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Tohoku University: Ferrous Superelastic Shape-Memory Alloy

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The research and development of shape-memory alloys is a field in which Japan is particularly strong, producing a significant number of innovative research findings in recent years. One of the most important of these has been the development of ferrous superelastic shape-memory alloy.

Shape-memory alloy is “memory metal”: it may undergo deformation at temperatures below a certain point, known as the transformation temperature, but has a “shape-memory” ability that allows it to return to its original, undeformed shape when heated above the transformation temperature. The scope of deformation possible in shape-memory alloys is significantly larger than that of traditional springs made from materials such as steel, which is the reason why the shape-returning property of this kind of shape-memory alloy is sometimes called superelasticity. Shape-memory alloy is metal that is able to return to its original shape when heated, but there are other metals that are able to return to their original shape without heat, as long as the force (stress) causing the deformity is removed. These metals are known as superelastic alloys. Research and development into shape-memory alloys and superelastic alloys began in the 1950s. In recent years these years of cumulative research have proven fruitful, with certain findings being developed into commercial applications. But many problems remain.

The shape-metal alloy most commonly applied in medicine and apparel is NiTi-based shape-memory alloys (nitinol). It has definite shortcomings, however, such as low cold workability and high production costs. Aside from nitinol, Fe-Mn-Si alloys are also slowly becoming commercially viable, but although they display shape-memory ability they do not show superelasticity, limiting their potential applications.

Dr. Yuki Tanaka has been heavily involved in the research and development into the new super alloy introduced here; he proposed that titanium be replaced by aluminum, as well as a number of other elements, in the composition of conventional ferrous shape memory alloy (Fe-Ni-Co-Ti alloy).

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( Tohoku University )