Abstract

Metallic wire used in medical devices contains small defects that must be accounted for in design to guard against failure. Sites of probable failure are often constituent inclusion particles, pores, or surface defects that behave as crack-like, stress concentrators. The aim of this research is to examine the effects of mechanical overload conditioning applied to medical-grade nitinol wire on fatigue performance. A mechanical overload conditioning treatment comprising a single axial tensile strain cycle of 11.5 % was applied at room temperature (300 K) to nominally Ti 50.9 at. % Ni wires with active Af 280 K. The conditioning strain cycle was applied to both plain wire samples with only process and melt-intrinsic defects and to samples which were milled by focused ion beam to produce a transverse 10×0.5×3 μm notch. Transmission electron microscopy was used to probe the root of the milled notch before and after overload conditioning in order to ascertain microstructural parameters responsible for property changes. Evidence of a plasticity-locked, mixed-phase, microstructure at the sharp defect root was found after conditioning. Samples were loaded in a rotary beam fatigue apparatus and cycled in air at 60 s−1 to a maximum of 109 cycles. The fatigue strain limit was increased by more than 20 % at 107 cycles in the conditioned versus non-conditioned plain wire.

1.
Myers
,
R. J.
, personal communication.
2010
.
2.
Duerig
,
T. W.
,
Pelton
,
A. R.
, and
Stöckel
,
D.
, “
The Utility of Superelasticity in Medicine
,”
Biomed. Mater. Eng.
 0959-2989, Vol.
6
,
1996
, pp.
255
266
.
3.
Shabalovskaya
,
S. A.
, “
Surface, Corrosion and Biocompatibility Aspects of Nitinol as an Implant Material
,”
Biomed. Mater. Eng.
 0959-2989, Vol.
12
,
2002
, pp.
69
109
.
4.
Holton
,
A.
,
Walsh
,
E.
,
Anayiotos
,
A.
,
Pohost
,
G.
, and
Venugopalan
,
R.
, “
Comparative MRI Compatibility of 316L Stainless Steel Alloy and Nickel–Titanium Alloy Stents
,”
J. Cardiovasc. Magn. Reson.
, Vol.
4
,
2003
, pp.
423
430
. https://doi.org/10.1081/JCMR-120016381
5.
McKelvey
,
A. L.
and
Ritchie
,
R. O.
, “
Fatigue-Crack Growth Behavior in the Superelastic and Shape-Memory Alloy Nitinol
,”
Metall. Mater. Trans. A
 1073-5623, Vol.
32
,
2001
, pp.
731
743
.
6.
Robertson
,
S. W.
, and
Ritchie
,
R. O.
, “
In Vitro Fatigue–Crack Growth and Fracture Toughness Behavior of thin-walled superelastic nitinol tube for Endovascular Stents: A Basis for Defining the Effect of Crack-Like Defects
,”
Biomaterials
 0142-9612, Vol.
28
,
2007
, pp.
700
709
. https://doi.org/10.1016/j.biomaterials.2006.09.034
7.
Schaffer
,
J. E.
, “
Structure-Property Relationships in Conventional and Nanocrystalline NiTi Intermetallic Alloy Wire
,”
J. Mater. Eng. Perform.
 1059-9495, Vol.
18
,
2009
, pp.
582
-
587
. https://doi.org/10.1007/s11665-009-9369-y
8.
Reinoehl
,
M.
,
Bradley
,
D.
,
Bouthot
,
R.
, and
Proft
,
J.
, “
The Influence of Melt Practice on Final Fatigue Properties of Superelastic NiTi Wires
,”
Proceedings of the International Conference on SMST 2000
,
S. M.
Russell
and
A. R.
Pelton
, Pacific Gove, CA May
2000
, ASM, Materials Park, OH, pp.
397
403
.
9.
Waitz
,
T.
,
Kazykhanov
,
V.
, and
Karnthaler
,
H. P.
, “
Martensitic Phase Transformations in Nanocrystalline NiTi studied by TEM
,”
Acta Mater.
 1359-6454, Vol.
52
,
2004
, pp.
137
147
. https://doi.org/10.1016/j.actamat.2003.08.036
10.
Kröger
,
A.
,
Wernhardt
,
R.
,
Somsena
,
Ch.
,
Eggeler
,
G.
, and
Wieck
,
A.
, “
In Situ Transmission Electron Microscopy-Investigations on the Strain-Induced B19-Phase in NiTi Shape Memory Alloys Structured by Focused Ion Beam
,”
Mater. Sci. Eng., A
 0921-5093, Vol.
438–440
,
2006
, pp.
513
516
.
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