Tech note: Nitinol Wire Braided Components

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A Historical Perspective

The history of steel dates thousands of years to ancient times, stainless steel has been prominent for about a hundred years, and Nitinol for perhaps a couple of decades. In this short chapter, Tom traces the ancestors of modern nickel titanium, dating to the 1930’s. Read on for more…

1.2 A Historical Perspective

This post is an excerpt from Nitinol: The Book, a working draft of an upcoming publication by Tom Duerig, Alan Pelton, and others. Visit the Table of Contents or Introduction for more information.

The discovery of metallic phase transformations that could lead to unusual macroscopic shape changes can be traced to 1932, to the work of Ölander in Au-Cd [1], even though one could hardly say that Ölander’s work constituted a proactive discovery of either shape memory or superelasticity. That claim instead can probably best be made by Chang and Read [2] during their study of the Au-Cd systems in 1951 [2] or by Reynolds and Bever’s studies of Cu-Zn in 1952 [3]. These later works clearly recognized that certain alloys could be deformed in their martensitic phases then heated to recover their original shape. Rachinger [4] discovered superelasticity in 1958 and even coined the term still in use today.

The observation of a shape memory effect in NiTi (Nitinol) is properly credited to Buehler, Gilfrich and Wiley [5] though they were certainly not the first to work with the near atomic NiTi alloy.  Interesting enough they completely misunderstood the origins of the effect they had accidentally discovered, despite all the previous work in Au-Cd and Cu-Zn that had correctly detailed the mechanisms involved. In fact, they failed to even recognize the link to martensite even though Purdy and Parr had previously reported the existence of martensite in Nitinol [6], dubbing it the “p phase.” Still, Buehler, et al’s discovery captured the imagination of the scientific world, and work began in earnest to try to understand the effect and what could be done with it. By 1971 it was recognized that the shape memory effect in Nitinol was the very same phenomenon that had been reported in the previously studied alloys [7].

The history of Nitinol’s commercial development is also rather irregular. Despite a plethora of governmental and industrial research programs and hundreds of patents, very few products were commercially launched in the 1960’s and 1970’s. A variety of factors were responsible for these failures, perhaps paramount of which were high costs and a lack of reliable material supply. Difficulties in manufacturing stemmed from melting accuracy, purity, forming and machining. There was also a clear and consistent lack of understanding regarding what could and could not be done with the material—or perhaps more importantly, what should and should not be done, for people learned the hard way that Nitinol could be used for a lot of things that it shouldn’t be used for. Attention began to turn to the much less expensive copper-based alloys. Not only were they less expensive, but the “cost of entry” was far less: melting could be done in simple, inexpensive furnaces and alloys were easily reduced to wire, machined, and even tested.

In the early 1980’s however, scientists and engineers were faltering on this front as well; failure to develop an acceptable combination of corrosion resistance, ductility and stability returned the commercialization torch to Nitinol. By this time, however, twenty years had passed without finding a commercially successful product and Nitinol had lost its sparkle in the eyes of both industry and funding agencies. It became a pariah, infamously known as a “solution looking for a problem.” While a cruel jab at those working in the field, in many ways it was right on the mark: The search for applications was being conducted by shape memory experts trying to find something worth doing, rather than by product designers and engineers that had profound knowledge of their field of application—this was a charge that had to be led by users, not providers.

The first Nitinol application (or any shape memory application, for that matter) to be sold rather than merely developed was fluid fitting couplings in the early 1970’s [8]. Though these were very high volume applications on tonnage basis, there were plenty of competitive conventional joining approaches, and the marketplace advantages were simply inadequate to command prices high enough to cover Nitinol’s high costs. Nitinol pipe couplings are still sold today, though the market has been flat for many years now, and the demand is largely historical. Still, it is important to recognize that the fluid fitting couplings did more than just pioneer commercial use: They provided the incentive to learn how to manufacture the alloys in large quantities, finally making high quality material available to researchers around the world. Once material was broadly available, product designers rather than shape memory experts began their search for applications—a much broader search that would soon bear fruit.

The first application of Nitinol in medicine is somewhat ill-defined, at least if one ignores orthodontic archwire. The Chinese get credit with pioneering human implants in the early 1980’s [9]. The first “for-sale,” agency-approved medical device was most likely the Homer Mammalok, by Mitek [10] and the first commercially marketed permanent implant was likely the Mitek bone anchor [11]. With the ice now broken, the 1990’s brought about a demand to make smaller, less invasive medical devices, which in turn led to an astounding growth of the Nitinol industry. Paramount to this growth was the birth of the stent, a device used to scaffold diseased arteries. Even though less than twenty percent of all stents are made from Nitinol, it is still by far the most commercially-important product ever made from Nitinol. Interestingly enough, the very first stent concepts were Nitinol [12], not stainless steel (though these first prototypes were never marketed).

It is also interesting to examine the growth of the scientific shape memory community itself. The year 1975 witnessed the first dedicated and published symposium on the shape memory effect, held in Toronto, though it was shared by Nitinol and copper-based alloys [13]. Here, for the first time, scientists shared their thoughts on applications, including damping, actuators, connectors, and even medical applications. The organization and conference known as ICOMAT (International Conference On MArtensitic Transformations) incorporated the shape memory effect into their 1979 conference in Boston [14]. These conferences (held every three years) were devoted to martensitic transformations in general, but invariably set aside an increasingly large forum for scientists to share their recent developments regarding shape memory alloys. The ICOMAT conferences and proceedings continue to be an invaluable resource to those interested in the latest work regarding shape memory alloy understanding and development.

In 1988, the first conference specifically and entirely devoted to the engineering aspects and application of shape memory alloys was held in Michigan [15]. The level of interest in the engineering aspects of these alloys was evidenced by the rapid sale of the entire printing of the proceedings. In 1992, a society was formed devoted entirely to the engineering aspect of these alloys, called the society of Shape Memory and Superelastic Technologies (SMST).  SMST held their first conference at Asilomar in California in 1994 [16], and has held a series of successful conferences since.  SMST was recently incorporated, and is now operated as an adjunct society to ASM International.  SMST has provided an invaluable forum for people to gather, share information, and publish results that are oriented more towards engineering problems rather than materials science

  1. A Olander, J. Amer. Chem. Soc. 54 (1932) 3819.
  2. LC Chang and TA Read, JOM 191, (Jan. 1951) 47.
  3. JR Reynolds and MB Bever, Trans AIME 54 (1952) 76.
  4. WA Rachinger, British J. Appl. Phys. 9, (1958) 250.
  5. WJ Buehler, JV Gilfrich and RC Wiley, J Appl Phys 34(5) (1963) 1475.
  6. GR Purdy and JG Parr, Trans Met Soc AIME 221 (1961) 636.
  7. RJ Wasilewski, Met Trans 2,(1972) 2973.
  8. JD Harrison and DE Hodgson, Use of TiNi in Mechanical and Electrical Connectors, in Shape Memory Effects in Alloys (J Perkins, ed.) (1975) 517.
  9. SMA-86, China Academic Publishers, (C. Youyi, TY Hsu and T Ko, eds) (1986) 379-446.
  10. JP O’Leary, JE Nicholson and RF Gatturna, in Eng. Aspects 477.
  11. RF Gaturna, JE Nicholson and J O’Leary, US Patent 4,898,156, (1990).
  12. CT Dotter et al., Radiology 147(1) (1983) 259.
  13. J Perkins, Shape Memory Effects in Alloys (AIME, New York) (1975).
  14. ICOMAT ’79, Cambridge, MA, edited by the MIT Dept. of Materials Science and Engineering.
  15. TW Duerig et al., Engineering Aspects of Shape Memory Alloys (Butterworth-Heinemann) (1990).

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