Monitoring Tensile Fatigue of Superelastic NiTi Wire in Liquids by Electrochemical Potential
Shap. Mem. Superelasticity
Monitoring Tensile Fatigue of Superelastic NiTi Wire in Liquids by Electrochemical Potential
Jan Racek 0 1 2
Marc Stora 0 1 2
Petr Sˇ ittner 0 1 2
Ludeˇk Heller 0 1 2
Jaromir Kopecˇek 0 1 2
Martin Petrenec 0 1 2
Corrosion Hydrogen 0 1 2
0 French Institute for Advanced Mechanics (IFMA), campus de Clermont-Ferrand , 27 rue Roche Genes, 63170 Aubiere , France
1 Institute of Physics ASCR , Na Slovance 2, 182 21 Prague , Czech Republic
2 TESCAN , Brno , Czech Republic
Fatigue of superelastic NiTi wires was investigated by cyclic tension in simulated biofluid. The state of the surface of the fatigued NiTi wire was monitored by following the evolution of the electrochemical open circuit potential (OCP) together with macroscopic stresses and strains. The ceramic TiO2 oxide layer on the NiTi wire surface cannot withstand the large transformation strain and fractures in the first cycle. Based on the analysis of the results of in situ OCP experiments and SEM observation of cracks, it is claimed that the cycled wire surface develops mechanochemical reactions at the NiTi/liquid interface leading to cumulative generation of hydrogen, uptake of the hydrogen by the NiTi matrix, local loss of the matrix strength, crack transfer into the NiTi matrix, accelerated crack growth, and ultimately to the brittle fracture of the wire. Fatigue degradation is thus claimed to originate from the mechanochemical processes occurring at the excessively deforming surface not from the accumulation of defects due to energy dissipative bulk deformation processes. Ironically, combination of the two exciting
Nitinol; Shape memory alloy; Fatigue; Tensile test; Electrochemistry embrittlement
-
properties of NiTi—superelasticity due to martensitic
transformation and biocompatibility due to the protective
TiO2 surface oxide layer—leads to excessive fatigue
damage during cyclic mechanical loading in biofluids.
Introduction
NiTi-based alloys have established themselves as key shape
memory alloy material for wide range of engineering
applications [
1
] exploiting their unique functional
thermomechanical behaviors due to the martensitic transformation
driven by the stress and temperature. The key phenomena are
the large recoverable superelastic and shape memory strains
and strong stress–temperature coupling. When trying to
utilize these phenomena in applications designs, NiTi alloys
are considered to be exposed to long-time cyclic variations
of large stress (\700 MPa), large reversible strains (\7 %),
wide range of temperatures (-150 C to ?120 C), or even
corrosive environments (water, blood, biofluids, oil, vapors,
hydrogen). Depending on the imposed strain amplitude, the
NiTi alloys exhibit thousands to millions of cycles to failure
in (thermo)mechanical cycling, no other metallic materials
can even approach such a superior fatigue performance at
comparable conditions. In successful practical applications,
however, the conditions are frequently limited. In this
context, the ‘‘limited thermomechanical fatigue performance of
NiTi’’ represents an obstacle hindering further development
of many engineering applications of NiTi (superelastic
stents, orthodontic implants and tools, vibration damping
elements, actuator springs, HTSMA actuators).
Fatigue of phase-transforming NiTi has been
investigated in the literature both experimentally [
2–8
] and
theoretically [
7, 8
]. Although the stress and/or temperature
driven martensitic transformation proceeds cyclically in all
fatigue tests, fatigue of superelastic medical stents,
orthodontic wires and implants, thermomechanical fatigue
of actuator wires or springs or fatigue of high temperature
actuators seems to be governed by different rules and hence
must be dealt with separately.
Functional fatigue (cyclic degradation of functional
response) [
3
] and structural fatigue [
6
] have to be
distinguished and possible links between them shall be identified.
In spite of the attention paid to the drift of the cyclic stress–
strain–temperature responses in the literature [
3, 4
], it is not
still clear how the drift is exactly related to the structural
fatigue. There seems to be a general agreement in the
literature on a dissipation energy-based fatigue criterion [8]—the
more dissipated energy is stored in the material during the
transformation cycle, the shorter is the fatigue lifetime. In
other words, if there is no energy dissipation, there should be
no fatigue and full strain superelastic cycling shall be feasible
for millions of cycles. But energy dissipation and hysteresis
are intrinsically related to the martensitic transformation and
lattice defects are created by phase interfaces propagating
during cyclic martensitic transformations [
9
]. The
accumulating defects lead to the microstructure evolution during the
thermomechanical cycling [
10
] which is believed to control
the fatigue performance [
3, 6
]. But is this really responsible
for the experiment (...truncated)