigh damping capacity with an internal friction value of similar to 0.35 (tan delta) at low frequencies in a wide temperature range. This alloy undergoes low-temperature aging after insufficient annealing from a severely deformed state (similar to 90% cold work). Via phase transformation analyses and microstructure characterization, we demonstrate that this high broad internal friction plateau originates from nanodomains and the stable coexistence of the B19'-phase and R-phase over a wide temperature range. The abundant phase boundaries and movable interfaces between nanodomains contribute significantly to this high broad internal friction plateau. Further analyses validate the critical role of low-temperature aging in inducing nanodomains, i.e., resulting in localized Ni segregation. The abnormal appearance of needle-like B19' martensites near room temperature is attributed to the defects that remain after insufficient annealing, after which low-temperature aging promotes localized depletion or enrichment of Ni. In addition, the results of this study indicate that high density nanodomains are generated from the two steps of insufficient annealing and low-temperature aging in the severely deformed Ni50.8Ti49.2 alloy. The idea of a high-damping microstructure design based on "domain engineering" is realized. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.