Nanocomposite NiTi shape memory alloy with high strength and fatigue resistance

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Abstract
Fatigue life
enhancement of NiTi-based shape memory alloys (SMA) has been an active topic
over the past decades, because many established and potential applications of
NiTi SMA in biomedical devices and recent solid-state refrigeration require
very long fatigue life (107-109). However, improving the
fatigue resistance of NiTi often compromises other mechanical and functional
properties. Here we provide a new strategy to overcome this dilemma through
hybrid heterogenous nanostructure. We produced superelastic NiTi nanocomposite with
crystalline and amorphous phases via severe plastic deformation and low
temperature annealing. We show that the NiTi nanocomposite has a recoverable
strain of 4.3%, yield strength of 2.3 GPa, and can endure over 108
reversible-phase-transition cycles under the stress of 1.8 GPa via compression
experiments of micropillars. We attribute the enhanced properties to the mutual strengthening of nano-sized amorphous and crystalline
phases where both dislocation slip in the crystals and shear band propagation
in the amorphous phase are suppressed. The synergy of
the properties of crystalline and amorphous phases at the nanoscale could be an
effective method to improve fatigue resistance and strength of SMA.