Fort Wayne Metals Research Products Corporation, Fort Wayne, Indiana, USA, and
Purdue University, West Lafayette, Indiana, USA
Grain refinement and microstructural size are critical in the design of nearly all metallic structures. In aircraft jet turbines, large crystals are grown along the load axis to create strength in a preferred direction. In fine medical wire, grain sizes are reduced to promote resistance to fatigue and to increase strength properties. Proposals by Hall and by Petch quantified the strengthening mechanisms of grain refinement in the familiar Hall–Petch relationship.1 In recent years, the microstructural size scale has generally been divided into three ranges:
- macroscale comprises the range from hundreds of microns to millimeters
- microscale comprises tens of microns
- nanoscale has arbitrarily been defined as 100 nanometres or less.
Today, microstructural process output can be monitored using advanced high resolution scanning electron micro-scopy (SEM) and transmission electron microscopy (TEM) with resolutions approaching several angstroms. These techniques have been used directly in the production of carbon nano-structures and in nanodeposition of metallic materials.2 To better understand the nanoscale regime, many researchers today are heavily involved in the study of microstructure and accompanying property relations at the nanoscale.3 The study described in this article provides an insight into the beneficial properties that can be imparted to wire for medical devices through microstructural control at the nano level.
Grain size and fatigue behaviour
Figure 1: Polyslip surface features and 200 nm PSB protrusion near crack front in System B alloy after 2.05 x 105fd cycles at 690 MPa.
System B (SPS Technologies, Jenkintown, Pennsylvania, USA).
Recent papers have given significant attention to the behaviour of nanoscale microstructures in fatigue.3,8,9 Some studies have reached phenomenological conclusions, and others have achieved physically quantifiable results. Studies that have achieved quantifiable results have been limited to pure metals such as copper, aluminium, titanium and binary intermetallics such as iron aluminides.8,9 Recent research has primarily focused on theoretical discussion of what should occur and secondly on a limited amount of experimental data. Hoppel et al. showed that ultra fine grain structures achieved through equal channel angular pressing (ECAP) in brass, copper and aluminium resulted in a significant reduction in fatigue life in strain-controlled low-cycle fatigue testing.9 This result may be due to the high dislocation or internal lattice defect density expected from the ECAP process. Deleterious effects on crack growth were found by Hanlon et al. in nanocrystalline (NC) nickel. In damage tolerant design in which fatigue lifetime is gauged by the remaining life in a cracked structure, this behaviour would result in reduced product life and warrant the use of a conventional microcrystalline (MC) structure. However, the results were positive in the high cycle fatigue testing where they showed an increase in the fatigue threshold.3 For the purpose of the study discussed below, low cycle fatigue is considered to be less than 105 cycles; high cycle fatigue is defined as greater than 106 cycles.
Figure 2: Typical wire diameter to grain size ratio in MC System B medical wire.
To be useful and predictable in engineering applications, structural metals and alloys require strict microstructural control. The level of control required is dependent on the application as well as the size of the finished component. As demonstrated in several recent studies,10,11,12 the importance of metallurgical consistency is critical to fatigue behaviour in medical devices that incorporate fine wire elements such as microcoils or cables used in cardiac rhythm management systems. Intrinsic inhomogeneities in the alloy matrix such as constituent particles (inclusions) and large grains as well as extrinsic surface defects can result in premature wire failure during fatigue loading.12 New thermo-mechanical processing technology (patent pending) has recently been developed that allows control of grain size in medical wire in the sub 1 micron range.12
Microcrystalline medical wire
Figure 3: Typical MC grain structure in System A medical wire.
Typically, medical wire processing involves microstructural refinement through repeated cold reduction and progressively shorter dwell recrystallisation anneals that yield increasingly fine distributions of grain.11 Despite continual refinement of grain size, as wire diameters become smaller, the average number of grains through a transverse section is typically reduced. Thus, in a relative sense, the microstructure becomes increasingly coarse as wire diameters become smaller. Grain structure of a typical medical wire of MC low inclusion alloy 35 Co-35 Ni-20 Cr-10 Mo, referred to here as System A is shown in Figure 3.
Table I: Comparison of ASTM F562 chemistry.
Figure 4: Grain size distribution in NC System A medical wire.
Figure 5: Surface microstructure in a System A failed specimen tested at an alternating stress of 1103 MPa. Section failed in 1 M cycles at this test level.
Figure 6: Stress-life (S-N) representation of ASTM F562 medical wire fatigue data showing realised increase in fatigue strength over the past decade.
Low dislocation density in the NC study with System A has not yet been confirmed with TEM examination, although this is in progress. Evidence of the low damage state and the effect of grain size on fatigue were confirmed through high cycle fatigue testing and uniaxial tension testing.
In the last decade, there has been significant growth in research on ultra fine microstructures and their physical properties. Ni is one of the more complex materials that has recently been examined as a candidate for nanoscale microstructural control to achieve high strength.17 Despite these advances, at the time of writing, little has been done to explore property relations in nanostructured materials of complex alloy systems such as high strength super alloys. The true study of nanostructure has only been possible since just before the turn of this century. Much more research is needed to fully understand and utilise the unique property capabilities of these highly engineered microstructures.
1. E.O. Hall, “The Deformation and Ageing of Mild Steel: III Discussion of Results,” Proc. Phys. Soc., B 64, pp. 747–753 (1995).
2. S. Zhiwei et al., “Grain Boundary-Mediated Plasticity in Nanocrystalline Nickel,” Science, 305, pp. 654–657 (2004).
3. T. Hanlon et al., “Fatigue Behavior of Nanocrystalline Metals and Alloys,” Intl. J. Fatigue, 27, 1147–1158 (2005).
4. D. Hull and D.J. Bacon, “Introduction to Dislocations,” Butterworth Heinemann, Burlington, MA, USA, 1st edition (2001).
5. K.S Chan,“A Microstructure-Based Fatigue-Crack-Initiation Model,” Metallurgical and Materials Transactions, 34A, pp. 43–58 (2003).
6. T. Kitamura and T. Sumigawa, “Slip Behavior and Local Stress Near Grain Boundary in High-Cycle Fatigue of Copper Polycrystal,” JSME Int. J., 47, pp. 92–97 (2004).
7. M.R. Lin et al., “Fatigue Crack Initiation on Slip Bands: Theory and Experiment,” Acta Metallurgica, 34, 4, pp. 619–628 (1986).
8. J.W. Morris, Jr., “The Influence of Grain Size on the Mechanical Properties of Steel,” Proceedings of International Symposium on Ultrafine Grained Steels, Iron and Steel Institute of Japan, Tokyo, pp. 34–41 (2001).
9. T. Sleboda et al., “The Possibilities of Mechanical Property Control in Fine Grained Structures, J. Mat. Proc. Tech., 177, pp. 461–464 (2006).
10. H.W. Hoppel et al., “An Overview of Fatigue Behavior of Ultrafine Grained Metals and Alloys,” Int. J. Fatigue, 28, pp. 1001–1010 (2006).
11. L. Kay et al., “Optimisation of Melt Chemistry and Properties of 35 Cobalt-35 Nickel-20 Chromium-10 Molybdenum Alloy Medical Grade Wire,” Proceedings of ASM International Materials and Processes for Medical Devices Conference, MPMD, Ahaheim, CA, USA, p. 1–7 (2003).
12. J.E. Schaffer, “A Probabilistic Approach to Modeling Microstructural Variability and Fatigue Behavior in ASTM F562 Medical Grade Wire,” Proceedings of the 9th International Congress, Fatigue 2006, Atlanta, Georgia, USA, Elsevier Inc. (2006).
13. J.E. Schaffer, “A Hierarchical Initiation Mechanism Approach to Modeling Fatigue Life Variability in 35Co-35Ni-20Cr-10Mo Medical Grade Fine Wire,” Purdue University, School of Mechanical Engineering, West Lafayette, Indiana, USA (2007).
14. Superalloys developed by SPS Technologies (Jenkintown, Pennsylvania, USA) for aerospace fasteners. Product brochure (1998).
15. S.C. Baik et al., “Dislocation-Based Modeling of Deformation Behavior of Aluminum Under Equal Channel Angular Pressing,” Mat. Sc. & Eng., A, 351, pp. 86–97 (2003).
16. P.A. Altman et al., “Rotary Bending Fatigue of Coils and Wires Used in Cardiac Lead Design,” J. Biomed. Mat. Res., 43, pp. 21–37 (1998).
17. H. Rumpf et al., “High Strength in Nano-Grained Superelastic NiTi Thin Films,” Mat. Sc. & Eng., A, 415, pp. 304–308 (2006).
Jeremy E. Schaffer M.Sc. is an Engineer at Fort Wayne Metals Research Products Corporation, 9307 Avionics Drive, Fort Wayne, Indiana 46809, USA, tel. +1 260 918 3772, e-mail: firstname.lastname@example.org
Some of the research for this work was performed at Purdue University, West Lafayette, Indiana, USA.