A Review on Micro‑mechanical Testing of NiTi‑Based Shape Memory Alloys

Abstract

NiTi-based shape memory alloys are considered as poten‑
tial candidates for various structural, functional and biomedical applica‑
tions. This is particularly related to their unique characteristics such as
pseudoelastic and shape memory effects. Considering the increasing
demand of NiTi alloys in miniaturized devices, the small-scale deforma‑
tion micro-mechanisms of such a material system is being thoroughly
reviewed in this article. At the frst hand, the fundamental characteris‑
tics of NiTi system is discussed briefy. The infuence of different factors
such as chemical composition, crystallographic phases and precipi‑
tates on the phase transformation and mechanical behavior of the
material are emphasized next. Subsequently, an extensive overview is
provided regarding the assessment of small-scale deformation behav‑
ior of NiTi alloys using two prime techniques: micro-pillar compression
and instrumented nanoindentation. Pros and cons for both the charac‑
terization techniques are analyzed as well. Interestingly, uniaxial com‑
pression of the micro-pillar reveals the pseudoelastic behavior in the
alloy with remarkable enhancement in martensitic transformation stress
and plateau strain with respect to those observed from macro-scale
testing. It is also evident from the studies that mechanical behavior of
the material strongly depends on different crystallographic orientation.
Contrary to the uniaxial compression, nanoindentation generates triax‑
ial state of stress beneath the tip of indenter, which is likely to infuence
the deformation micro-mechanism for the NiTi system, as well. Varia‑
tion in hardness, elastic modulus and recoverability of NiTi system are
primarily assessed through nanoindentation-based studies. In addition,
recent studies highlight the importance of optimizing the nanoindenta‑
tion parameters such as tip confguration, tip radius and load level for
precise estimation of pseudoelastic activity in the alloy. Moreover, a
simplifed approach is generated for estimating the indentation stress–
indentation strain curve. Overall, this paper generates a thorough and
comprehensive insight about the micro-mechanical testing of NiTi-based
shape memory alloys