  Molecular dynamics simulations are performed to provide a detailed
understanding of the functional degradation of shape memory alloys at small
scale. The origin of the experimentally reported accumulation of plastic
deformation and the anomalous sudden increase of the residual strain under
cyclic mechanical loading are explained by detailed insights into the relevant
atomic scale processes. Our work reveals that the mechanical response of
shape-memory-alloy pillars under cyclic compression is significantly influenced
by the presence of an amorphous-like surface region as experimentally induced
by focused ion beam milling. The main factor responsible for the observed
degradation of superelasticity under cyclic loading is the accumulated plastic
deformation and the resultant retained martensite originating from a synergetic
contribution of the amorphous and crystalline shape-memory-alloy regions. We
show that the reported sudden diminishment of the stress plateaus and
hysteresis under cyclic loading is caused by the increased stability of the
martensite phase due to the presence of the amorphous phase. Based on the
identified mechanism responsible for the degradation, we validate reported
methods of recovering the superelasticity and propose a new method to prohibit
the synergetic contribution of the amorphous and crystalline regions, such as
to achieve a sustainable operation of shape memory alloys at small scale.
