New Stainless Steel Shape Memory Alloys
 |
Some 800 couplings made of shape memory nickel-titanium alloys are used in this U.S. Navy F- 14
Tomcat   Low-cost shape memory alloys could find more uses |
Stainless steel could broaden the commercial appeal of shape memory alloys from fighter jets to
more common applications
Nickel magazine, Jun. 01 -- Stainless steel may soon provide a low-cost alternative to
alloys that are valued for their ability to snap back to original form but are too expensive for widespread
use.
"Shape memory" alloys behave like no other material. They can be twisted and stretched out of shape at low
temperatures, but when reheated, they instantly revert back to their original shape. Although scientists have
known about this useful trick for decades, the high cost of even the commercially successful titanium-nickel
and copper-base alloys have limited applications to high-end ones, such as fighter jets and blood-clotting
filters.
Now, newly developed stainless steels, containing from 4.4 to 6.3% nickel, promise to expand the market for shape memory materials, says Chenxu Zhao at Xi'an Jiaotong University in China. These steels are more corrosion-resistant, easier to work with and, moreover, much less expensive than shape memory alloys currently in use.
So why isn't stainless steel a bigger player in the shape memory market already? Until recently, thermo-elastic transformation was thought to be a necessary condition for the shape memory effect. This phenomenon is characterized by a small hysteresis between the temperature at which the material begins to retain its distorted shape (the martensite phase) and the temperature at which it remembers and flips back to its original cubic structure (the austenite phase). The effect of thermo-elastic transformation is too weak in stainless steel to be considered worthwhile.
But researchers have found that certain steels (FeNiC, FeMnSi, FeMnSiCrNi) undergo a non- thermo-elastic transformation but still exhibit good shape memory effect. These materials rely instead on a stress-induced transformation to restore their shape. The reversible strain is less than that of traditional shape memory alloys (3% compared with 8% for titanium-nickel and 6% for copper-based alloys) but strong enough to warrant further research into commercial applications.
Chenxu Zhao says pipe couplings are the most practical use for shape memory stainless steels. Couplings that shrink instantly to provide an effective seal over smaller diameter pipes could have a welcoming market at the right price. Traditionally, this market has been limited to high- end applications, as mentioned.
"The lower-cost shape memory stainless steels have great potential for industrial applications in this field, because 2% reversible strain is sufficient for pipe joint applications, says Chenxu Zhao in the February edition of Advanced Materials and Processes.
One Chinese company has already capitilized on this discovery. Shanghai Tianhe Shape Memory Material Corp. claims to have developed a stainless steel coupling that, when heated to 250ºC, shrinks to fasten two separate pipes together.
Shape memory stainless steels could also prove to be useful in marine environments where corrosion is a concern.
Photo: U.S. NAVY
|
Chenxu Zhao |
