Reference Library

Nitinol isn’t the only shape memory game in town, though its unique combination of properties has made it the most commercially successful. Did you know that there are alloys of brass, bronze, and iron, that also exhibit shape memory effects? In this post, Tom discusses some of the other materials with unusual shape memory properties. Read on for more…

1.3 Other Alloys Exhibiting Shape Memory Effects

Nitinol is rare in showing the required thermodynamic conditions for the shape memory effect, but by no means is it unique. Some alloys have garnered more attention than others and warrant greater discussion.

Beta titanium alloys (Ti-Nb, Ti-Mo, T-V, etc):

Memory effects in these alloys have been well known since the early 1980’s.  There are many concoctions that can be used, but all are similar—the idea is to stabilize the beta phase of titanium to just the right extent—any of a dozen or so “beta stabilizers,” alone or in concert, will serve that purpose.  Perhaps the most work has been done on Ti-35 wt.% Nb [1] and Ti-10V-2Fe-3Al [2]. The resulting alloys are not ordered like Nitinol, but rather have a body centered cubic (bcc) austenitic structure that transforms to an orthorhomobic martensite on cooling.  The alloys exhibit a large hysteresis with poorly defined shape recovery attributes compared to Nitinol. They also generate relatively little force and a boast a relatively small transformational strain (on the order of 3%). What makes them interesting is that they are easier to hot and cold form than is Nitinol, and they are free of nickel, which can at least be of emotional concern in medical implants. To date, no devices using the shape memory effect of beta titanium have been commercialized, but this probably represents the most active area of investigation outside of Nitinol itself.

Beta brass (Cu-Zn-Al and variations thereof):

As discussed above, knowledge of these alloys pre-dates Nitinol by over a decade, having been extensive studied in the early 1950’s [3]. These are ordered alloys, with rather complicated martensitic structures. Theoretically their memory effects are very large, however ductility limitations prevent the full effect from being realized. The alloys are also easily melted and transformation temperatures are much more easily controlled than in Nitinol. Thus during the 1970’s and early 1980’s, a great deal of effort was devoted towards commercializing Cu-Zn-Al alloys in actuators and fastener applications (often incorporating fourth elements, such as manganese to improve stability). Repeated disappointments were encountered due to the aforementioned ductility problem, poor corrosion resistance and perhaps most limiting, severe instability in transformation temperatures even at room temperature—ageing the martensite phase at room temperature severely stabilized that phase, leading to increased A_s and A_f temperatures. As Nitinol became more available, nearly all work on the Cu-Zn-based systems was halted.

Bronzes (Cu-Al-Ni, Cu-Al-Be, etc.):

Like the brasses, these alloys were extensively studied prior to Nitinol. They have one significant advantage over both Cu-Zn-Al and Nitinol in that one can achieve reasonably stable shape memory effects at high temperatures (well above 100°C). Unfortunately, despite a great deal of effort, little if any success was made in achieving useable ductility in wrought polycrystalline materials. Some success has been gained through powder metallurgical efforts [4], and certainly single crystals [5] are commonplace with realizable transformational strains well in excess of those that can be obtained Nitinol. While the use of single crystals in commercial applications may seem far-fetched, at least one application has been reduced to commercial sale (eyeglass frames). Forming without disturbing their single crystal stature is difficult, but still, one cannot exclude the possibility that these problems can be solved.

Iron-based alloys (Fe-Mn-Si, Fe-Co-Cr, etc.):

Even though the memory strains in these systems are small and the forces weak, these alloys continue to be studied due to their low costs. With typically 3% memory strain available even under ideal conditions, it is unlikely that they are suitable for fasteners, and their large hysteresis makes them a poor choice for actuators.

Nickel-Aluminum:

Ni-Al, near the equiatomic composition exhibits all the attributes one would think are needed for a significant shape memory effect, but with a transformation temperature well above room temperature. While there has been a great deal of interest because of the high transformation temperatures, ductility and toughness are exceedingly poor by any standards. To the authors’ knowledge, little progress has been made to mitigate this problem despite a great deal of work by highly skilled researchers.

Of course this is not a completely list of alloys exhibiting the effects. Others tend to be more esoteric, including In-Tl, U-Nb, Au-Cd, Ag-Cd and Ru-Ta. The authors do not anticipate any of these as being of commercial value in the pre-Startrek era.

1. C Baker, Metal Science Journal 5 (1971).
2. TW Duerig, et al, Acta Metall. 30 (1982) 2161.
3. E Hornbogen and G. Wassermann, Z. Metallkunde 47 (1956) 47.
4. TW Duerig, J Albrecht and GH Gessinger, J. Metals (December 1982) 14.
5. H Sakamoto and K Shimizu, Phase Transformations ’87, Cambridge (1987) 6.