Its good to be back on the blog.
This post is motivated by a news item in the R&D section of my professional house journal ,IOM3's Materials World (MW Aug 2011) entitled "High-strength alloy takes the strain" It reports briefly on Tohoku Univ. (Jp) work on superelastic materials.
Superelastic Effect in Polycrystalline Ferrous Alloys
In superelastic alloys, large deformation can revert to a memorized shape after removing the stress. However, the stress increases with increasing temperature, which limits the practical use over a wide temperature range. Polycrystalline Fe-Mn-Al-Ni shape memory alloys show a small temperature dependence of the superelastic stress because of a small transformation entropy change brought about by a magnetic contribution to the Gibbs energies. For one alloy composition, the superelastic stress varies by 0.53 megapascal/°C over a temperature range from –196 to 240°C.
T. Omori*, K. Ando, M. Okano, X. Xu, Y. Tanaka, I. Ohnuma, R. Kainuma, K. Ishida
Science 1 July 2011: 68-71.
Expanding the Repertoire of Shape Memory Alloys
The exceptional properties of many materials often come at the expense of limited performance in other areas. For example, conventional metals and their alloys are strong—they are good at resisting stress (i.e., an applied load)—but they tolerate only a very small amount of strain (i.e., deformation) before they are
irreversibly deformed. Rubber can easily return to its original shape, even after large deformations, but is much weaker than conventional metals. However, some metal alloys exhibit “shape memory”; they are strong but can recover from being deformed when heated. This process seems counterintuitive, but these alloys take advantage of solid-to-solid “diffusionless” phase transitions: The atoms rearrange how they pack into crystals in an orderly fashion, and this process changes the material's macroscopic shape. Few other materials possess this combination of strength and flexibility (see the figure), and clever engineering has exploited these properties—for example, in implanted medical devices such as stents. On page 1488 of this issue, Tanaka et al. (1) report on a superelastic alloy that almost doubles the useful range of deformation that can be induced in such alloys.
Ji Ma and Ibrahim Karaman, Science 19 March 2010: 1468-1469.
Ferrous Polycrystalline Shape-Memory Alloy Showing Huge Superelasticity
Shape-memory alloys, such as Ni-Ti and Cu-Zn-Al, show a large reversible strain of more than several percent due to superelasticity. In particular, the Ni-Ti–based alloy, which exhibits some ductility and excellent superelastic strain, is the only superelastic material available for practical applications at present. We herein describe a ferrous polycrystalline, high-strength, shape-memory alloy exhibiting a superelastic strain of more than 13%, with a tensile strength above 1 gigapascal, which is almost twice the maximum superelastic strain obtained in the Ni-Ti alloys. Furthermore, this ferrous alloy has a very large damping capacity and exhibits a large reversible change in magnetization during loading and unloading. This ferrous shape-memory alloy has great potential as a high-damping and sensor material.
Y. Tanaka1, Y. Himuro1, R. Kainuma2,*, Y. Sutou1, T. Omori1 and K. Ishida1,
Science 19 March 2010: Vol. 327 no. 5972 pp. 1488-1490
DOI: 10.1126/science.1183169
MATERIALS SCIENCE
OTHERS WHO REPORTED THESE FINDINGS
- Fellow blogger : http://gblogger-metallurgy.blogspot.com/2010/03/ferrous-polycrystalline-shape-memory.html
- My professional house journal, IOM3’s Materials World.
FUNDAMENTAL APPROACHES
REVIEWS OF TOPICAL PROBLEMS PACS numbers: 62.20.Fe, 75.30.Kz, 75.80.+q, 81.30.Kf
Shape memory ferromagnets
A N Vasil'ev, V D Buchel'nikov, T Takagi, V V Khovailo, E I Estrin
SEARCH SCIENCE
KEY WORDS: alloy,superplastic,forming
