Plasma Arc Melting (PAM) and Corrosion Resistance of Pure NiTi Shape Memory Alloys
Shap. Mem. Superelasticity
Plasma Arc Melting (PAM) and Corrosion Resistance of Pure NiTi Shape Memory Alloys
A. Tuissi 0 1
G. Rondelli 0 1
P. Bassani 0 1
0 CNR IENI Unit of Milan, National Research Council of Italy - Institute for Energetics and Interphases , Via R. Cozzi 53, 20125 Milan , Italy
1 CNR IENI Unit of Lecco, National Research Council of Italy - Institute for Energetics and Interphases , Corso Promessi Sposi 29, 23900 Lecco , Italy
Plasma arc melting (PAM) as a suitable noncontaminating melting route for manufacturing high-quality NiTi alloy was successfully examined. The corrosion resistance of PAM Nitinol was evaluated by both potentiodynamic and potentiostatic tests and compared with lower purity NiTi produced by vacuum induction melting (VIM). For the electro-polished surfaces, excellent corrosion resistance of NiTi comparable with the Ti alloys was found with no pitting up to 800 mV versus saturated calomel electrode in simulated body fluid at 37 C. Potentiostatic results of PAM Nitinol indicate slightly better corrosion resistance than the lower quality VIM alloy.
NiTi processing; Corrosion resistance; Plasma arc melting; Microstructure
Introduction
Due to the highly reactive nature of titanium, the synthesis
of NiTi shape memory alloy (also known as Nitinol)
requires non-contaminating processing methods. The
gathering of C, N, and O on melting causes the formation of
inclusions such as TiC, TiO2, and Ti2NixOy on bulk level
and hot-working oxidation may leave distributed inclusions
at surface level of semi-finished products. A recent
observation, the surface particulates of NiTi and in biomedical
devices, is reported by Shabalovskaya et al. [
1
]. The
inclusions, and their sizes, could affect both the NiTi
functional properties and the processing aspects such as the
workability of low dimension semi-finished products.
Nowadays, the role and the amount of inclusions affecting
specific properties such as the fatigue life,
biocompatibility, or corrosion resistance of devices made in NiTi
are not univocally explained. Regarding TiC inclusions,
which are found typically in VIM processed Nitinol due to
carbon crucibles employed in this kind of melting plant,
their presence is particularly detrimental to pitting
corrosion resistance, as asserted in Kimura and Sohmura [
2
],
Shabalovskaya et al. [
3
]. Also, regarding oxide-type
inclusions (i.e., Ti2NiOx) Liang and Huang [
4
] have found
that their presence is the main reason for pitting. Coda et al.
[
5
] characterized the main types of inclusions of
commercial Nitinol produced by VIM and vacuum arc melting
(VAR) routes; by means of extreme value statistics,
Urbano et al. [
6
] showed that the maximum inclusion
dimension in the highly strained volume of a fatigue test
contributes to a decrease in the fatigue performance of
0.3mm-diameter superelastic wires. Morgan et al. [
7
] reported
about extra low inclusions content (ELI) alloys appeared to
have improved fatigue performances. Rahim et al. [
8
]
investigated the effect of C and O impurity levels on fatigue
lives, of pseudoelastic NiTi, reporting that the fatigue
resistance is affected mainly by surface aspects and that
lower C and O contents will not result in greater fatigue
lives. However, cleaner processing decreases the
probability for fatigue crack initiation due to inclusions, but it
cannot suppress it completely. It is a matter of fact that
non-contaminating melting route should be used for the
production of high-quality NiTi alloys for biomedical
applications, according to the ASTM standard specification
F 2063-12 [
9
] that limits both the impurity contents as well
as the amount of particulates. Among non-contaminating
melting methods, PAM represents one of the most
important achievements in the field of high-quality alloy
production over the last three decades. The elevated working
pressure of PAM prevents the selective evaporation of the
high-vapor pressure elements avoiding chemical alterations
of final alloys composition. Moreover, inert operating
conditions of PAM, in conjunction with the transfer of a
high energy density, permit alloying of a great number of
metallic elements. For over 20 years, industrial PAM
furnaces have been employed for the production of reactive,
high melting points, refractory metals, and superalloys, as
reported in [
10–15
]. Thus, PAM seems particularly suitable
for melting ‘‘cleaner,’’ near equiatomic NiTi shape memory
alloys (SMA), not only on laboratory scale NiTi-based
button preparations [
16, 17
], but also for Nitinol industrial
production.
The aim of this work is the evaluation of the corrosion
resistance of a Ni50.8Ti49.2 alloy produced by a pilot scale
PAM furnace in comparison to one prepared through a
conventional VIM melting technique by using the same
starting Ni and Ti metals. Microstructures of PAM and
VIM NiTi alloys were investigated by FEG-SEM, and the
corrosion resistance was determined by potentiodynami (...truncated)